Learned lesson about overdoing a Good Thing !

[HaroldBrooks]

Recently I've been trying to extract as much lower Bass as I could from the power section of my smaller amps, figuring if some bass is good, then more would be better !

Well I think I caught myself "Over doing" it once again, this time it has to do with adding a huge Cathode Bypass cap to the power tubes in a push pull 6V6 amp setup (amp has a single 12" Jensen speaker).

I learned that you can get away with a huge caps on single preamp tubes (as in 470uf 50v) to reduce hum, by creating a kind of short to dump the heater noise to ground. I figured a large cap might be good for the push pull power section as well, to get that last bit of low end... But it seems I caused an obnoxious swirly noise on the decay of low, and very high gain power chords. I just noticed this effect tonight, and it was driving me nuts as to why that was happening, as all seemed well with the amp for the last month or so, playing at lower gain settings.

The clue came when I backed off the bass settings severely using a stomp box EQ, and the nasty sound lessened quite a bit, so on a hunch I removed the huge cathode bypass cap I had added from the power tubes all together, and that seems to have fixed it, the nasty sound is gone !

So I am going to give myself the same advice I routinely give others, don't try to achieve unnatural levels of bass from your smallish amp. If you need a "Bigger" sound with more bass, hook up your amp to a bigger multi speaker cabinet, with perhaps bigger speakers with more natural bass.

A small amp and speaker has a limit as to the bass it can generate.

Any comments regarding this are welcome, as I am still learning a lot about amps and mods, and only know the basics, but enough to cause trouble for myself it appears !

[Helmholtz]

The clue came when I backed off the bass settings severely using a stomp box EQ, and the nasty sound lessened quite a bit, so on a hunch I removed the huge cathode bypass cap I had added from the power tubes all together, and that seems to have fixed it, the nasty sound is gone !

Even a "huge" cathode bypass cannot produce more bass than no bypass cap. So there must be a different reason. It might be associated with the bias shift caused by the cap which lets the tubes run cooler at large output compared to no cap.

[pdf64]

Cheaply made amps tend to have OTs that are barely adequate for the application, and may get pushed close to saturation when overdriven. Increasing the cathode bypass cap value (along with the increase of mains voltage in North America) may act to push the OT fully into saturation, which may be characterised by a ratty thin tone.
May describe what happened here?

How does it sound if the stock value bypass cap is fitted, and the mains is adjusted so as to achieve a heater voltage of 6.3Vac?

[Chuck H]

I interpret that Harold ADDED a cap to the existing circuit. We don't know if there is an existing bypass cap at this time.

Another possibility is that higher levels of LF in the output generate more current in power amp leads. Leads that may be interacting with more sensitive low level signal leads in the amp. What you heard could have been an oscillation. But I like pdf64's angle too.

[R.G.]

It's possible that you were on the edge of motorboating. This is the low frequency equivalent of high frequency oscillation. You sometimes run into it if you keep increasing bass response in the circuit until the power string bypasses are no longer completely effective in removing those low frequencies from the power supply to the tube. It's a little odd to get right on the edge of it, though

[HaroldBrooks]

I interpret that Harold ADDED a cap to the existing circuit. We don't know if there is an existing bypass cap at this time.

Another possibility is that higher levels of LF in the output generate more current in power amp leads. Leads that may be interacting with more sensitive low level signal leads in the amp. What you heard could have been an oscillation. But I like pdf64's angle too.

I think you may have hit upon it Chuck, and here is why. Yes I added a cathode bypass cap to the shared cathode resistor of the power tubes where there was none, and I also noticed that when I leaned over the amp with my guitar boosted to the moon, and created an audio feedback because of the close proximity. The feedback tone oscillated like a tremolo, and when I removed the cap again, that oscillation of the feedback was gone, so I have the strong suspicion you guessed correctly. The amp's layout is a hodge podge of wires and never intended for the big guitar signal I am giving it. Good show Chuck.

Actually there are other things perhaps afoot, as both pdf64 and Helmholz indicated as well. The amp 6V6 power tubes are currently biased from the factory using a common 220ohm resistor, and this puts both tubes very, very close to the edge of 14watts each for plate dissipation, although there is no red plating, so that may be a factor that contributed. Again, I noticed from the start that the output transformer is also a bit small for this type of amp, and the amp is from 1957 so yes, the wall voltage in my house is around 126vac these days, and that is a factor.

I ignore nothing you gents see fit to comment on, as you all have a tremendous wealth of experience individually, and collectively.

Thanks for the excellent observations so far, but please keep them coming if you have any more !

[Helmholtz]

Yes I added a cathode bypass cap to the shared cathode resistor of the power tubes where there was none

So this means that you compared a "huge" cathode cap to no cathode cap as I assumed?

