akavalve

Guitarcon 2011 in Fitchburg, MA

2011-04-09T07:05:00.000-07:00

I'll be joining my friend Adam from Stompbox Sonic at his booth at Guitarcon 2011.

I'll be bringing along my modded C600 and a number of other small tube amps.
Bring your guitar and check 'em out and bring your amp questions in person.

Drawing up a component layout from a tube amp schematic

2009-11-12T03:45:00.000-08:00

I'm building up a low wattage tube practice amp roughly
based on an old Gregory Gemini 700 / Mark 5.
Since the circuit is pretty simple, the process may help you to learn
how to translate a schematic diagram into an actual tube amp layout.
Here's a schematic for the No. 2 version of the Gregory:

So how do I get from that schematic to this:

It's not too difficult if you take it one step at a time.
I'll show you loosely how I approached it for this one.

First have a look at the schematic and try to divide it into functional blocks.
That will keep you from getting confused as you go along.
It'll also be much easier to troubleshoot if everything doesn't work on the first try.

Below I've boxed the preamp stage in red, the tremolo section in orange,
and the power and output section in black:

Now you need to figure out what the components are actually
going to be physically and electrically attached to.
I find that some kind of tagboard or turret board makes the planning
and subsequent wiring neat and easy to follow.

I picked this board from Antique Electronic Supply.

Now on a sheet of graph paper I drew in the components
for the individual amp stages in colors that match
the sections I indicated on the schematic.
Then I drew in all the connections that would need
to be made according to the schematic.

At this point check and double check you work.

This drawing is done imagining the tagboard oriented vertically.
Each of the rectangles relates to one resistor or capacitor in the circuit
and the lines represent connections between them.

I do this stuff pretty regularly, so you'll see I'm not super fussy about neatness or corrections.
You'd want to be a bit more careful yourself until you can rely on your instincts to catch errors:

There also components which will not be mounted on your board
(control pots, tube sockets, transformers etc).
I've been lazy here and only indicated the pots.
They're in green on my hand drawing and circled in green on the schematic below.
You should probably be careful to include them all if you haven't done this before.
Thoroughness will pay off in the end.

Now turn the layout drawing in the same direction as the tag board
and you're ready to start soldering:

I install all the components first and then move onto the connections.
Here are the caps and resistors installed on the board:

And here's the board with the electrical connection in place:

Now all that's left is to install the board and make the connections
to the chassis mounted components and fire it up to see if it works!

If you're paying very close attention, you'll notice that my layout
drawing doesn't match the schematic drawing exactly
(and the board doesn't exactly match the drawing).
I've made some changes as I been working on these amps
but I haven't drawn up a revised schematic yet.
The one here should serve well enough for the example though.

Low Wattage Tube Practice Amp - Plywood Mockup

2009-11-02T03:34:00.000-08:00

I've been working out a circuit for a low wattage tube practice / recording amp inspired by the Gregory Gemini 700 / Mark V . This weekend I took a break from the electronics and built a plywood mockup for the cabinet to see how I liked the size feel of it at home. Here it is in my living room:

I'm going to give it some kind of tilt back legs. Here it's just propped up on an Altoids tin to see how it looked tipped back a bit. I'm pretty happy with the size of it in the house. Once I wire up the chassis I can see how it sounds with a speaker in there. If all's well I'll build up the cabinet in real wood with real joinery.

Here it is with the cat for scale (and because, oddly, they match):

Our cat is a mammoth - 14 lbs fit and trim - so the cab is actually a bit bigger than it looks. I'm planning to put a Weber 8" in there, but there's enough room for a 10" in case the 8" doesn't cut it. The baffleboard is designed to be easy to remove so I can try a few different speakers without a lot of fuss. I may even try a pair of Weber 6's in there for kicks.

Epiphone Valve Junior - the Gain Matrix Mod Part II

2009-10-21T17:23:00.000-07:00

I'm going to start describing the Valve Junior Gain Matrix Mod with the physical side of the job. There are three gain stages in the Valve Junior. I'm installing one switch for each of these stages. Here's the chassis marked out for drilling the switch holes:

The switches will change the gain and frequency response by selecting and disconnecting the cathode bypass capacitors. Here is a front of chassis view the three switches installed:

And here they are from inside the chassis:

In the following post you'll see that the wires are color coded the wires to make it a bit easier to follow the wiring.
The yellow wires are for the "stock" configuration for each stage.
They connect to the left hand terminal of their respective switch.
The blue wires are for the "mod" value caps.
They connect to the right hand terminals.
The black wires are for ground and they connect to the center terminals:

Next up I'll show the actual circuit wiring and in the fourth I'll draw up a schematic for the mod.

Champion 600 Speaker Upgrade Comparison

2009-10-20T03:06:00.000-07:00

I installed four different speakers in my Champion 600 and ran some frequency response sweeps for the sake of comparison.*

What I saw in the graphs pretty much confirmed what I heard in each of them.

First the stock speaker compared to a Jensen Mod:

Just as my ear was telling me - the Jensen is a touch more efficient and a touch smoother but they really didn't sound all that different. I found the difference to be hardly worth the upgrade cost.

Here is the Weber alnico with a late breakup cone compared to the stock Champion 600 speaker:

Notice that the Weber late breakup alnico is a good bit more efficient than the stock speaker up to about 3K, where it drops off dramatically. This probably accounts for some people experiencing the Weber as dark sounding. What isn't evident from the graph is that in addition to the increased bass response the Weber is more able to handle the bass so it doesn't fart out the way the stock speaker does.

The next graph is interesting. It compares the Weber late break up ribbed cone to the Weber early break up smooth cone:

They're pretty similar in the bass but in the upper mids and treble they're very different. So in addition to the breakup characteristics they have quite different upper range response. Personally I prefer the smooth cone sound - it sounds a bit more vintage to my ear. The ribbed cone sounds tighter and more modern. They both sound great though and are both worth the upgrade.

Here is the smooth cone Weber compared to the stock speaker:

It look much more like the original in the treble response. Kind of like the stock speaker with increased bass and mids (and better bass handling).

Here's plots for all four speakers on the same graph - just for ease of comparison:

Incidentally, I found that the stock grill cloth really does flap around at higher volumes making some nasty farting and flapping along with the low notes. I cut mine out when I cut down the baffleboard for the Weber speaker installation and am looking for something to replace it with.

* These graphs are for the sake of comparison between the speakers. Since they aren't anechoic chamber tests the graphs include the room effects so don't pay to much attention to each tiny peak and dip. They don't show isolated speaker response but work fine to illustrate how the speakers differ from each other under the same room conditions.

Epiphone Valve Junior - the Gain Matrix Mod

2009-08-03T19:07:00.000-07:00

Here's a gain reduction mod I recently completed on an Epiphone Valve Junior. The owner was looking for less volume and less gain. This mod has 3 switches each with three positions:

With all the switches set to the left the amp is stock. Flipping a switch to the center position reduces the gain by taking the cathode bypass cap for that stage out of the circuit.

Flipping it all the way to the right switches in a different value cap for the cathode bypass. In the right position the upper two switches act as differently voiced bright/body switches and the bottom one acts as a deep switch.

The amp can now be made quiet enough to be used at home. It can be cleaned up for less gainy sounds and brightened for more clarity. And it can be quickly returned to stock if you miss that muddy sound it had when it was new. The various combinations of three switches give a total of 26 alternate voicings.

I shot pictures of this mod but I haven't written up a whole description. If anyone is interested in seeing the whole process, let me know in the comments section of this post and I'll put the details together.

How to fit a Weber alnico speaker into a Champion 600 with a JJ 6V6

2009-07-26T06:08:00.000-07:00

The original owner of this Champion 600 decided to move to a larger amp . I'd put so much time into the mods that I decided to buy it from him to try a few things he hadn't been quite ready to spring for himself. The first was to try a couple Weber 6" alnico speakers. If you'd like to see the index of mods I've done on this little amp, it's here. This is the back of the amp with the "early breakup" smooth cone alnico speaker installed:

Notice the nice bit of clearance between the magnet and the JJ Tesla 6V6. In a stock Champion 600 cabinet the big magnet on the Weber Alnico barely fits if you have a larger bottle 6V6 like the JJ in there. To get the clearance I moved the baffleboard over.

The baffleboard that is in there already has a bit of play. If you unscrew it you can slide it to the left and remount it. That'll give you just enough space to get the speaker in without it touching the tube. Here's the front of the amp with the new Weber installed:

It's easy to see the shifted speaker because I've also removed the grill cloth. The original cloth is so thick that it causes some farting out on low notes. It made a good bit of difference to get rid of it. I plan to find something appropriate to replace it with once my speaker experiments are done.

The stock baffleboard won't shift quite as far as you see in the photo above. It was close enough but I wanted to have a healthy bit of space between the speaker magnet and the power tube so I cut the baffleboard down by a half an inch and slid it over even further. You can get away with this because the TV style front actually covers a good bit of the baffleboad. I used silver sharpie so the cut and drill markings would be clear in the picture:

I marked off one half inch from the right edge and marker centers for new mounting holes a half inch to the left of the originals. That's about 13mm for people sensible enough to be on the metric system.

Here's the baffleboard cut down to size and with the new mounting holes drilled out.

If you do this yourself it's important to note that this view is from the FRONT of the baffleboard. If you do the same thing looking at the back you'll move the speaker in the wrong direction!

Currently on hand I have the stock Champion 600 speaker, a Jensen Mod and two Webers (early and late breakup). I'll post frequency response and sensitivity comparisons in the next round.

Where is the dangerous voltage in a tube amp?

2009-07-24T05:13:00.000-07:00

"UNPLUG YOUR AMPLIFIER AND DISCHARGE THE FILTER CAPS BEFORE YOU START WORK INSIDE A TUBE AMPLIFIER. THERE ARE LETHAL VOLTAGES INSIDE THAT CAN KILL."

Everyone who's started working with tube amps has heard that a hundred times. And it's absolutely essential advice. But obviously you need to turn the amp back on to test your work. Is it safe to run the amp while it's out of the cabinet as long as you're don't poke around inside?

Not necessarily. Dangerous voltages can appear where you least expect them - either through circuit design or component failure.

Here's an Ampeg V4 / VT22 that's in for a tune up. The reverb pan for these amps is under the top cover and if you're doing extensive work it's much more convenient to unplug it. Here your can see the RCA cables that connect to the pan dangling above the chassis:

Now normally one doesn't associate reverb leads with high voltages, so you might be inclined to let them hang where ever they may. Lets take a look at what happens when the amp is switched on:

The meter here is measuring the DC voltage between the center pin of a reverb lead and the amplifier chassis. It's showing 0.013 volts. That wouldn't shock a flea. Nothing wrong there. What happens when the standby switch is thrown into the operate position?

181 Volts! You'll be glad if that's not dangling out the back of the chassis touching the screwdriver you're about to pick up! This voltage isn't there for all that long. The sequence below shows 1 second intervals.

It drops from 181.2 to 104.7 in just a couple seconds. In 15 seconds the voltage drops to 12 volts and keeps dropping from there. Because of the grounding scheme in the standby circuit of the V series Ampegs high voltages appear all over the amp in places you wouldn't usually expect them - even when the amp is in standby!

The point is, never assume that there is no voltage at some point in a tube amplifier just because there's no reason for it to be there. If you're are unsure, check with your meter. If your meter says it's safe, don't assume it's always safe. Things change. Never let wires dangle unprotected from the chassis. You never know what kind of surprise they might have in store. And always work with one hand behind your back. In case you do make a mistake the current is less likely to go straight through your heart.

Federal Tube Limiter Release Time Adjustment

2009-07-22T02:22:00.000-07:00

The top unit in the picture here is a big old Federal tube limiter from Kissy Pig studios.

This unit was new to Kissy Pig. It certainly looked great but it didn't sound nearly as nice. That's because it was designed and built for the US military for use as a broadcast limiter to prevent overmodulation of communications broadcast signals. When transplanted to the modern recording studio it's got a couple of problems. First is that if it's just plugged into the patchbay it has way too much gain and the limiter triggers very early. This is because it was designed for the 600 ohm world when most audio equipment had a 600 ohm impedance on both the input and the output. Interfacing directly with the high input and low output impedance of modern gear can cause operating level and frequency response problems. I strapped a 620 ohm resistor strapped across the output and made a cable with 10K resistors in series with each of the legs of the input to fix the impedance mismatch which in turn cured the operating level problem.

It had a second issue that made it pretty unusable in a studio situation. The release time was very long. According to the manual it was 2 seconds. That's because the Federal limiter was designed to react slowly - to keep the level of a speaking voice under control. That makes the device sound pretty weird when you put something like drums through it. The first hit sounds great when the limiter kicks in. But since the release time is so long you don't hear the limiting kick in again until it's had a chance to recover. So if there was more than a second between drum hits it sounded pretty great. Otherwise it just sucked the volume down and kept it there. Not so good.

Fortunately the circuit is not very complex. There is one capacitor that can be changed to adjust the time constant for the limiter circuit. It's a 1.0 uF Micamold cap. It's the the huge shiny silver box shown below:

It's C1 in the schematic (click on the schematic for a larger version):

I removed the lead form the left hand terminal of the original cap and used that lead to connect a new cap from the tube cathode pin to the base of the cathode resistor. It actually physically connected to the other terminal of the original cap, but that made a more convenient connection point. With the other lead disconnected the cap is out of circuit so there's no reason not to.

