Federal Tube Limiter Release Time Adjustment

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

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?

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?

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

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

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?

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.