Drawing up a component layout from a tube amp schematic

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.

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

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.

Desoldering Tool for PC Board Mounted Pots

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

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.

Fender Champion 600 cathode bypass mod Pt 2

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 Tone Stack Bypass/ Fat Switch Mod

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

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 Presence Plus Mod

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"