1940's Amp High Volume Oscillation

[bobloblaws]

I have an amp here built by Hammond of Guelph Ontario. The (attached) schematic drawing dates to 1946 and apparently this specimen was built in 1949. The owner brought it to me to have the inputs changed to 1/4". I replaced the 6N7 tube (my tester showed the existing one was bad) and I replaced a couple of leaky caps and a resistor that had drifted from 100K to 160K. The "Pickup" channel sounds fine but is not very loud, there is one less gain stage than the other channel. Turning the volume up on the "Mic" channel, it sounds very warm and robust, starts to clip around 5, then just past 7 it starts to squeal. I swapped all the tubes save for the 6C5 (didn't have a spare), swapped out all filter caps and the cathode bypass caps on the 6L6's and the 6N7, tried 470 ohm resistors on the 6L6 screen grids (a la Fender), connected a 1 Meg resistor from input to ground, and even disconnected the other channel (lifted pin 6/plate #2 on the 6N7). After doing all that I discovered that while I could get rid of the squeal by turning down the tone knob, when I turned the volume up yet a bit more the amp started to oscillate in a medium speed tremolo like fashion. I checked the B+ and I can see it moving up and down in time with the oscillation. I swapped in another rectifier but the problem persists. Does this suggest a bad power transformer?

B


Here's the schematic and an audio file from my iPhone (I think the Voice Notes app compression makes the noise level seem louder than it is, the amp is actually pretty quiet at moderate levels). In the recording the tone knob is down and I'm turning up the volume until it oscillates. then I turn the tone knob up and you can hear the high squeal along with the thump. At the end I turn the volume back down to 5 or so and tap on the guitar strings a bit.

New Recording 112.m4a

Edit: FWIW, I should maybe clarify regarding swapping out caps. The caps in question are electrolytics and could very well be original. Yes, they should be replaced as a matter of course, but I haven't had that conversation with the owner yet. Anyway, I did not have new ones in all at once, rather, I'd swap in a pair or a single and when that didn't alleviate the problem I'd reverse swap and move on to the next caps. In hindsight maybe I should have done it cumulatively. Or maybe the problem has nothing to do with old caps?

[olddawg]

If you plug a shorted plug into the input jack and turn the volume up does it still oscillate? A guitar isn’t a 1940s mic. Have you tried a guitar with a different pickup? You may just have too much gain and an impedance mismatch in the preamp that wasn’t designed for guitar pickup...

[Helmholtz]

As you describe the oszillation as medium speed tremolo effect, it would be a kind of motorboating. This is often caused by a positive feedback between stages via unsufficiently decoupled/filtered supply voltages, in other words too small filter cap C-values. 4µ and 8µ are already extremely low and electrolytics lose capacitance over time. You should try new filter caps and if necessary increase the values moderately.

[Enzo]

I am not a fan of wholesale parts replacement as a starting point, however in something 70 years old, those caps are definitely in need of change. Caps were expensive back then, so they used 4uf instead of 10uf or 20uf. SO I have to agree with helm, change your filter caps, you are probably suffering from lack of decoupling.

[bobloblaws]

I am not a fan of wholesale parts replacement as a starting point, however in something 70 years old, those caps are definitely in need of change. Caps were expensive back then, so they used 4uf instead of 10uf or 20uf. SO I have to agree with helm, change your filter caps, you are probably suffering from lack of decoupling.

I have this amp again to do the cap job so I re-read this thread. Just curious, my understanding is that the terms coupling and decoupling refer to doing just that to different stages or sections of a larger circuit. So when we use the term decoupling in this context is it simply referring to the decoupling of the DC powered section (or sections) from the AC power source?

[Helmholtz]

I have this amp again to do the cap job so I re-read this thread. Just curious, my understanding is that the terms coupling and decoupling refer to doing just that to different stages or sections of a larger circuit. So when we use the term decoupling in this context is it simply referring to the decoupling of the DC powered section (or sections) from the AC power source?

In this case decoupling means preventing any spill-over of signal voltage between stages via the plate supply. Effectively shorting low frequency components to ground requires some minimum filter capacitance.

[bobloblaws]

In this case decoupling means preventing any spill-over of signal voltage between stages via the plate supply. Effectively shorting low frequency components to ground requires some minimum filter capacitance.