We tried a few different values in studio to hear what sounded best and ended up with 1/10th of the original value. That's the one you see in the circuit now. It's a Mallory 150 series .1 uF cap. Now the unit sounds great and is operating at normal levels. While I was there a shot pictures of the entire manual. If there's anyone looking for a copy, email me and I'll send it along as a PDF.

An angled desoldering tool for cleaning tube socket terminals

2009-07-20T03:26:00.000-07:00

Removing components and solder from tube socket tabs is not difficult. But it's a tedious business. If you're removing a lot of components you'll be glad for anything that speeds the process up. Here's a standard solder sucker or solder pump:

It's a common tool used to suck molten solder away from a solder joint. It's spring loaded inside. After it's cocked, pushing the black button releases the internal plunger and causes a vacuum effect at the tip to suck solder away into the body of the pump. It works well in plenty of situations but frequently the flat tip doesn't quite fit the bill. I keep a collection of desoldering tools suited for specific jobs (here's another one I modified for use for PC board mounted pots). On this one I've cut away part of the tip for use at tricky angles. Here's the stock tip and the angled one:

The angled tip is hugely helpful for getting a good seal against the old solder joint on a tube socket terminal. Here it is pressed against one side of the solder joint with the soldering iron heating the joint from the other side:

Note the angle of the tip. In tight quarters it's impossible to get a straight tip to sit this snugly against the tube socket tab. With a nice tight seal against the joint the terminal comes clean with just one activation of the pump:

This may seem trivial, but when you have enough joints to do it saves serious amounts of time. There are around 50 joints for me to desolder and clean on this Ampex 601 rebuild. Futzing around with awkward positioning of the desoldering pump makes for a poor seal and it can easily take a half dozen shots to clean the joint out well in a cramped chassis. If you fiddle with each joint for 2 or 3 minutes you could be spending a couple hours just sucking the solder out. Getting the remaining wires and component leads off of the terminals without damaging them can still be fiddly. But having the old solder removed quickly speeds the process up immensely and gets you on to the interesting parts of the work.

Is the Ampex 600 / 601 a good vintage tube mic preamp bargin?

2009-07-12T04:57:00.000-07:00

The Ampex 600's and 601's were mono reel to reel decks with tube electronics common to many units of the time. Not many people have a use for a mono tape deck these days, but these units have both a mic and a line input jacks and a line out. So they're being bought as a way to get a real tube microphone preamp for not much dough. Not a bad idea, but you do need to be pay attention to what you're actually getting for the money. A tube mic pre requires an input transformer to work well with modern low impedance microphones. In vintage tube mic pres this transformer frequently plugs into an octal socket like the ones used for large power tubes. Here's the empty socket from an Ampex 601 that came in for a recapping job:

Good quality input transformers don't come cheap. If this socket is empty, think of spending just shy of $100 to put a transformer in there. Here's what the socket looked like in this unit when it came to me:

That's how the input transformer plugs into the socket. Here's the base of the part that came in this Ampex:

The base looks just like an octal power tube and it plugs in in the same way. Let's look a little closer at this one. Here is the piece standing on it's base:

Here I've popped off the metal shield so you can see the transformer windings:

If you can't make out the windings, don't be concerned. There aren't any. This is a simple dummy plug inserted into the transformer socket. Since transformers are so expensive most of these Ampex 600 and 601 tape decks shipped with no input transformer. All they had was this jumper plug. This was cheaper and worked fine with the high impedance microphones designed to work with this sort of input back in the day. It won't work well with microphones you are likely to own now though. So if you're thinking of buying one of these vintage tube pres for recording ask if there is a microphone input transformer before you make an offer, and remember that you're going to be spending close to an extra $100 if there's not one.

Are tubes with broken bases still good?

2009-07-08T12:46:00.001-07:00

Here's a pair of EL34's that came out of a very unhealthy Traynor YVM-1. Take a close look and compare the bases:

The black cylinder in the center of one of the tube bases is broken off and the bottom of the glass envelope is poking through the bass. This central post is commonly called the key, guidepost or locator pin. It's a bit easier to see here:

So is there anything wrong with using a tube with that black center piece broken off?

Well, yes and no. It doesn't have any electrical function, but you do need to be careful. To explain why I'll start with the bottom of the unbroken tube:

You may be able to make out the numbers labeling the pins, You can also see that the center piece is not a perfect cylinder, it has a small protrusion between pins 1 and 8. I've diagramed both in red here in case it's not completely clear from the photo:

That little protrusion is meant to lock into the groove in the tube socket. With it in place the tube can only plug in in one orientation.

Someone must have tried to force a tube into this socket in the wrong orientation. You can see a chip in the socket made by the tubes key as it was forced in:

My guess is that that is what broke the key off of this tube:

With the key broken off you don't have to fiddle around and align the tube. I can fit it in 8 different ways. That certainly makes things easier. Let's look at the connections made when the tube is inserted. Here's the same socket rotated 180 degrees so the key slot point down:

Now I've overlayed the schematic diagram for an EL34 to show how it's internal elements are connected to those pins.* The black block at the bottom of the diagram indicates the key. I've lined that up with the groove in the socket to show a properly inserted tube. I've also labeled the socket connection with ballpark voltages you would expect to find on those sockets.

Now imagine that the tube is inserted so that the key in the diagram lines up with the chip made in the socket when the tube was forced in. The voltages present on the sockets stay the same the tube elements that those voltages are connected to would be different. I've rotated the tube diagram below to illustrate the point:

The most obvious problem here is that the 445 volts on socket #4 is now connected to pin #7 on the tube.

Inside the tube pin #7 is pin # 2 by the heater element. That effectively connects the 445 volts on socket #4 directly to socket #2. In the amplifier circuit socket #2 is connected to ground through the heater transformer.

This ends up giving the 445 volts a very low resistance path to ground, which translates into a huge amount of current:

Grounding the screen voltage should have blown the fuse but apparently it didn't. It could be that the chip in the socket was just the first attempt and once the key was snapped off the tube was plugged in again in yet a another orientation (once that key is there's only a 1 in 8 chance you'll get it right by guessing). If you do this and you're real lucky the fuse will blow before you cause major damage. This amp wasn't lucky. One side of the output transformer blew. The open transformer winding caused the arcing between pins 2 and 3 on the socket on the other side:

In total the output transformer, the output tubes, screen resitors and tube sockets all needed to be replaced. Not cheap!

So can you still use a tube with the key broken off? Yes, sure. It'll still work if it's aligned right. Just keep in mind the steep price you could pay if you are off by a pin or two, you may not get a second chance.

* You may have noticed that the numbers on the bottom of the tube run clockwise while the numbers on the base and the diagram run counterclockwise. Tube diagrams assume that you are looking at the top of the tube socket. Usually the tube diagram will match the tube base but since we're looking at the bottom of the tube socket in this I've flipped the tube diagram to match the view. Doing that causes the numbers to progress in the reverse direction.

Peavey Classic 30 Reverb Howl

2009-06-07T05:29:00.000-07:00

This is another one from the slightly odd repairs file. Here's the reverb pan from a Peavey Classic 30 which came to me with a complaint that the reverb was "humming and feeding back".

When the reverb knob was turned up past 3 the amp made a pulsing howl which got louder the further the reverb knob was turned up.

Suspecting some magnetic coupling I took the reverb pan out of the amp, leaving the wires connected. By orienting the tank in different ways the problem could be greatly reduced but I couldn't get it to go away completely.

Disconnecting the RCA plug from the input jack on the reverb tank itself didn't affect the howl at all. Reconnecting it and disconnecting the outplug plug stopped it completely.

The quickest next step would be to swap out the tank to see if it was bad. Below is the Accutronics reverb tank part number 4EB2C1B printed on the tank itself:

Fortunately I happened to have another on of these around the shop. I plugged in the substitute and the amp was back to normal with a perfectly working reverb.

Now here's the odd part (and the reason I'm bothering to post such a run of the mill repair). I always try recheck once a repair is finished to make sure what I've done has actually fixed the problem. So for good measure I plugged the original tank back in. Now the amp worked fine with the original tank! Even though I'd plugged and unplugged the orginal tank a number of times somehow the act of plugging in the replacement tank caused the original tank to make a good electrical connection. I've seen oxidized or poorly toleranced plugs cause reverbs to stop working or have intermittent signal, but causing this howling was a new one on me. It just goes to show, it always a good idea to clean jacks and plugs even if you don't have a reason to suspect them.

Fender AB764 Vibro Champ Output Transformer Measurement

2009-05-10T14:28:00.000-07:00

Here's a chart showing the measurements I took on the output transformer in a Fender AB764 Vibro Champ that was in for repair. The measurement procedure is the same as in the Champion 600 output transformer post.

Look at the boxed impedance ratio 2,343 : 1
That's the impedance ratio at 1,000 Hz (or 1KHz).

It indicates a 2,343 ohm plate load with a 1 ohm speaker load. But obviously the speaker load isn't 1 ohm. For the Vibro Champ it's meant to be about 4 ohms.

To figure out what the transformer ratio is with a 4 ohm speaker load, just multiply both sides of the ratio by 4.

( 2,343 times 4 ) : ( 1 times 4)

is

9,370 : 4

9,370 is about 9.5K. That's a bit lower than the 11K I measured on the Champion 600 I modded back in December. The plate voltage in the Champion 600 is higher as well (366 volts as apposed to the 342 volts in the Vibrochamp). The AB764 serves as a template for the Champion 600. I'd assume the higher impedance ratio in the Champion 600 was made to compensate for that higher voltage.

That's not a capacitor that's just two wires twisted together. Right?

2009-05-04T04:05:00.001-07:00

Here's a picture of the inside of a Fender AB764 Vibrochamp that was in for repairs:

An new JJ 6V6 got it running again but even with it's Weber alnico speaker it didn't sound all that good. I replaced the tone stack and coupling caps and that helped. But the interesting part of this repair was a more elusive capacitor problem.

Many later Fender amps have capacitors in the output section that were put in to suppress parasitic oscillations. I like to remove these if possible to brighten the amp up a bit.

In this particular one there were actually two ceramic caps (one dark brown and one tan) connected in parallel from pin 5 to pin 8 on the 6V6 power tube. The arrow indicates them in the picture and I've highlighted the cap in red on the schematic to the right.

The trick about removing these caps is that they were originally installed to fix a problem. So sometimes when you remove them the problem rears it's head again.

That was the case in this one. When I removed the parasitic oscillation cap the amp got a bit dirtier and actually sounded darker not brighter.

You'll notice the picture of the Vibrochamp circuit that there are two wires twisted together sort of like these two I've twisted together here:

One of these two wires carries the B+ voltage to the preamp stages from the power supply and the other connects the output of preamp to the input of the 6V6 power tube.

They're going in the same general direction. So why not twist them together? Nice and neat, isn't it?

Well let's hook the two wires I've twisted together up to the capacitance meter. Here are the leads of the meter connected to two ends of the wire:

And here's what the capacitance meter reads with the probes connected to the ends of the wires:

That's almost 30pF that would be connecting the preamp B+ to the 6V6 input at high frequencies! Doesn't sound like a great idea does it?

The fact is that capacitors exist in all sorts of places where we don't intend them to be. And this can cause problems, especially at high frequencies. Capacitance is, after all, just an electrical characteristic of physical materials. The components we call capacitors are just things that are carefully manufactured to optimize and control the capacitance of those physical materials. There are plenty of capacitors that we buy and install into our amps. But there are plenty of others that happen through accidents of layout and lead dress.

Suspecting unwanted capacitance in the twisted pair of wires in this Vibrochamp I unhooked them, straightend them and hooked them up again.

Above you can see them reconnected. The amp sounded more open, a bit less gritty. And as I'd hoped I was also able to remove the parasitic suppressor cap to brighten it up a touch.

Gregory Mark V / Gemini 700 practice amp

2009-04-29T12:38:00.000-07:00

Once in a while a repair comes in that's a real surprise. This Gregory tube practice amp is one of them. When it was fixed up it really sounded fantastic. It's not as aggressive as most small amps. The treble is tamed and the distortion is fairly smooth and laid back and the clean sound is very sweet even at very low volumes.

I was really sorry when I had to give this one back!

Since the result is so nice, I thought I'd spend some time going though some of the oddities of the circuit.

Here's the schematic:

There's a number of unusual things here.
The first is the input stage.

A run of the mill 12AX7 triode input would look
something like this:

But the 12AU7 in this Gregory has two more elements in the preamp tube.

Here's what it looks like in the schematic:

The plate, cathode and control grid are elements found in a triode tube.

I've labeled in red the additional elements the additional elements found in a pentode tube:

Since it has five elements instead of three, we call this tube a pentode. Pentodes are common as output power tubes. The EL34 is one. But pentode inputs on guitar amps are pretty rare birds (one example is the 6SJ7 in the early Fender Champs with the 5C1 circuit). The pentode input is certainly not sole contributor to the sound of the amp, but it makes me curious to try a 6SJ7 in the Champion 600.