No cathode bypass cap in the original circuit would actually be surprizing. Acc. to Langford-Smith (R.D.H.) a class A PP stage doesn't need a bypass cap as cathode signal currents of both tubes compensate and there is no signal voltage across the cathode resistor (similar to the tail of a LTPI). But a class AB stage should have a cap for best performance.

Please post schematic!

The feedback tone oscillated like a tremolo

This strongly supports R.G. Keen's hypothesis of latent motorboating. It also means that your power supply filter caps have critically low capacitance wrt. the bass response of the amp.


It is probably a combination of several effects.

[Leo_Gnardo]

Wondering what was the capacitance of this "huge" bypass cap that caused the problem?

[HaroldBrooks]

Wondering what was the capacitance of this "huge" bypass cap that caused the problem?

470uf 50v cap. I must admit that I was driving the amp input with a Seymour Duncan "Flat" boost (25db gain) and a with about another 6db of gain from a stomp box EQ, and at the front of all of that, a compressor. LOL, a bit over the top. At such high levels I've found with almost ANY amp there will be some type of raucous noise along for the ride, making it hard to use in a performance. The metal type amps like Soldano, Mesa, etc... are more carefully thought out I believe.

I only own and modify the old stuff, as a matter of fact, my newest amp is a 1986 Marshall JCM 800, and the next is a 1965 Gretsch 6162. Most of my amps are from the 1950's and 1940's, so you can see I am trying to fit a square peg into a round hole, but still have the square peg at the end of it all. Still, I have had some good success so far, so I am encouraged that I can have an amp that sounds good at a modest crunch level, but sound at least 'OK' when you drive the crap out of it !

[HaroldBrooks]

So this means that you compared a "huge" cathode cap to no cathode cap as I assumed?

No cathode bypass cap in the original circuit would actually be surprizing. Acc. to Langford-Smith (R.D.H.) a class A PP stage doesn't need a bypass cap as cathode signal currents of both tubes compensate and there is no signal voltage across the cathode resistor (similar to the tail of a LTPI). But a class AB stage should have a cap for best performance.

Please post schematic!




This strongly supports R.G. Keen's hypothesis of latent motorboating. It also means that your power supply filter caps have critically low capacitance.


It is probably a combination of several effects.

Interesting comment on the power supply caps Helmholz, I just checked those early today in the only way I know how, that is to parallel a good big 450v cap of at least the same value, and see if the wide open volume hum changes at all. So far they all passed that test, but I realize this test is superficial and a bit flawed, as it misses another test and something I only know a little about, "ESR" entirely.

I agree that it's probably of few things, one being that I was pushing a 1958 Zenith Hi-Fi amp I am making into a guitar amp beyond what was intended gain wise, the other is my lack of any coherent respect for the prior stage resonances. I know nothing about this, but I read it in a search for similar problems with too large Cathode bypass caps.

By the way, the oscillation is still there, but much, much lower. Now I have to put my guitar pickup practically on the amp to get it to oscillate. But since I removed the 470uf cathode bypass cap I installed, the funky swirly sound on power chord decay is gone, as in 100% gone, and that is good. The oscillation I can invoke doesn't seem to effect the tone or response one bit. But then still, it was much worse with the big cathode bypass cap in place, so there is that.

Here's the schematic, and I think you've seen it from my other post. Sorry, wasn't trying to keep it a secret, I just sometimes limit details until someone asks, but from now on I will try to post all my schamtics up front to avoid the mystery. You guys can read deeply into these things, me, not at all at this point.

Thank you for your help, and I have already changed the Volume control negative feedback as per your suggestion, and the distorted amp tone sounds even better than before.

[Helmholtz]

Oh, this schem looks familiar (from your other thread). Thought this was a different amp.

[Enzo]

Just my opinion...

I think smaller amps often limit bottom end because it makes more demand of the power supply and speaker. A 3" speaker in a table radio is never going to make a big bottom end, so why task the amp with it? Same really with an 8" Champ speaker. It can still be PLENTY loud without the big bottom. We try to drag more bottom out of the amp, and we bang up against the power supply limits.

[Helmholtz]

that is to parallel a good big 450v cap of at least the same value, and see if the wide open volume hum changes at all.

Paralleling a good cap is a good test, but open volume hum at idle is no good indicator for sufficient capacitance. While higher capacitance should reduce ripple (120Hz), the hum you hear is most probably not (only) ripple.

Rather I would test if the paralleling stops the oscillation at high output.

[Mick Bailey]

I wonder if the sound is a beat frequency caused by PSU ripple (due to increased current draw) and imperfect cancellation in the OT.

[Helmholtz]

I wonder if the sound is a beat frequency caused by PSU ripple (due to increased current draw) and imperfect cancellation in the OT.

As for very low frequency beating the two frequencies involved can only differ by a few Hertz, this effect would only occur at special notes. (BTW, at power stage clipping the OT can't cancel ripple any more.)

I still tend to the motorboating at high output theory.