Oh, that is interesting. I had no idea the filter caps had an effect on signal voltage. I thought they strictly served the purpose of filtering AC ripple. So I guess my takeaway is that disregarding ancient filter caps simply because I'm not hearing 120 Hz hum is a misinformed strategy. Thanks.

[Enzo]

But signal is AC. If my first stage puts a little signal on its B+, then that can affect the other stages on that B+ node. Thus the two stages can "couple" through the B+. And if they are out of phase, it can oscillate. So the "filter cap" there is a "decoupling" cap as it prevents the signal from coupling between stages. So decoupling does not refer to the AC power source.

If you want, we can call those caps both filtering and decoupling. But by the time we get down to the preamp tube B+ node, there isn/;t any ripple left anyway, and all it does is decouple. The job of the cap is to keep the DC supply totally smooth, whether from ripple or from signal.

[g1]

Also to mention, the terms 'coupling caps' and 'decoupling caps' are not interchangeable, but are sometimes confused with each other.
Coupling caps are used between stages to block DC but let signal through.
Decoupling caps are generally filter caps. They decouple the signal from the supply rail by providing AC ground for the signal.

[TimmyP1955]

In addition to new caps, you might try adding grid stoppers on the first and/or second stages.

[Randall]

Also to mention, the terms 'coupling caps' and 'decoupling caps' are not interchangeable, but are sometimes confused with each other.
Coupling caps are used between stages to block DC but let signal through.
Decoupling caps are generally filter caps. They decouple the signal from the supply rail by providing AC ground for the signal.

That clears up things a lot for me as far as decoupling goes, but one thing... if signal is decoupled off to ground through the last filter cap, how is there any signal getting thru the coupling cap on it's way to the next grid? (I'm guessing the plate supply resistor has a role here).

[Enzo]

The decoupling/filter cap is at the B+ end of the load resistor. The coupling cap to the next stage is from the plate end of that resistor.

Imagine a guitar string, it vibrates in the middle, but the ends are held tight to solid body. Now imagine that instead of the string tight to the bridge, it is free to vibrate down to the tailpiece. In either case, the middle of the string still vibrates, but we normally prevent the vibration from getting past the bridge. yeah, I know, crappy example.

[Helmholtz]

if signal is decoupled off to ground through the last filter cap, how is there any signal getting thru the coupling cap on it's way to the next grid?

The purpose of filter caps is to provide a low impedance path for AC currents (including the signal plate current of the connected tube) to ground, thus minimizing AC voltage components (ripple+signal) across the cap.
If the ESR of the cap is too high and/or the capacitance is too low, this cannot work properly.

A 4µ cap has an impedance of around 330 Ohms at 120Hz. This may just be enough to suppress audible ripple. But the cap's impedance increases with 1/f and at 6Hz the impedance will be as high as 6.6k, meaning that such low fequencies cannot be decoupled but will show as voltage across the cap, also coupling to other nodes. The thereby coupled stages thus may become a very low frequency oscillator.

This is the reason why motorboating always shows a very low pulsing frequency and why it can mostly be cured with good caps of higher capacitance.

[bobloblaws]

Hey guys, I just got back to this project. I first changed the the 4uF and 8uF caps to 10uF each and the rest to 22uF. I was still getting the motor-boating effect at high volume so based on the advice given I tried 20uF where the 4uF was originally, the first preamp tube node. That eliminated that problem and I bumped up the other 10uF to 20uF as well for good measure. However, at high volume the high pitched squeal is still occurring. Again, I can get rid of it by turning down the tone pot slightly below the half way notch, but ideally I'd like to correct this problem as well. Originally I had thought that whatever was causing the motorboat type low frequency oscillation was the same thing that was causing the high pitched squeal, because both would occur at roughly the same volume level. But now that I've eliminated one problem and the other is still there I'm thinking they are (were) unrelated. The other thing is that with no guitar plugged in there is no squealing. And with the guitar plugged in if I turn the volume control on the guitar down to zero the squeal goes away. But it doesn't sound like conventional feedback between guitar pickup and speaker. There is at least one microphonic tube but it doesn't sound to me like it's coming from that either. Is this where I should consider the grid stopper suggestion per TimmyP1955?

[g1]

Which tube is microphonic, is it the 6N7? Is it metal? I sometimes have a hard time with microphonics from metal tubes.