I'll go through some more of the circuit in future posts (and hopefully get some pictures from the owner - I forgot to take some myself before I sent it back).

Trippy Reverb Trick for Two Channel Fender Amps

2009-04-27T11:47:00.000-07:00

This is a trick to get a deep and spacey reverb sound out of most 2 channel Fender Amps.

Here's the set up with the "Normal" channel jumpered to the "Vibrato" channel:

Generally people DON'T jumper channels on their Fender amps because
the two channels are phase reversed*. This means the two channels cancel each other out.
So instead of getting more volume from the second channel
when you turn it up, you get less.

This trick takes advantage of that cancellation to get rid of a lot
of the straight guitar sound leaving mostly reverb.

----Video Quality Disclaimer----

The video makes it sound like the amp has loads of buzz.

It doesn't actually - it's the foolish AGC
circuit on the camera.

Soldier through and you'll get the basic idea though.

--------

This video shows the basic setup:

The sound on the video will give you the "how to" but doesn't capture the depth of the reverb, so I suggest you try it yourself to hear what it sounds like in person. It's much more dramatic.

You can also toy with the tone control settings to get more variations from the reverb sound.
It can get pretty wacky with some tweaking (and it can get loud quick so be careful).

* Technically they are "reversed polarity" or "180 degrees out of phase".
Though incorrect "phase reversed" or "out of phase" are the more colloquial usage
and they're the ones you're more likely to hear in music circles.

Are these two resistors different?

2009-04-27T09:44:00.000-07:00

One of the first steps you'll read in any set of tube amp troubleshooting instructions is "visual inspection". It just means looking over the components for any obvious signs of stress. Here I've stuck two 100K resistors in a piece of foam for the sake of comparison.

Looking at the two 100K resistors pictured an astute observer might notice the slightly darker midsection of the upper resistor. It's fairly subtle here and even more so when it's still in the circuit, not positioned right next to a normal looking one.

These resistors are plate resistors taken from a Fender '65 Twin Reissue that came in with a broken Normal channel. The resistor that shows slight signs of overheating was completely open. It was passing no current at all, effectively shutting off the tube. It might as well have not have even been in the circuit. I replaced it to get the channel working again.

So why did I take out both resistors? Well the second one is actually completely open as well, it just doesn't happen to show any visible signs of fatigue.

That's the trick with visual inspection. It's a very good first step. But troubleshooting effectively means knowing how to use your meter to see for you. Parts can fail in many ways that you never have any hope of catching with your eyes.

Fender Champion 600 Preamp Bias Part 6a

2009-04-18T05:55:00.000-07:00

Sound is change. That is, what we perceive as sound is our ears registering variations in air pressure. Air pressure is changing with the weather all the time. But what our ears are sensitive to are very quick fluctuations - between 20 and 20,000 times a second. If there are variations in air pressure back and forth between a higher and lower pressure at 440 times a second what we hear is an "A" note ("A 440" to be specific). It doesn't matter what the barometric pressure is that day, what our ears register as sound are the changes.

For an amplifier to pass "sound" the signal inside it has to be electrically equivalent to these fluctuations. You can think of it as an electrical picture of the sound we hear. The changes in air pressure have corresponding "electrical pressures" (voltages) inside the amplifier. When the output voltage of the amplifier is applied to the speaker the movement of the speaker produces variations in air pressure which we perceive as sound. If a voltage varies back and forth between 1 volt and -1 volt at 440 times a second, the speaker will vibrate at that same rate and we hear an "A" note.

I mention this in order to introduce the concept of alternating current (AC) which will be essential in completing our discussion of preamp bias.

To uderstand AC let's look first at a graph of direct current (DC) voltage over time:

If you were to measure the voltage at an instant it time your reading would be the same at 1 second as it would be at 2 seconds, 3 seconds or 4 seconds. That's the definition of DC. No change.

Now imagine a voltage that rises and falls as time passes:

If you could measure the voltage at an instant of time your reading would depend on what instant in time you chose. At 1 second the voltage would be 2 volts.

But a second later it would be 1 volt:

We don't talk about AC in terms of "+1 volt at 2 seconds". It's really not practical to measure a voltage at an instant in time. Instead we have a few different ways of quantifying an AC voltage. The one we'll use here is probably the easiest to understand. It's called "Peak to Peak". It's simply a measure of how much the voltage changes from it's lowest to it's highest point:

Just subtract the lowest voltage from the highest voltage and you have the Peak to Peak (abbreviated P-P). The above example is pretty straightforward. Since the lowest voltage is zero the P-P voltage is the same as the voltage at the high point - 2 volts.

Let's look at a slightly more complicated example.

Let's make the highest voltage a negative value, -.7 volts. The lowest value will have to be lower than that, or even more negative. Let's make it -2.7 volts:

The difference between the two is 2 volts, the same as in our first example.

You may have noticed anther change in this example. I've changed the time scale on the bottom of the graph. Instead being 0 to 4 seconds it's 0 to 2.3 milliseconds. The graph shows one complete cycle of the sine wave, going from the mid point to the high peak, to the low peak and returning to the mid point. In the previous example one cycle took 4 seconds to complete. In this one it takes 2.3 milliseconds or .0023 seconds - much quicker.

If one cycle takes only .0023 seconds how many cycles happen in one second?
It's 1 divided by .0023. Rounding up that's 440 Hz - the "A" note mentioned in the opening of this post.

We'll get to why I chose those voltages and what it has to do with our whole bias investigation in Part 6b.

Fender Champion 600 hum reduction mod

2009-03-19T03:19:00.000-07:00

The Champion 600 I've been working on came back in with a request for the cathode referenced heater mod to reduce the hum. For a single ended amp this particular one is actually pretty quiet already. It certainly hums less than any of the vintage Champs I've seen. I don't mind a little bit of hum myself but the owner is new to tube amps so he's a bit more sensitive to it than I am. The mod is a real simple one though so I thought I'd take a crack at it and see if it did anything.

R15 and R16 are the resistors that reference the heater filaments to ground. A old trick for single ended amps is to move the end of these resistors to the cathode of the output tube.

Since I've moved the cathode bypass cap as part of another mod, I had a convenient place to connect the resistors. If the cap is still there the resistors could be connected to the top of R10 instead (indicated by the red circle).

Here's a measurement taken off the output with a 4 ohm dummy load connected:

You'll notice that there's almost no reduction in the fundamental or even any of the lower harmonics. It's not until about 2K Hz that we start to see a reduction in the hum. As I mentioned earlier, this amp was pretty quiet to begin with. Curiously the owner actually perceived it as humming slightly more with the mod installed so I'm going to put it back to stock when it comes back in to get a Weber Alnico speaker put in.

Fender Champion 600 Preamp Bias Part 5

2009-03-08T05:31:00.000-07:00

Now I'm finally going to get down to talking about biasing. Instead of giving a wordy definition of bias I'm going to try to focus on some details of the circuit that will give you an intuitive sense of what bias is and why we need it. Once you have a grasp of those the textbook definitions should begin to make more sense.

Part 4 of the preamp bias posts showed where to connect your meter leads to measure the B+ voltage, plate voltage and grid voltage in the first 12AX7 gain stage in a Champion 600. Those measurements will all come into play later on. Right now we're going connect the meter in another way to read the cathode voltage.

Zooming in on the schematic, we see this label (which I've highlighted in blue):

It says +1.7 VDC / TP6. This means we should see a reading of positive 1.7 volts of DC at Test Point Six. The line extending from the label box indicates where the meter would be connected to take this reading.

All of the test point measurements (and in fact the majority of the measurements you'll take inside any amp) are "referenced to ground". What that means practically is that the point you connect the black lead of you meter should have no components between it and ground.

This is the symbol for ground on a schematic:

So looking back at the schematic above you'll see that the way our meter is connected is measuring the cathode voltage "referenced to ground".

Now back to our cathode resistor. Since a capacitor blocks DC current and it's a DC measurement we're concerned with we'll ignore the cap for the moment:

That leaves us with 1.7 volts measure across a 1.5K resistor. That means we have enough information to calculate the current through the resistor using Ohm's law.

1.7 volts divided by 1,500 ohms is .0011 amps - or 1.1 ma.

It's no mistake that this 1.7 volts at the grid and 1.1 ma of current fall right at point C on the graph of plate curves we've been looking at:

The circuit designer chose point C to be the bias point. Given the load line they had to choose the 1.5K resistor to make that happen. The next post will explain why.

Fender Champion 600 Preamp Bias Part 4

2009-03-06T10:13:00.000-08:00

In the last post we left off with three points whose combination of current and voltage fell along the load line:

This whole load line business gets a bit abstract and that can make it hard to think about it in terms of the actual circuit. To try to bring it back to earth a bit (or at least back to the schematic) here's a picture illustrating where that plate voltage appears in the circuit*:

If you were to connect your meter's red and black leads to the points indicated in the schematic the voltage you'd be reading on the meter would be the voltage that appears on the X axis of the plate curves graph we've been working with. We'll see later that this voltages changes a great deal depending on the bias point and the nature of the signal that is connected to the input. The fact that this voltage changes according to the input is exactly what makes the tube function as an amplifier.

Move the red lead to the other side of the plate resistor and you'd be reading the B+ voltage. This voltage should remain pretty much constant with signal applied or without:

Now move both leads and you'll be reading the grid voltage:

The grid voltage is indicated on the plate curves as well. Here is the graph with the load line again. Have a look at the top end of the curves - you'll see a voltage label on each of them:

By following the grid voltage curve to it's intersection with the load line, you can see what grid voltage (-0.5 volts) that corresponds to the plate voltage and current we've chosen at point A:

Since the grid voltage curves are pretty widely space you have to guesstimate when the points on the load line fall in between curves. I've done that here for points B and C.

Using these points and the three values associated with each of them we can calculate the value of the cathode resistor in order to set the bias. We can also use them to calculate the gain of the stage. We'll do that in the next post.

* This is the plate voltage relative to ground. There are times when you need to measure the plate voltage relative to the cathode but this measurement will work fine for what we're concerned with here.

Fender Champion 600 Preamp Bias Part 3

2009-03-04T02:12:00.000-08:00

In Part 2a we plotted the maximum current for the first 12AX7 gain stage in a Champion 600. That point was connected to the maximum voltage point to find what's called the load line for the stage.

This is the place to come to terms with a essential concept. Every point on the graph of plate curves represents a possible combination of plate voltage and plate current for a 12AX7. Let's choose a few for the sake of illustration. Let's pick Point A at 1.1 milliamps of plate current and 140 volts at the plate. Here's that point graphed onto the 12AX7 plate curves:

Now we'll pick another one we'll call Point B at 0.4 millamps of plate current and 230 volts at the plate. Here's that point graphed onto the 12AX7 plate curves:

Now we'll a third, Point C, at 1.1 milliamps of plate current and 230 volts at the plate. Here's that point graphed onto the 12AX7 plate curves:

Here's the fundamental concept. The load line we've drawn crosses all the possible combinations of voltage and current that can occur in our circuit given the 100K plate load we've chosen - so any dot which plots a combination of current and voltage must lie on this line for it to be a possible occurance in the circuit. We've plotted three different points but only Point C lies on the load line. When the circuit is drawing 1.1 mA of plate current the plate voltage must be the 230 volts indicated by point C NOT the 140 volts indicated by Point A. And with 230 Volts on the plate the circuit must draw 1.1 mA as indicated by Point C, NOT the .4 mA indicated by Point B.

The load line indicates that Point A and Point B do not exist for the circuit given our chosen 100K plate load. Points A and B can still exist as long as we adjust either the current or the voltage so that they lie on the load line:

This all may seen somewhat abstract, but it's essential to understanding the biasing of the preamp stage. It should all become clearer as we begin to discuss bias in the next few posts.

Fender Champion 600 Preamp Bias Part 2b

2009-03-02T09:59:00.001-08:00

In Preamp Bias Part 2a we derived this load line for the first 12AX7 gain stage:

To find the maximum current we imagined that the full preamp B+ voltage was across the plate resistor. This post is an addendum to for those interested in why that is a reasonable way to estimate the maximum current. First we should look as the path of the current from the B+ power supply to ground:

From the 340VDC power supply the current flows through the plate resistor, on through the tube itself and then through the cathode resistor to ground.* So here's what he circuit "looks like" for DC current flow:

So there are three separate resistances to the flow of current. All three are in series so the total resistance will be all three of those resistance added together. Using Ohm's law again:

The total current flowing in the circuit will be 340 volts divided by the the total of those three resistances.

From both Ohm's law and from intuition we know that more resistance will mean more opposition to current flow. Hence less resistance will mean more current flow.

So to find the maximum current flow we want to be thinking of the condition under which the circuit has the least possible resistance.

This means that we have to make all three of those resistances as small as possible. The plate resistor (Rp) is a fixed value of 100,000 ohms - nothing we can do about that.

The cathode has a fixed resistor too (Rk) - 1500 ohms. It's value is not going to change either.

The internal plate resistance (ra) is the resistance the tube itself contributes to the circuit. This value varies very widely - practically from zero to infinity. This change in resistance is actually key to it's functioning as an amplifier.

We want to be calculating the point at which the total resistance is smallest. Since that will occur when the tube's internal resistance is smallest we'll use the smallest possible internal resistance in our calculation. That makes things easy. That's effectively zero.

So that leaves us with the following:

That means the total resistance is just the sum of the plate resistor value and the cathode resistor value.

But in Part A I said all we used to calculating the maximum current is the plate resistor value. Now we're using the cathode resistor too. Why the change?

There's no change really. The cathode resistor is very small compared to the plate resistor - only 1.5% of the value so we can ignore it for the calculation. If you're designing a stage from scratch you won't pick the cathode value until after you've drawn the load line so it's easiest to be in the habit of ignoring it for the common stages you'll find in most guitar amps.

* Capacitors block DC. Since we're looking at the DC current flow, no current will be flowing through any of the capacitors.

Fender Champion 600 Preamp Bias Part 2a

2009-03-02T04:38:00.000-08:00

The the last post we found one of two points we need to draw the load line for first 12AX7 gain stage in a Champion 600. We'll use the load line to find the bias point and later reset that bias point for the 12DW7 mod.

The first point was found by simply plotting the B+ voltage on the X axis of the 12AX7 plate curves:

Now we need to find the maximum plate current. To do this we use Ohm's Law to calculate how much current will flow with the full B+ voltage dropped across the plate resistor.

We know the B+ is 340 volts and the plate resistor is 100,000 ohms.

340 divided by 100,000 is .0034 amps or 3.4 milliamps

Now we plot 3.4 mA onto the Y axis and connect that dot to the one we made on the X axis:

The line connecting these two points is what's called a "load line" and it's what we use to determine the bias point for the tube. I'll cover the bias point in an upcoming post.

Fender Champion 600 Preamp Bias Part 1

2009-03-02T02:43:00.000-08:00

In order to understand what's behind the new the bias point for the 12DW7 mod, we'll first have a look at how the initial 12AX7 gain stage in the stock Champion 600 is biased.

There are three critical factors in the effecting the 12AX7 stage - the power supply voltage, the value of the plate resistor and the value of the cathode resistor. They're indicated on the schematic below. Keep the first two immediately in mind. The cathode resistor will come in later.

Like all the images here, you can click on the schematic image for more detail.

Now we'll reference the B+ and the plate resistance to "plate curves" for a 12AX7 to determine the "load line" and from there determine the bias point.

Don't let this scare you off. It sounds much more complicated than it actually is. It's really quite straightforward once you grasp the basic concepts.

The graph of plate curves is part of the collection of tube data generated by the tube manufacturer. You can find them posted in a host of places on the web. I frequently use Duncan Amps TDSL.

Here's an image showing the plate curves for a 12AX7:

The X Axis shows the voltage at the tube's plate, the Y axis shows current flowing through the tube and the numbers labeling each of the curves indicate different bias voltages.

The first thing we need to do is determine the maximum possible plate voltage.

That one's easy. Since the plate voltage will never exceed the available supply voltage we can simply use the B+ voltage for the preamp section. We can read that off the schematic - 340 Volts. So we place a dot on the plate voltage axis of the graph:

Now we need to find a place to a dot on the plate current axis. This dot should indicate the greatest amount of current that will flow through the tube.

We can't just read this number off the schematic - we'll have to calculate it.

We'll do that in the next post.

Ampeg Curiousites Part 5

2009-02-27T14:52:00.001-08:00

Here's the EQ circuit card removed from an Ampeg B25B. If you look in the lower right you'll see a big rectangular component that looks sort of like a big ceramic capacitor:

If you look a bit closer you'll notice that it has seven leads - definitely not a simple capacitor or resistor. So what the heck is it? The answer can be found in the schematic.

Here's the preamp side of the schematic:

Have a look at the EQ part of the circuit - that's what we're looking at in the first picture above. The EQ section is highlighted below:

Zooming in on the lower of these you'll see the resistor / capacitor network that makes up the EQ section. There's a dotted line around the EQ section which I've highlighted in red. You'll notice that unlike R20, which is outside the dotted line, none of the capacitors and resistors within the dotted line have component numbers. That's because all of these components are contained within that little rectangular component in the first photo.

I've also highlighted the numbers that follow the dotted line. The numbers are 1-7 and correspond to the seven leads on the circuit card in the original picture.

These same little seven lead components are found in the tone circuits of a number of Ampegs. When these parts fail, the circuit has to be rebuilt with discrete components - it's an easy enough job if you just follow the schematic.

Fedner Champion 600 12DW7 / ECC832 Mod Part 1 - Update

2009-02-26T04:04:00.000-08:00

I'm focusing here on the 12DW7 / ECC832 partly out of curiosity and partly because it's half the work to rebias for the new tube. What is a 12DW7? Technical details can be found in the 12DW7 mod part 2.

The goal of the mod is to get a good bedroom level clean sound with a more useful range in the volume control. You can lower the gain and get more headroom by substituting a 12AT7,12AY7 or 12AU7 for the 12AX7. It won't be biased properly, but you may like the sound anyway and you're not going to hurt anything.

You can also plug a 12DW7/ECC832 in place of a 12AX7. A 12DW7 / ECC832 is one half 12AX7 and one half 12AU7. If you like the sound, try this mod to bias the lower mu half of the tube correctly.

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

All it takes is a 12K resistor jumpered in parallel with the stock 100K plate resistor R8 and a 1K resistor in parallel with the 1.5K cathode resistor R2. They're the two dark brown 3 watt IRC resistors shown in the middle of the picture.

The effect of putting 2 resistors in parallel is to reduce the overall resistance:

So the stock plate resistor in parallel with the additional 12K one gives a combined value of 10.7K for the plate. This greatly increases the current through the tube, which is what we need for a 12AU7.

With the current increased we need to change the bias of the stage to suit a 12AU7. With the stock resistor it will be running quite cold. The added 1K resistor in parallel with the stock 1.5K one yields a total resistance of 600 ohms.

If you're interested, the working through of the parallel resistance formula is covered in more depth in the Fat Boost Resistor Value post.

Here's a close up of the plate resistor. This jumpering method makes it very easy to remove the mod if you decide to go back to stock.

You can see below that the joint on the left hand of the plate resistor is not perfect - the solder hasn't flowed onto the lead of the original as completely as one would like. This is just for a listening test though so I didn't bother touching it up.

With this mod in place the amp is much cleaner and much quieter. It's important to note that you should NOT plug a 12AX7 back into the socket with the mod in place. A 12AX7 triode can't handle the amount of current that a 12AU7 triode can, so half of your 12AX7 will be toasted by the rebiased stage.

Fender Champion 600 Input Voicing Mod "Kit"

2009-02-25T06:12:00.001-08:00

Instead of selling "kits" for the Champion 600 mods on this site, I'm putting together a series of posts with direct links to sources for the parts involved.

Below is a chart of capacitor values and -3dB points for the Champion 600 low input revoice mod.

This mod will work in a great number of amps with high and low inputs including the 5E1, 5F1 Champs and a host of other Fender Amps. You can find the the measured frequency response for a .012 uF cap in Part 2 of the mod post.

Clicking on any of the capacitor values in the chart should take you directly to a suitable part for the mod on the Mouser Electronics website. You can order directly from there.

If you have any questions about the parts links here, feel free to drop me an email.

Fender 5E1 / Champion 600 Coupling Cap Mod "Kit"

2009-02-22T12:07:00.000-08:00

The stock capacitor in a Fender Champ 5E1 and 5F1 circuits is .022uF. This allows a great deal of bass through the circuit and the cap is frequently changed to restrict the low end and make the distortion a bit less muddy.

If you'd like to try changing this cap you can use the chart below to help select a value. In case you have trouble finding a place to purchase caps of the right value, each of the cap values in the chart has a link directly to a suitable part on the Mouser website.

Just click on any of the values and you'll see how it works.

The values in blue indicate that the cap values are in microfarads (uF). As the values get smaller I've switched to picofarads (pF) indicated in black.

If you are a bit foggy on the meaning of different capacitor values, see the capacitor values post.

The capacitor values here are good for a stock 5E1 Champ or a 5F1 Champ with a cathode bypass cap on the first stage (the frequencies shift a bit if that cap is not present and stock 5F1 champs don't have the bypass cap).

They'll also work in place of the tone stack in a Champion 600.

Fender Champion 600 akavalve Mods

2009-02-21T02:37:00.000-08:00

Since the blog format is a bit awkward to navigate through, I thought I'd make up a table of contents for my Fender Champion 600 Mod posts. First are the essential safety posts and then the mods listed in order of difficulty with the easiest ones first.

Discharging the filter caps:

Part 1
Part 2

Tube Upgrade:

New preamp and power tubes

Input Voicing Mod

Part 1

Part 2
Frequency Response Measurement

Input Mod "Kit"
Cap values for different frequency response and
Mouser Electronics Parts Links.

Presence Plus Mod:

The Presence Plus Mod
The Presence Plus "Mod Kit"

Cathode Bypass Mod

Part 1
This one includes a couple charts for bypass cap values
and their frequency response for both
the stock circuit and the Frondelli mod.

Part 2
This is the installation of the mod.
It uses large cap values
but smaller ones detailed in Part 1
could easily be substituted.

Part 3
This shows the measured frequency response
with the four different settings of the
cathode bypass lift described in part 2.

Tone Stack and Fat Boost Mods

Tone Stack Test

Installation Details and Frequency Response Graph

Explanation of the Fat Boost Mod resistor values

Tone Stack Bypass Cap Values (same as 5E1 Champ)

The 12DW7 / ECC832 mod

Part 1

Part 2

Miscellaneous:

Animated GIF of the disassembly

A look inside the chassis

Stock Speaker response measurement

Comparison of the Champ 5E1 and 5F1 schematics

Stock Output Transformer Measurement

I'm pretty sure that's all of them.
I'll add new ones to this list as I post them.

Fender Champion 600 Presence Plus Mod Kit

2009-02-21T02:18:00.000-08:00

I've had a number of requests to provide kits for the mods described here. The components are generally quite simple - it's finding the sources and actual part numbers that may give people some trouble.

So instead of bagging up components, figuring out how to price them and sending them out, I'm going to try to provide links to Mouser for the exact parts needed for each of the mods.

This post is for the Presence Plus Mod.

If you're not making it switchable, the only part you need is the 16uF capacitor. Here's the link to the part on Mouser's website.

This is a lot easier for me than trying to make up "mod kits" and mailing them out. And it's cheaper for anyone interested in doing the mods too since you won't be paying for my time in the price of the kit. Let me know if it works well for you.

Desoldering Tool for PC Board Mounted Pots

2009-02-07T13:36:00.000-08:00

This picture's from an Ampeg B25B that came in for intermittent/scratchy performance and one broken channel. The channel 2 bass pot wouldn't turn at all. When I opened it up I found this dead bug and a bunch of what looks like it might have at one time been a clutch of eggs or something sprayed around the outside of the pot. Inside the pot there was another bug ground up and mixed with the pot lubricant which through some foul miracle turned into a sort of cement that held the pot stationary.

But that's not what this post is about.

The pot had to come out to be opened and cleaned. Taking resistors and axial caps off a board isn't so tough because they flex enough for you to heat the solder connection at one end of the component and gently pull that side out while the solder is still liquid. Then you just repeat for the other end.

Stiff components can be a pain though - especially ones held to the board at more than two points.

For this kind of job I frequently use this slightly modified desoldering iron. This style is just a regular iron with a suction tube attached to a hollow angled tip. You squeeze the red suction bulb and press the end of the iron against the joint. When the old joint becomes liquid you release the bulb and the solder is ostensibly sucked away.

The original tip has a fairly small opening so the only option for desoldering larger leads was to press the tip to one side of a joint, clear it and the do the other side. The seal against the board wasn't very tight this way. This meant that the suction wasn't great either so it usually would take a few passes to get enough solder off to free a joint.

To make the iron more useful for amp work I pulled the tip off and drilled it out from the back to 3/32 of an inch. Now the tip fits right over the ends of most components leads - even the fairly large ones on these Ampeg pots. With the lead fed up inside the tip the the whole joint is heated very evenly. Once the solder melts the opening of the iron can be pushed flush with the board. The tighter seal makes suction good enough to pull the solder off the joint, off the lead and out of the mounting hole in one fell swoop.

The desoldering tip goes right over the protruding lead. As soon as the old solder liquifies, release the bulb and the old joint is almost completely removed.

Part of the trick to having it work well is to tin the hole over with solder before using it. Tinning is just melting new solder onto the hot iron to form a molten solder coat over the tip. This protects the tip from corrosion and aids in the heat transfer. The old joints heat much more quickly this way. Most take only about 2 seconds. This is great becuse the shorter time and more even heat means less chance of damaging the board.

Here I'm melting a bit of fresh solder into the tip:

It's important to squeeze the suction bulb in before you start and then hold it there while you do the tinning - otherwise you'll just blow the solder out when squeeze the bulb before cleaning the joint. You can see below that the hole is completely filled with molten solder.

I find it's not necessary to re-tin the tip for every joint. I just do it when it starts taking a bit longer for the old solder joint to melt.

I ended up removing all the pots for good measure. Here are the joints from the treble pot. You can see the backplate for one of the disassembled rocker switches floating above suspended by it's leads.

These joints came clean with just one pass from the iron. The old pot pops right out without having to heat the joint again.

The mounting holes are generally clear enough that they don't have to be touched up at all before the pot goes back in - makes the whole business go a lot quicker.

Epiphone Valve Junior - Gain Reduction due to R6 and R7 in a Stock VJ

2009-02-06T13:15:00.000-08:00

This post explains a particular part of the Epiphone Valve Junior circuit. If you're looking to reduce the volume and/or gain in your VJ, I started a series of posts for a gain reduction mod.

Here's a schematic for the stock Epiphone Valve Junior:

There is a voltage divider in the preamp to reduce the overall gain of the amplifier. R6 and R7 are 1 Meg resistors used to form the circuit:

These two resistors are in series with the output feeding the next stage being taken at their junction. This a straight ahead voltage divider circuit. R6 and R7 are indicated in red on the schematic below Click on it to see it in full detail.

If you follow the math in the voltage divider posts, you'll see that with two resistors of equal value, as R6 and R7 are here, the voltage at the output will be 50% of the voltage at the input.

That would mean that with 2 Volts output from the first gain stage (V1) the voltage seen by the second stage (V2) would be just 1 Volt.

Things aren't quite that simple in the Valve Junior circuit though. If you look at the schematic you'll see that the 1 Meg volume pot is connected in parallel with R7:

So the voltage divider is really composed of three resistors - R6 in series with the total resistance of R7 and the volume pot in parallel.

Ok, so that's a bit more complicated. We really want just two resistances to calculate the voltage divider output. Fortunately two resistors in parallel can be treated as a single resistance. We can find the effective resistance of R7 and VR1 in parallel using the following formula:

If this formula seems too daunting, the Champion 600 Fat Boost Mod Resistor Values post goes though the details of how to apply it.

Using this formula you'll find that any two resistors of equal value connected in parallel will have a combined value of one half the value of a single resistor.

Incidentally, this is what guides the rule of thumb about connecting speakers in parallel (e.g. two 8 ohm speakers connected in parallel yields a 4 ohm load).

Here we're luck and the two are equal with R7 being 1 Meg and the end to end resistance of the volume pot being 1 Meg. One Megaohm is one million ohms. Half of one million is 500,000 ohms. One thousand ohms is one Kilohm, so the effective resistance of the two in parallel is 500 Kilohms or 500K.

Here is a modified schematic with the parallel resistance of R7 and VR1 shown a a single 500K component:

As far as the Valve Junior's functioning is concerned this simplified circuit is the same as the original circuit even though it doesn't indicate the actual physical components. This is what's called an equivalent circuit, and we use it to make the functioning of the circuit easier to comprehend.

So after all of that we have new values for the voltage divider. The top half is still the value of R6 - 1 Meg. The bottom half is now the equivalent resistance of R7 and VR1 in parallel, or 500k.

Using a form of the voltage divider formula we'll find that the output voltage will be about 30% of the input voltage. So with the 2 Volts input given in the example above, our output voltage should be 2 Volts times .3 which equals .6 volts.

You may notice that this .6 Volts is itself the input to another voltage divider - the volume pot itself. You can see how a pot acts a voltage divider in part 3 of the voltage divider post.

So after all that math, it's time for a reality check. Here the right hand meter is connected from the bottom of R7 to the top of R6 - effectively measuring the input voltage to the divider. The left hand meter is connected across R7 - effectively measuring the output voltage feeding the volume pot:

With enough signal applied to achieve 2 Volts from stage 1, the output voltage feeding stage 2 is .6 Volts - right in line with our calculations.

That's a gain reduction in the stock Valve Junior of about 10 dB. That means, of course, that eliminating the voltage divider by jumpering over R6 will result in a 10 dB increase in gain.

So now you know what that 1 Meg R6 is doing in your Valve Junior. Whether or not you want to keep it there is another story entirely.

What is a voltage divider? Pt3

2009-01-31T03:46:00.001-08:00

Part 1 of this post showed a voltage divider made with two resistors connected end to end:

It's a useful circuit, but it's action is fixed - determined be the value of the resistors chosen for the circuit. A potentiometer (commonly called a "pot") is a variable resistor and it can easily be wired to form a variable voltage divider.

A pot generally has three pins. The outer two pins connect to the opposites ends of the pot's resistive track. The resistance from one end of a pot to the other never changes. In this example it's 500 ohms.

The pot's center pin is connected to the wiper. The wiper moves along the resistive track as the pot is turned. Because the wiper simply splits total resistance into two parts, the sum of those two resistances is always equal to the total resistance from end to end. In this case 250 ohms + 250 ohms = 500 ohms.

To the circuit this looks like two 250 ohm resistors connected in series. Putting volts across the ends of the pot as we did in part 1 causes 1.5 volts to appear from each end of the pot to the wiper.

So what happens when the pot is turned?

The resistance from end to end stays the same but the way that resistance is divided by the wiper changes:

Here the position of the wiper makes the pot function like a 100 ohm and a 400 ohm resistor connected in series. Following the rules from part 1 again, 3 volts in will produce 2.4 volts out.

If the pot is turned toward the other end of it's rotation and the resistances are reversed:

Now 3 volts at the input yields only 0.6 volts at the output:

This is just what is happening when you turn down the volume knob on your guitar or amplifier.

The Epiphone Valve Junior has both a fixed voltage divider for reducing the gain between stages and a variable voltage divider (the volume pot) wired in parallel. In the next post I'll take a look at that circuit as a real world example.

What is a voltage divider? Pt 2

2009-01-30T11:34:00.000-08:00

To help visualize the concept from part 1 of this post, here's a quick circuit mock up of the circuit diagrammed there. On the left is a 10 ohm resistor with the left hand meter's probes connected across it. Connected in series to this resistor is a 90 ohm one with the right hand meters leads connected across it.

Below the meters are reading the actual values of their respective resistors (10.4 ohms to the left and 91.4 ohms to the right). This works out to pretty much the 10% and 90% of total resistance as shown in part 1.

Now the power supply is connected to the circuit and turned on.

The left hand resistor is seeing o.233 volts and the right hand one is seeing 2.112 volts. The sum of the voltage drops across the two resistors is 2.345 volts.

o.233 divided by 2.345 is .101 or about 10%
2.112 divided by 2.345 is .901 or about 90%

The voltage is raised to about 4.5 volts on the power supply.

Now the left hand resistor is seeing 0.439 volts and the right hand one is seeing 3.99 volts. The sum of those voltage drops 4.429 volts.

0.439 divided by 4.429 is .099 or about 10%
3.99 divided by 4.429 is .903 or about 90%

The voltage is raised again to about 10 volts on the power supply.

Now the left hand resistor is seeing 1.010 volts and the right hand one is seeing 9.05 volts. The sum of those voltage drops 10.06 volts.

1.01 divided by 10.06 is .1 or about 10%
9.05 divided by 10.06 is .899 or about 90%

So no matter how the voltage changes the ratio between the voltages across the resistors stays the same.

With an output connected across the 90 ohm resistor (as shown in part 1), any voltage input to the circuit will produce 90% of that voltage at the output. Now you can see how the voltage divider is effective for the constantly changing AC signal generated by your guitar as well as for a simple DC circuit.

What is a voltage divider? Pt1

2009-01-30T11:30:00.001-08:00

You'll find voltage dividers in a whole host of places throughout your amp, your pedals and even in your guitar itself.

So what is it? And what does it do?

There are other applications, but one common use is as attenuators to reduce the voltage between stages. Once you understand the basic concept it's use in more complicated instances (like the divider formed with your plate load resistors) will be much easier to grasp.

The simplest sort is formed with two resistors hooked up in series:

You'll see that the total resistance from the top of the first resistor (R1) to the bottom of the second (R2) is just the sum of the two resistors (Rtotal).

Rtotal makes up 100% of the total resistance of the circuit. The 10 ohm resistor R1 makes up one tenth of the total resistance - so .1 or 10%. R2 makes up the remainder, or 90%.

So let's put a voltage across the two resistors, say 3 Volts:

The full voltage, 3 volts, could be measured across the two resistors. If you add the voltages that appear across each of the resistors individually they must equal the total voltage of 3 volts. How much voltage appears across each resistor?

Take resistor R1. It makes up 10% of the total resistance of the circuit. Take 10% of the total voltage (that's 3 times .1) and you'll get the voltage drop across R1. That's .3 volts.

Do the same for R2 (3 times .9) and you'll get 2.7 volts.

Add those two and you'll get 3 volts. So things check out.

Now because we're looking at voltage not current we can change the resistances and get the same results as long as the ratio between R1 and R2 stays the same.

Here's the same circuit with R1=50 ohms and R2=450 ohms:

When you do the math:

R1 divided by Rtotal = 500/50 = .1 or 10%
R2 divided by Rtotal =500/450 = .9 or 90%

You can see this is the same as in the previous example.

So why does all this matter? Let's imagine the resistors in a more complete circuit:

Here imagine that the input is coming from one amplifier stage the output is feeding the next stage. If we look again at how the voltages divide across the resistors:

You'll see that with 3 volts of input you a .3 volt drop and the output voltage is reduced to 2.7 volts. Changing the values of the resistors can change the amount of drop. If you use a variable resistor (a potentiometer or "pot") you can vary the drop by turning the knob on the pot. This is exactly how the volume knobs on you guitar and amplifier work. I'll cover that in more detail in part 3.

Fender Champion 600 - Discharging the Filter Caps Pt 2

2009-01-23T13:08:00.000-08:00

Here's a more detailed look at discharging the filter caps in a Fender Chapion 600.

First I'll get this out of the way:

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

With the amp turned off connect the black lead of the multimeter to ground at the speaker jack. This will be the bare wire connected to the sleeve connection on the jack. You can use any ground connection really, but this on is very easy to get a clip onto. Connect the red lead to the top of R11. Set the meter to read DC Volts. Now flip the amp on.

You'll see the voltage on the meter rise quickly to around 420 volts. If you don't see any voltage reading then you've connected things incorrectly or you meter is not set properly. Don't proceed until you've figured out why you have no reading, and be very careful when when moving the probes as there is now very high voltage in the circuit.

When you have the meter connected properly and are getting a 400+ VDC reading shut off and unplug the amp.

When you shut the amp off you will see the voltage dropping slowly on the meter. You are now ready to connect the clip end of the discharge probe to ground. I find the easiest method is to clip it to the sleeve of the input jack.

Mote that the voltage is still dropping slowly even though the probe hasn't been fully connected. This is because the capacitor is bleeding it's charge though the circuit and through the meter itself. This is exactly what we want the discharge probe to do - but faster.

Now touch the other end of the discharge probe to a source of B+ voltage. Here I'm about to make contact with the red lead of the output transformer primary:

Now you will see the voltage drop quite rapidly. This picture covers the change in just a few seconds (click on it for a close up):

Finally all the voltage will be drained out and you're ready to work on you amp.

Note: Turning the amp on is not really not part of the discharging process. The reason I include it is because it insures that you are getting a good voltage reading and that you can watch that reading drop. That way when your meter shows no voltage you know it's because there is no voltage left in the caps not because the meter isn't connected properly.

Second note: If your speaker is still connected to the amp during this procedure you will notice that the voltage drops fairly quickly. This is because the connected speaker is doing the job of the discharge probe. This is actually a perfectly reasonable way to avoid the discharge probe all together. Discharging with the probe is an excellent habit to get into though because not all amps bleed off their caps voltage so quickly on their own. If you decide to forgo the probe, make sure you are absolutely confident in your voltage measurements in order to stay safe.

Fender Champion 600 - Discharging the Filter Caps Pt 1

2009-01-23T11:30:00.000-08:00

Here's the procedure for discharging the filter caps in a Fender Champion 600.

The video is pretty small so I'll put up another post with pictures so the detail is a bit clearer.

I'll cover the construction of the probe in a later post.

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Discharging Filter Caps - The Basics Pt 2

2009-01-23T11:29:00.000-08:00

Here's hows how a discharge probe is used to bleed voltage off of a capacitor. This cap is completely removed from the circuit for illustrational purposes, but the procedure is the same when a cap is installed in an amplifier. The meter is tough to read here but I hope it's good enough to get the general idea.

The resistor at the clip end of the probe is a 56K 3 Watt one covered in two layers of shrink tubing for safety. The resistor will limit the current from a 450 Volt supply to under 10 ma but it is still a very good idea not touch the alligator clip end of the probe while connecting the other end to the B+. I'll cover the probe construction in a later post.

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Discharging Filter Caps - The Basics Pt 1

2009-01-23T11:07:00.000-08:00

Here are the basic reasons and methods for discharging filter caps. The meter is a little tough to read here but I think it's clear enough to get the basic idea. This is just for illustrational purposes, it's NOT the the method I recommend. I'll cover that in part 2.

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Fender Champion 600 Free Installation of the Mercury Magnetics Mod

2009-01-22T08:59:00.000-08:00

Doing the Champion 600 mods in my previous posts left me with a good bit of curiosity about the specifics of the Mercury Magnetics kit. If anyone in the Boston area has a Champion 600 with a Mercury Magentics mod kit they need to have installed, drop me an email. For the opportunity to get in there and see just what's going on I'd be glad to do the put it free of charge.

Just putting it out there in case anyone's interested...

How to Remove Tubes / Another Way PC Board Fail

2009-01-19T06:10:00.000-08:00

The first half of this video shows how to remove preamp tube from your amplifier.
The second half shows a rather extreme example of what can be happening to the circuit board in the process.

To be fair, most pc board mounted sockets are better supported than this one from an Ampeg V4B. The same principle applies though. It doesn't take much motion to make a hairline crack in on of the solder joints on the other side of the board. The worst thing is that these tiny cracks are likely to cause intermittent failures. So while the amp may work most of the time you won't be able to trust it to gig with or bring a recording session.

Tube Amps and the Freezing Cold

2009-01-17T17:36:00.000-08:00

I suppose this one's from the "do as I say not as I do" file. This is what can happen when you leave your amp in the trunk in the freezing cold. I was playing a gig that ended in the middle of a snow storm and I left my amp (a custom thing built on a Teisco Checkmate 50 chassis) in the trunk for a few days.

In the morning the trunk got warm enough for water to condense on the cold transformers and then got cold enough for it to freeze there.

Of course one would worry about rust and moisture damage, but the real danger here is not the frost and condensation on the transformers. It's the danger of the amp being switched on with that condensed water shorting the circuit inside the amp.

This can happen as a result of condensation inside the chassis or by water dripping there from the outside. Switching an amp on in this state is a great recipe for arcing and damage inside the amp.

If you are gigging in cold weather it's a very good idea to keep your amp covered in the trunk or van and ideally keep it covered while it comes up to room temp to avoid condensation. If you bring a cold amp into a warm humid room it will come up to room temperature in a fairly short period, but the water that condenses on the cold transformers and chassis can remain for hours or more. The above picture is of an unusual case. Remember, the condensation generally occurs after the amp has been brought inside. It may be perfectly dry coming in from the van. After 10 minutes in the indoors it can become dripping wet.

Now of course this has been happening to amps for decades and they haven't all gone up in smoke. But the danger is there. If you really want to play it safe you could build a simple series outlet current limiter and keep it in you gig box. Use that when you warm the amp up (and dry it off) after a particularly cold trip.

Fender Champion 600 Fat Boost Mod Resistor Values

2009-01-16T03:14:00.000-08:00

In the Fat Switch Mod the circuit sees 3 different resistances for the mid resistor in the tone stack: 15K (stock), 30K (Frondelli Mod fat boost value), and 47K (for a little extra boost). The actual resistors on the switch are quite different values. The 47K value is there but the other two are 68K and 22K. Why not the 15K and 30K that the circuit needs to "see" for the mod?

In order to use a simpler switch I approached the mod a bit differently. I decided to replace the standard mid resistor with a 47K one. This sets the max mid resistor value. The fat switch then selects one of two resistors and connects it in parallel with the 47K resistor, lowering the effective resistance. In the center position both of the additional resistors are disconnected so the total resistance remains 47K.

This should be clear from a schematic drawing:

Too find the effective resistance when one of those resistors is switched in use the formula for finding the total resistance of any number of resistors connected in parallel. Incidentally, this is the same formula you would use when connecting speakers in parallel:

Since we only have two resistors connected at any time, it's a bit simpler. All we need is R1 and R2. Here's how the formula for the 47K resistor in parallel with the switched in 68K resistor is solved in detail:

That 28K value is plenty close to the Frondelli Mod value of 30K. If you're wondering how close, take a look at the graph at the bottom of the Fat Switch Mod post. You'll see from comparing the curves for the three fat boost resistor values that that 2K difference doesn't matter much.

Here's the same equation for the 22K resistor in parallel with the 47K one:

Solve that equation and you'll see that the 22K in parallel with the 47K results in 15K - the same effective value as the original R19. So switching in the 22K resistor puts the tone stack back to stock.

Reading Capacitor Values Part 1

2009-01-14T03:40:00.000-08:00

Reading capacitors values from schematics or even parts lists can be confusing. There are a few basic rules to keep in mind that will make it a bit less daunting. First off the basic unit of capacitance is the farad. A farad is so large that you will never encounter a capacitor measured in farads. Most capacitors in tube amps are measured in microfarads. It takes one million microfarads to equal one farad. If a unit for a capacitor in a tube amp schematic is not specified the value is almost certainly microfarads (abbreviated uF). The rest will for the most part be picofarads (pF). I'll take some examples from the Fender Bassman 5F6A. Below is the complete schematic.

Here's an excerpt showing the 5F6A tone stack. The cap show in red is labeled .02-400. The first number indicates that the capacitance is .02 microfarads, the second number indicates the voltage rating, 400 volts in this case.
The treble cap is a much smaller value, .00025 microfarads. If you're looking though a parts catalog you are not likely to find a .00025 microfarad cap. That's because a cap of this size will more often be expressed in picofarads (abbreviated pF). There are on million picofarads in one micofarads. To find the value of a cap rated in micofarads in picofarads simply multiply by 1,000,000.

In this case .00025 microfarads x 1,000,000 = 250 pF.

Looking at the splitter of the phase inverter, there is a cap bridging the output of the inverter:

This cap is shown as at 47MMF. What does this mean? I've said already that the caps in tube amps are going to be fairly exclusively rated in microfarads (uF) or picofarads (pF). What does MMF mean?

In short, it is the same as picofards (pF). To understand why takes a bit of explanation. The abbreviation for "micro" is taken from the lower case of the Greek letter "mu".

Now you can see that the upper case "mu" looks just like an "M".
But what about the lower case"mu"?

Frequently you're not using the symbol for "mu" especially if you're typing. Convention has "mu" written as "u" so microfards is written uF. But in a way it's logical to think of the lower case "mu" as "m". This is why you'll sometimes see microfarads written as mF. Technically mF means millifarads (one thousandth of a farad) but since caps are not generally rated in millifarads you can safeley assume that if you see mF it is microfarads that is indicated.

MF techically means megafarad - one million farads. This is a value so ridiculously large that a cap this large could not even begin to be manufactured. If you see MF you can also assume microfarads (uF).

So the MMF from above can be considered to be mmF or micromicrofarads. "Micro" means "one millionth" or "divided by 1,000,000). A picofarad is a microfarad divided by 1,000,000. So saying micromicrofarad is really the same as saying picofarad.

So to sum up:

The majority of tube amp caps will be indicated in microfarads (uF, mF, MF) and picofarads (pf, mmf, MMF). Occasional you will see nanofarads (nF). A nanofarad is simply one thousanth of one microfarad or one thousand picofarads.

If a unit for a capacitor in a tube amp schematic is not specified the value is almost certainly microfarads.

Fender Champion 600 Cathode Bypass Mod Pt 3

2009-01-08T02:47:00.000-08:00

Here's the response curve for the switched cathode bypass caps in part 2 of this post. The first graph is for the stock tone stack:

From top to bottom the curves show:

6V6, 1st stage (12AU7) and 2nd stage (12AX7) engaged (stock).
6V6 and 1st stage (12AU7) engaged.
6V6 bypass cap engaged.
All bypass caps disconnected.

The curves in the next graph follow the same pattern but this time the Tone Stack Bypass is engaged. There is a great deal more gain with the tone stack bypassed. If you click to enlarge the graphs and look at the right hand scale you'll see that the peak response in the modified amp is about 10dB higher than in the stock one.

This amp has the 12DW7 mod installed so the 1st stage has less gain and correspondingly less increase in gain with the cap engaged. In a stock amp the jump from the aqua curve to the green curve would be increased to about the same as the jump between the green curve and the dark purple curve on top.

Fender Champion 600 cathode bypass mod Pt 2

2009-01-06T03:05:00.000-08:00

When I wrote part 1 of this post I'd been planning to lower the cathode bypass capacitor values in order to clean up the up the bass response. What I ended up doing was removing the cathode bypass capacitors all together in order to get a bit of compression for a fuller low volume clean tone. The effect is a more roundness and a good deal less volume - very nice for a bedroom level clean sound. This meant I needed to find some other ways to control the low end (see the Input Voicing and Presence Plus mods).

For this mod I put in a three pole four position rotary switch that adds the cathode bypass caps in one by one. With the rotary switch the gain of the amp goes up with each setting until it reaches the stock (fully bypassed) position:

This could easily have been three separate switches, but I thought the single control made the operation a bit clearer and didn't clutter up the chassis as much. Since I'm limiting bass response in a couple other ways I ended up keeping the caps at their stock values. The first and second stages could easily have values from part 1 substituted if you have a need for greater bass reduction.

Here is:

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

And here's the internal view of the switch with the ground wires in place:

This is how the ground wires hook up:

The wire with the black arrow connects to the lower side of the C4 space on the pc board.

The wire with the blue arrow connects to the lower side of the C10 space on the pc board.

The wire with the red arrow connects to the lower side of the C3 space on the pc board.

Here is the rest of the switch wiring. Notice that the leads for the large blue cap (which serves as C4) jump three pins on the switch.

The stage 2 wire jumper two pins. This wire attaches to the shrinkwrapped end of C10 (indicated by the red arrow).

The wire connecting stage 1 to the switch connects to only one pin. It connects to the shrinkwrapped end of C3 (again, indicated by the red arrow).

If you've removed these caps and are reinstalling them, make sure you observe the proper polarity when you put them back in. The indented end of the cap lines up with the indent in the white outline on the pc board.

For convenience of installation, I replaced the original C4 with a axial lead cap of the same value. It's the large blue cap in the photo below. The negative side of the cap connects to the switch and the positive side is connected to the high side of the pc board connection for C4. The negative lead of the cap holds it pretty well in place but there's a dab of silicone underneath just for good measure.

This switch functions kind of like a staged clean master volume. If you're interested in getting distortion out of your preamp circuit, this mod could be rearranged to serve as a sort of dirty master volume instead. If you're interested let me know and I'll post the details.

Fender Champion 600 Input Voicing Mod Pt 2

2009-01-05T17:02:00.000-08:00

Here's a graph showing the frequency response for the Input Voicing Mod. The measurement is taken at the amplifier output with a 4 ohm resistive load.

The top curve is the stock "High" input and the bottom curve is the stock "Low" input. The middle curve shows the response of the "Low" input with the mod installed. Notice that the modded "Low" input is within 1dB of the "High" input for the high frequencies but the lows are rolled off significantly.

I didn't include the modded "High" input response on the graph because it is so close to the unmodded "High" response that it's almost indistinguishable. I would swear I hear a subtle difference between the modded and unmodded "High" input. This may be because the tone generator used for the frequency sweep is non inductive and doesn't react to the input impedance the way a pickup does. Or I may be crazy. If you do this mod yourself, let me know what you hear.

Fender Champion 600 Tone Stack Bypass/ Fat Switch Mod

2009-01-03T05:20:00.000-08:00

This pic shows the shows a few of the mods I've made to this Champion 600. The two small red switches on the bottom of the chassis are for my Presence Plus and Input Voicing modifications. This particular post concerns the tone stack mod associated with the two bigger switches to the upper right.

This mod takes the coupling cap mod from the Tone Stack Test and hardwires it to a switch so the amp can use the stock tonestack or bypass the tonestack with a single coupling cap to eliminate the mid cut in the stock circuit. The right had switch below handles that job. The left switch acts as a three position "fat" switch.

First....

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Here's the finished mod from above:

The combination of resistors on the "fat" switch are selected to combine for three settings:
15K (stock), 30K (Frondelli Mod fat boost value), and 47K (for a little extra boost). The boosts effect the mids most dramatically, but they provide extra gain across the whole spectrum too.

All three resistors connect to the same pin on the right hand switch - the one just to the left of the center pin with the black wire.

The switches I used here each have one more set of contacts than are needed to make the mod. I frequently do this when I'm experimenting so that if I decide to add something to the switch later on I don't need to disassemble the circuit and solder in a new one.

The resistors in the fat boost circuit replace R19 on the pc board. Here's how the wiring runs:

Here's the coupling cap, prepared for installation:

And here it is soldered in place. Since only one end is really fixed and the other will be supporting a wire I put a bead of silicone underneath to make sure it stays in place.

Here's how the wires are run to the switch. Point "A" on the switch runs to point "A" on the board. Same for "B" of course.

Here's the frequency response for the four settings measured at the amp output with a 4 ohm resistive load:

Click on the graph for a high res version. From the bottom to the top the curves are for Stock, Fondelli Mod Fat Boost, Extra Fat Boost and Tone Stack Bypass. The tone stack bypass curve is up about 16dB up at the mid cut frequency!

A final note...if you try this one yourself, be careful to place the switches low enough so they clear the cabinet when you put the chassis back in. It a tight fit.

Fender Champion 600 Tone Stack Test

2009-01-03T03:17:00.001-08:00

The stock Champion 600 circuit is roughly based on the Vibro Champ AA764 circuit (without the vibrato circuit of course). Fender has incorporated a tone stack from the AA764 but eliminated the bass and treble knobs by replacing the pots with fixed resistors. The result is a tone with a strong mid cut and a significant gain loss between the first and second stages.

This was my quick test to see how the amp sounded with the tone stack removed from the circuit and a coupling capacitor substituted for it. I've used a fairly large value here - .1 uF. The 5E1 and 5F1 Champ circuits both used a .02 uF coupling cap instead of a tone stack. This is a good place to start if you're trying to get your Champion 600 closer to one of those earlier circuits.

The coupling cap is tagged in from the top of C1 to the bottom of R21. The end of R19 needs to be lifted to disconnect the tone stack. The result is a more even frequency response and a good deal more drive to the second stage. I've done my 12DW7 mod to this amp, so I needed a bit of extra volume. With the tone stacked bypassed in a stock amp the breakup of the extra gain of the dual 12AX7 stages will make the amp lot dirtier. I'm looking for more clean headroom but a lot of people might go for the added drive.

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Like most of the mods I've done on this Champ 600, I'm going to make this one switchable. You can leave the mod like this if you choose but if you do it's good practice to tidy up the end of R19 by either removing it completely or insulating the lifted end with shrink tubing.

Fender Champion 600 Input Voicing Mod

2009-01-02T02:50:00.000-08:00

This is probably the simplest of the mods I did to this Champion 600. Since I put in the 12dw7 the "low" input wasn't really very useful (I never use the low inputs myself anyway). Just one capacitor can revoice it to make it more useful. With the cap in place the "low" input gets a big volume boost for everything but the bass frequencies making it sound more like another channel than a lower volume input. The effect is a more dramatic for the "low" input but to my ears it gives a very subtle boost to the "high" input as well. Here's a plot of the frequency response.

To test the mod and see if I liked it, I just curled the ends of the leads on this orange drop cap and hooked them underneath the leads of R5. The leads are spread a bit wide and then compressed when the cap is hooked in. The spring tension of the leads holds the cap in place for a listening test. There isn't really any significant voltage in this part of the circuit. So worst case, if the cap falls over and shorts a nearby lead, there isn't going to be any damage.

This mod will actually work with any amp that shares this input circuit: That includes a whole lot of Fender amps. The schematic above was excerpted from the Bassman 5F6A - but you'll see that it's identical to a host of others. The red resistor is the one being jumpered over with the .012 uF cap.*

I put the cap on a switch just for the sake of comparison. Here's a modified schematic for the Champion 600:

Now, before the actual modification....

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

So here it is - just a cap a switch and one wire!

Here's the underside showing the switch. The switch for this mod is to the left. The one to the right is for the "presence plus" mod.

This is probably the quickest and easiest mod I've done for this amp. It's real simple to drop a few different caps in and see if you like it. Higher values will give a bit more low end, smaller values cut more low end and hence sound brighter. Unless you regularly use your "low" input (and who does?) the switch is really unnecessary. To obtain a suitable capacitor, see the input voicing mod "kit" post.

*Since you are essentially bypassing the grid stopper resistor at high frequencies on an older amp, you may need to add a new grid stop resistor. If you don't have trouble with radio frequency interference and it sounds fine, the don't worry about it. The Champion 600 already has an independent grid stopper (the 10K resistor connected to pin 2) so it's not an issue.

Fender Champion 600 Tube Upgrade

2009-01-01T07:42:00.001-08:00

I don't think of myself as a huge tube snob but the stock 6V6 tube in this Champion 600 was really pretty bad. It was inarticulate and mushy both overdriven and clean. I installed a JJ Tesla 6V6. If you put in new 6V6, play for a while and then put the stock one back. You'll chuck it right in the trash without hesitation.

The stock 12ax7 was nothing to write home about either. The amp will improve with an upgraded 12AX7. I've swapped in a 12DW7/ECC832 for more headroom.

I liked the effect of the 12DW7 / ECC832 so I modded the circuit to properly bias the tube.

Fender Champion 600 Presence Plus Mod

2009-01-01T05:18:00.000-08:00

This post is one in the series of mods I made to this Champion 600. I wasn't sure what to call this one so for now it's dubbed "Presence Plus". It pretty much the same as a presence boost except that the boost has a much wider frequency range so it effects more than just the traditional "presence" frequencies. It extends from the high frequencies down to just above the resonant bump in the speaker response. The idea is to have the feedback loop maintain control over the low end looseness of the speaker but be transparent to the high and body frequencies. The effect is to give the amp a bit more gain and openness without boosting the low end flab.

Here's the modified schematic. The added components are shown in red:

There are a few other mods in progress here so don't let the altered circuit board throw you off. The presence mod consists only of the huge black cap in the upper right labeled 16 uF 350 VDC, the red switch it's connected to and the black jumper wire that completes the connection across R23 in the pc board. That's all there is to it.

The 16 uF cap is ridiculously over rated at 350 VDC. I really wanted a 16 uF cap. I have 10's and 22's in stock but nothing in between, so I pulled a 16 from the parts bin and used that. There's so little voltage across R23 that pretty much any voltage rating you find will work (and take up a whole lot less space).

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Here's a view from the top:

And here's the switch on the bottom. You can see the silver Sharpie line I've drawn to position pilot holes for future switches.

Note: be careful when placing the switch on the outside edge. Mine just barely clears the wood support for the chassis inside the cab. I stupidly forgot to check before I drilled and if it had been another 1/8" out the chassis wouldn't have fit back in.

Here's a closeup of a schematic for the mod. In the full schematic a the top of the post the feedback loop path to ground is highlighted in blue. Below you can see the added components in red.

I find that the Champion 600 has plenty of high end response (especially with the sort of lousy stock speaker). If you find otherwise, you can fiddle with the value of the 16 uF cap. I don't think think you'll find much use in going lower, but raising the value will restrict the low end response and the effect will be more of a high mid / treble boost.

If you want to order the cap for this mod, have a look at the Presence Plus Mod "kit"

Fender Champion 600 12DW7 / ECC832 Mod Part 2

2008-12-28T07:07:00.000-08:00

Here's a look at the 12DW7 / ECC832 tube I used in the Champion 600 12DW7 Mod Part 1 .

Starting with the pin out diagram:

You can see from the diagram that, like other 12A*7 tubes, the 12DW7 has two triodes. Each triode has three connections - pins 1,2,3 and 6,7,8 respectively.

On most 12A*7 tubes (12AX7, 12AT7, 12AY7, 12AV7, 12AU7) the two triodes
in a single tube have identical characteristics.

In a 12DW7 the two triodes are different.

The section listed as Section 1 on the data sheet uses pins 6,7 and 8 and has characteristics identical to one half of a 12AX7.

The one listed as Section 2 uses pins 1,2 and 3. This section is identical to one half of the lower gain 12AU7 .

As an illustration, here's a Traynor YB1A with a 12DW7 in the first valve position:

Input 1 feeds the 12AU7 half of triode and input 2 feeds the 12AX7 half. The input is about 120 mV RMS and the meters are reading the output voltage from each of the two triodes.

The left hand meter reads 6.3 V for the 12AX7 stage output - that's a gain of 53 (6.3 V output/120 mV input).

The right hand meter reads 1.6 V for the 12AU7 stage output - that's a gain of 13 (1.6 V output /120 mV input).

So you can see the most obvious difference when swapping a 12DW7 for a 12AX7 is going to be a lower overall stage gain with the 12DW7.

This is also apparent if you look at the Amplification Factor in the data sheet (highlighted in yellow).

Amplification Factor is related to the gain of the stage the triode is used in. It should not be thought of as the actual gain of a stage - it is actually the maximum possible gain in a stage designed to use that triode. As you can see in the measurements in the YB1A above, the actual stage gain for the 12AX7 was just above 50, even though the Amplification Factor for a 12AX7 is 100. The reasons for this are too complicated to get into here, but I'll post them independently at a later date. Suffice it to show that the 12AU7 stage has a significantly lower gain than the 12AX7 one (the combined gain for the two stages will be roughly the same as if you substituted a 12AY7 for the 12AX7).

The second and slightly more subtle difference is evidenced in what is listed as the Grid Voltage (highlighted in blue). Doubling this number will give a very rough sense of the possible input headroom available to the stage. It a very inexact number but good enough for the sake of illustration.

For Section 1 then the input headroom would be 4 volts. For Section 2 it would be 17 volts. A typical 12AX7 stage can output over 4 volts with an input of only 70 millivolts. At 120 millivolts it easily clears 6 volts:

Many guitars can output over 500 millivolts. And those voltages are in RMS - the peak to peak voltage is 2.828 times the RMS voltage (peak to peak voltage is what you really have to look at if you're concerned with the headroom of a stage).

So you can easily see that if the second stage is also a 12AX7 triode and there is no attenuation between stages, the second stage is going to be clipped. A 12AU7 has 4 times the input headroom of a 12AX7 (even more if it's run into the nonlinear part of it's curves). So using a 12AU7 as a second stage has some advantage if you're looking to keep things clean.

Plugging a 12DW7 into a Champion 600 actually places the 12AU7 section before the 12AX7 section, which is why the first stage is the one I rebiased in Part 1 of this posting.

Spring Clip Tube Removal Tool

2008-12-23T10:06:00.000-08:00

Tight spring clips do a good job of keeping your power tubes in place. They can be a pain when you're trying to get the tubes out though - especially when the amp is still is it's cab and you're trying to wrestle two hands around a hefty output transformer. Here's a tool I came up with to make the job a bit easier.

Fender Champion 600 Output Transformer

2008-12-23T09:09:00.000-08:00

Another difference between the new Fender Champion 600 and the older Fender Champ 5E1 and 5F1 circuits is the higher supply voltage (also called the B+ voltage) and an accompanying higher impedance primary on the output transformer in the Champion 600. I've never measured an original myself but replacements like the TF103-48 have a 5K ohms primary impedance. My measurement for the Champion 600 transformer at 1kHz is about 11K ohms.

I was a bit surprised by this so I called Mercury Magnetics to ask them about the impedance of the output transformer in their C600 Champion Upgrade Kit. They said their measurement was around 9K for the original and that they raised it a bit to 10K for the upgrade.

I don't have the details for their measurement, but here's the details of mine:

The tone generator (center) is set to input 1 volt @ 10Khz into the secondary of the output transformer (shown on the left side meter). The right hand meter shows the voltage output on the primary with 1 volt on the secondary. So here's how the primary load is calculated.

A transformer impedance ratio is equal to the square of it's voltage ratio.
Here the voltage primary to voltage secondary is 59.4:1
59.4 squared is 3,528
So for a 1 ohm load, the load reflected at the primary would be 3,528 ohms.
But the Champion 600 doesn't have a 1 ohm speaker for the load, it's 4 ohms.
That will raise the the impedance reflected at the primary 4 times.
So 3,528 times 4 is 14,112 or about 14k ohms.

But didn't I say I measured about 11K?

Yes - but at 1kHz. The above measurement was for 10Khz.
Transformers (especially cheap ones) are imperfect devices and their response varies with frequency.

Below is the same procedure, but at 1k.

With 1 volt on the secondary at 1 kHz, the reading on the primary is 52.94 volts (rounded to 53).

Here the voltage primary to voltage secondary is 53:1
59.4 squared is 2,809
So for a 1 ohm load, the load reflected at the primary would be 2,809 ohms.
So 2,809 times 4 is 11,236 or about 11k ohms.

Here's the reading at 100Hz:

With 1 volt on the secondary at 100 Hz, the reading on the primary is 44 volts.

Here the voltage primary to voltage secondary is 53:1
44 squared is 1,936
So for a 1 ohm load, the load reflected at the primary would be 1,936 ohms.
So 1,936 times 4 is 7,744 or about 8k ohms.

From this you can see that if you load the 4 ohm secondary of a TF103-48 with a 8 ohm speaker you raise the primary load to 10k - same as the Mercury Magnetics. Of course there's a whole lot more to a transformer than just the loading, but if you need to replace the OPT in a Champion 600 and are short on dough an 8 ohm speaker on the 4 ohm tap should get you back in business.

UPDATE: For comparison, I've posted measurements for an AB764 Vibro Champ output transformer here.

Float Your Screen Resistors!

2008-12-23T04:07:00.000-08:00

Here's a pretty extreme example of what happens when you
screen resistors are too close to the pc board. This is why it's important
to "float" higher power resistors off the circuit board.

The amp here is an Ampeg V4B. The board was so burned that the
screen resistors were moved off to the power tube sockets.
The resistors that burned up were replacements,
the originals would have been underneath the pc board.

In the center of the picture below is a floated resistor in a
Fender Champion 600. The breathing room beneath
the resistor reduces the chance of the pc board being
damaged if the resistor overheats or flames out.

It's shocking that some new production amps don't bother
to do this as their pc boards are generally very delicate.
There is no really effective way to repair a pc board
once it goes. Your tech can kludge in jumper wires
but the amp will never be "like new".

Fender Champion 600 12DW7/ECC832 Mod Part 1

2008-12-23T04:03:00.000-08:00

This post has been updated.

I'll leave this old page in place until I've checked all the links.

Fender Champ - comparing the 5F1 and 5E1 circuits

2008-12-21T05:54:00.000-08:00

In modding their Champion 600's, some people are trying to get closer to earlier versions of the Fender Champ circuit, the 5E1 and 5F1. This post is to clarify the differences between those two early circuits. There are two significant ones.

The first is the missing cathode bypass capacitor in the 5F1. Eliminating this cap reduces the first stage gain. Reducing the value instead of eliminating it completely will give a bit of a treble boost (see my earlier post "Champion 600 cathode bypass mod" for a chart showing how different capacitor values will effect the response).

Secondly, in the 5F1 champ the choke has been eliminated and a screen supply node consisting of a capacitor and resistor has been added. This saved Fender some cash, as a choke is a good bit more expensive than a cap and a resistor. In a Class A amp a choke doesn't contribute to sag they way it does in a Class AB amp, but the change still has an effect. The screen supply resistor in the 5F1 drops the voltage for the screen below that of the plate. Since the screen voltage actually has more effect on plate current than plate voltage, this changes the character of the output section. If you really want to hear if the choke makes a difference between the two circuits you need to put the choke in series with the screen supply resistor, not eliminate the resistor completely.

Fender Champion 600 cathode bypass mod

2008-12-17T08:35:00.001-08:00

This post is from the series of mods I've done to this Champion 600. I'm finally getting down to changing the circuit a bit. My main goal is to clean the amp up a bit, especially in the low end. It's probably going to require a speaker replacement eventually, but I'm starting with the cathode bypass caps.

With most 12AX7 circuits, smaller bypass cap values will reduce the low end response by a bit less than 6dB. The frequency at which the response is down by 3dB is, logically, the "3dB down point" or the "knee frequency". Changing cathode bypass caps can be a bit more subtle than changing coupling caps since the low end response is never reduced more than 6dB - only the knee frequency is changed.

There are two cathode bypass caps in the preamp section of the Champion 600. Either or both of them can be changed out to shape the frequency response of the amp.

Here they're indicated on the pc board layout - C3 is for the first stage and C10 is for the second:

A standard tuned 6 string guitar isn't going to produce anything below 80Hz and even the low frequencies it does produce aren't going to reproduce well though the Champion 600's speaker. Below is a chart with values calculated for the 3dB down point with a series of common cap values. Note the stock value of 22uF has a -3dB point of 9Hz. I'm going to start with a 1.5uF in the first stage and a .68uF in the second and work from there.

If electrolytic caps are used (and they probably will be for the larger values unless you've got deep pockets) it's very important to maintain the polarity of the cap or it will explode under use.

This amp came to me with a printed copy of the Frondelli Mod . The mod itself is fine, but a number of the changes detailed there do the opposite of what this amp owner is looking for. He's looking for a cleaner sound with more low end headroom. Even though Frondelli is limiting the low end somewhat, he is also increasing gain for a more saturated sound. If that's what you're interested in, though, you can tweak the bypass cap value to your taste there as well. The blue resistors in the diagram below indicate the ones changed for the 1st stage in the Frondelli mod (R8 and R2). The bypass cap associated with that stage (C3) is indicated in orange.

Jon Frondelli recommends a .68 uF cap here, but you're free to change it to taste. Below is a chart showing calculated -3dB values adjusted for the new plate and cathode resistor values in the Frondelli mod.

If you're interested in how these values are calculated, drop me an email - or visit Randall Aiken's site.

Inside the Fender Champion 600

2008-12-16T14:18:00.001-08:00

There are a few relatively pleasant surprises inside the Champion 600.
Granted, the bar is pretty low for current production budget amps,
but here's a few things I was glad to see.

The 3 higher power resistors are all floated well off the circuit board
(one of them is shown in the middle above).
This allows them to dissipate heat more effectively and keeps them
from cooking the pc board when they get hot.

Even though the octal socket for the 6V6 is mounted to the pc board, the socket also screws to the chassis when the amp is fully assembled. When it's screwed down it has a pretty decent mechanical connection. Not perfect, but better than average for today's amps.

There are plenty of amps in which the only mechanical support for the tube socket is the solder that makes the electrical connections for the tube pins. These inevitably get strained when tubes are taken in out. Usually they cause an annoying intermittent failure long before they crap out completely.

The 9 pin preamp tube socket isn't quite as good but better than a lot I've seen. It's mounted directly to the board with no independent mechanical support. The top ring on the socket fits pretty snugly into the hole in the chassis. The pc board mounting screws are close enough to the socket that the board doesn't flex too much when the tube is wiggled out of place.

The plastic jacks are better than I expected them to be. And the way the small front panel circuit board is implemented it would be easy to eliminate the card completely and replace the jacks and the volume pot with more robust components. There's also plenty of real estate immediately available for switches and such - both under the chassis and on the front panel. I'm planning to have a few switches in there, at least for the testing phase, so it's nice to have the space.

Disconnecting PC Board Spade Connectors

2008-12-15T11:57:00.005-08:00

PC board spade connectors can be difficult to pull off gently.
Here's how I do it when there's enough room to get pliers in there.
The board here is from a Fender Champion 600.

Opening a Fender Champion 600 for modding

2008-12-15T11:09:00.000-08:00

Getting ready to make some circuit changes to this little Champion 600.
The gif is just for fun really.
If anyone is interested in more detail let me know.
I have photos in much higher res.

Fender Champion 600 vs Epiphone Valve Junior

2008-12-13T18:11:00.000-08:00

Here's a frequency plot of a stock Epiphone Valve JR and a stock Champion 600, each using the Eminence Legend 12" speaker in a Fender Deluxe Cab.

Notice the relative low frequency roll off and the 725 HZ bump on the Valve Junior plot.

What's not evident here is that the gain setting on the Champion 600 is at about 12:00 the gain on the Valve Junior is about 9:00 for the same output.

Fender Champion 600 Part 1

2008-12-13T15:27:00.000-08:00

Below is a frequency plot for a Fender Champion 600 using it's own speaker first, then with the internal speaker unhooked and the Champion amp connected directly to the speaker in 4 different combos.

Green is the stock speaker

Purple is the stock alnico in and Ampeg Jet Cab

Red is an Eminence Legend 12" in Fender Deluxe Cab

Yellow is a Celestion G12 Century in a Hughes & Kettner Puretone Cab

Three things strike me about the stock speaker plot:
• The 90Hz low frequency resonance of the speaker is quite strong.
• The high frequency response is extended above 5KHz.
• The speaker sensitivity is the lowest of all of the four tested.

The low frequency bump on the is certainly not helping the muddiness or lack of low end headroom that characterize the Champion. The next step will be to modify the circuit to limit the low frequency response.

Fender Pro Junior Cathode Bias Mod

2008-12-09T06:09:00.000-08:00

I got this little Pro Junior in trade a while back. These things sound pretty decent even stock. This one was cutting out intermittently though. Turns a few solder joints were cracked where the sockets were mounted on the PC board (pretty common with board mounted sockets). The board itself was OK so the fix was quick and easy.

Here it is with the back off:

When I looked over the main circuit board, I noticed that the wire marked W1 was a jumper to ground for the power tube cathodes. That would make for a quick and easy modification to cathode bias. So I figured I had it open, why not. And if I'm going to bother, why not make it switchable fixed/cathode bias.

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Here's the 22oK resistor assembly. They'll serve as the bias feed resistors in fixed bias mode and as ground reference for the power tube grids in cathode bias.

I futzed a bit with the cathode resistor value. I was planning on selling this amp, so I ran it slightly on the cool side so that whoever ended up with it wouldn't have a tube eater on their hands. Here's the final assembly.

The black wires run off to the right hand switch. You can see I also added a switchable
negative feedback loop. It was a bit inconvenient having the switches under the chassis but I didn't want to change the faceplate and it was a much shorter run for the wiring.

When I was done the amp sounded pretty cool. The pair of switches made it a good bit more versatile. In cathode bias with the feedback loop disengaged it was much looser, browner and more touch responsive - a much better blues sound. Engaging the feedback loop and or the fixed bias would make it tighter, cleaner and brighter.

I sold this particular one awhile back so I can't get more photos. I think my Dad still has a stock one though. If anyone's interested in a more detailed procedure, drop me a line - he might be convinced to let me use it for a step by step demo.

The Tube Amp Book by Aspen Pittman

2008-12-08T06:10:00.000-08:00

The Tube Amp Book has always been a sort of infomercial for Groove Tubes. I've never had much use for the first half it - glossy photos of classic guitar gear and a catalog of Groove Tubes product line. The second half though is a good source of schematics - which is what I've always valued it for. My old copy of Volume 3 is broken in half and shedding pages from the split. I got the Deluxe Revised Edition for Christmas last year and what I love about it is that it's spiral bound so it lies flat without breaking the spine.

Here's the old and the new:

The schematics are also printed a bit larger (the book is much bigger but now there are two schematics per page). This is what's left of my Volume 3 - I'm missing a few pages of Fender Champ schematics:

The Deluxe Revised Edition actually has fewer printed schematics than the older editions. It's down to 368 from 408 in the past. Ampeg V4's seem a lot more common to me than some of the others they chose to include. Maybe it's a Boston thing. It's conspicuously missing from the new edition though. It is still on the included CD of schematics. There are 800 on there. It's not as useful as the printed ones, but it's still handy if you don't have a web connection near your bench.

The Deluxe Revised Edition also has a slightly expanded technical section. It's a bit random and only slightly useful IMHO but now includes an article on Class A by one of my favorite technical writers Randall Aiken. There's some tube data there too, which is handy to have around but it doesn't have curves for all the tubes listed which is a disappointment. Like schematics, tube data is easy to find elsewhere - the Tube Data Sheet Locator at Duncan Amps is one good hub.

Before schematics were widely available electronically, TTAB had a bit of a corner on the market. Now, since all of the schematics in are easily found at such places as Schematic Heaven, TTAB has taken some shots for selling material that anyone get for free on the web. As I see it you're really paying for the printing, binding and organization. To me, it's worth the $35 or so cover price to bypass the downloading, printing, punching and binding of 350+ schematics. Sure, not everything you're looking for is in there but it's a handy reference for most amps you're likely to run into.

Ampeg V4 curiousities part 2

2008-12-07T06:29:00.000-08:00

This is a look a some of the less common tubes used in the preamp section of the V4. There's also a view of the circuit board revealing one of the major differences between the V4 and the V4B.

Ampeg V4 curiousities part 1

2008-12-06T13:55:00.000-08:00

A quick tour of the power supply end of the underside of the circuit board and a look at the unusual plate resistors.

Adjustable Bias for an Ampeg V4B

2008-12-05T11:08:00.000-08:00

This is a shot from an Ampeg V4B that's currently in for repairs. It blew the flyback protection diodes and then shorted a power tube. I thought it would be better off not biased quite as cold as these V series amps are stock.

The change is pretty simple - I replaced R49 (a 75k resistor) with a 47K resistor in series with a 20 turn 25K cerment pot. The wiper of the resistor is connected to the supply diode and one leg is attached to the 47K resistor. The other leg of the pot is unused. A dab of silicone under the pot holds it in place and keeps adjustments from stressing the leads or the solder joints. It gets the bias voltage down as low as about -40 volts. Stock its about -60 volts. The nice thing about this arrangement is that if the pot were to fail with an open circuit on the wiper the bias voltage would actually go up and the tubes would be protected.

carbon comp v.s. metal film resisitors

2008-12-05T10:12:00.000-08:00

This is another test post of sorts - seeing how well the video works.

The videos show a comparison of the heat stability of metal film resistors compared to the traditional carbon comp ones.

After watching the video, imagine what those carbon comp screen resistors hanging over the top of the tube socket of your old Fender are doing as your amp heats up. They aren't going to get as hot as they do when hit with the heat gun. But if you imagine all the carbon comps in your amp drifting a bit with heat you can imagine the amp's not going to sound the same. Might be better might be worse but it won't be consistent.

Here's the IRC metal film one:

And the carbon comp:

Ampeg V4B bias mod

2008-12-04T11:12:00.000-08:00

A bit of a test post really - trying out the photo upload.

This is the inside of a Ampeg V4B that had burned out the screen resistors. The pic shows a mod done I found in there done by a previous tech. Stock V series amps derive the bias from the B+ supply. The transformer next to the power tubes was added across the heater supply to kludge an independent bias supply winding in the power supply.

getting started

2008-12-04T08:51:00.000-08:00

When I was starting to learn about electronics, finding good introductory tube circuit material was no small task itself. There were plenty of old EE texts but that's a tough place to start if advanced math is not you idea of a relaxing Saturday morning. There were also a smattering of "guru" sorts of books but they were more collections of tips and trivia than solid technical introductions. Nowadays tube info on the web is so ubiquitous that the veteran RCA Tube Manual seems awkward and incomplete and many of the old gurus serve as punching bags for website FAQ's. Actually learning the ropes isn't all that much easier though. First there's sorting out the bogus information. That's a bit of catch 22. You need a certain level of understanding to work through it and if you have that understanding then chances are you already have that information. Then there's struggling with the fundamentals of electronics and the peculiarities of vacuum tubes. That's a study that never ends but it can be particularly difficult to get rolling. This blog'll be sort of an offhand collection of work that passes over my bench along with bits of "tube amp basics" for people trying to get through the fundamentals and onto the fun stuff. Hope you enjoy it.