Parasitic oscillation problem untraceable

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Bieworm
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Parasitic oscillation problem untraceable

Post by Bieworm »

I am effortlessly looking for a problem that sounds like parasitic oscillation. I just can't find it, whatever I do. I even changed the PI from paraphase to LPT, added gridstoppers of various values. Ofcourse the chopstick procedure, lead dress, snubber caps ,... no source found. It occurs on the lead channel at moderate volume and gets worse when overdrive is turned up more and more. But it's always there... on the clean channel it's there on serious volume into overdrive

The amp has no screen resistors on the 6V6's they're fed directly
20th_Imperial_SC200HB_pg1-1.pdf
by B+ node 3. Is it possible the oscillation is to be found there?
Another thing I noticed was the 5 ohms 10W resistor between B+ node 2 and 3 rattles against the chassis. Could that be the problem causing the annoying high frequency on the notes by vibration? Even with the amp on the bench, loose from the cab? I know I'm just guessing, but I'm running out of ideas and patience...
Just wanted to know if those things are related or if anybody has experienced this before...
Btw there are grid stoppers on every grid now.
20th_Imperial_SC200HB_pg2.pdf
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Post build photos. Post voltages. Post a sound sample of the issue you're experiencing.

Pull preamp tubes 1 at a time, and see if the problem persists. Does it still happen with the reverb tube(s) pulled?
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Oscillations can be difficult to diagnose. Do you have a scope or other test gear?
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

TriodeLuvr wrote:
Sat 01/02/21 12:32 pm
Oscillations can be difficult to diagnose. Do you have a scope or other test gear?
I bought one recently, but it's not accurate. Josh was right about the UTD equipment..
I'm bringing it back and get me the siglent 1102 cmv..
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Well, I was going to suggest a few circuit points to check, but I guess it will have to wait. You might find it by following Josh's advice, but oscillations are often the result of two or more stages interacting. Does it only do this when you apply signal? Is this a new build, or is it a new problem in an older amp?
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Re: Parasitic oscillation problem untraceable

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It's a new build. Tried various tubes in all spots . From 12ax7 to 12au7.. no change ever. All wires wide enough separated and all. Only does it with enough signal applied. Soft touch doesn't engage it. Harder strums or picks and it's there. Very frustrating...
But all in all I don't know if I really like the OD character, even without the high freq fizz on top. But I'd like to know if it's the power stage. Because I intend to build another preamp on it and keep the power section. I would hate it if I end up having the same problem when it turns out it's the power stage that's the culprit
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Sat 01/02/21 3:41 pm
It's a new build. Tried various tubes in all spots . From 12ax7 to 12au7.. no change ever. All wires wide enough separated and all. Only does it with enough signal applied. Soft touch doesn't engage it. Harder strums or picks and it's there. Very frustrating...
But all in all I don't know if I really like the OD character, even without the high freq fizz on top. But I'd like to know if it's the power stage. Because I intend to build another preamp on it and keep the power section. I would hate it if I end up having the same problem when it turns out it's the power stage that's the culprit
I could be wrong, but that doesn't sound like a typical PO to me. You should post some samples of the sound. IME a Parasitic Oscillation is usually either always there, or made obvious when adjusting the knobs. I've never experienced one that was entirely based on the source signal level...unless you mean turning up the volume of the amp? We need to hear it.

Wire separation alone isn't necessarily going to solve problems. The angles and physical position/locations of the wires are always important, and sometimes you need to move them into a different position. But it helps if you can narrow down the area with the issue.

I didn't mean swapping tubes, I meant remove 1 tube at a time, and leave that 1 tube out. Start with the reverb tube(s). The amp doesn't need the reverb to operate, so remove them completely, and see if the problem persists. Then try removing V1 and see what happens. Turn up the amp's volume and see if you hear anything.

If it is a PO in the power amp, you can try adding screen grid stopper resistors. And/Or raising the control grid stopper value that you have.

Thanks,
Josh
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

Thanks Josh for pointing these things out. I will look into those..
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Bieworm wrote:
Sat 01/02/21 3:41 pm
Only does it with enough signal applied. Soft touch doesn't engage it. Harder strums or picks and it's there.
Are you sure it isn't the speaker? Could the voice coil be rubbing at higher output power?
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

TriodeLuvr wrote:
Sat 01/02/21 11:08 pm
Bieworm wrote:
Sat 01/02/21 3:41 pm
Only does it with enough signal applied. Soft touch doesn't engage it. Harder strums or picks and it's there.
Are you sure it isn't the speaker? Could the voice coil be rubbing at higher output power?
Yes. I have a variety of speakers and tubes at hand to rule this out.
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

Here's a sound clip. Reverb tubes removed.. made no difference

https://www.dropbox.com/s/iaifnpishg09c ... 3.m4a?dl=0
16096764278276381473099472993888.jpg
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Sun 01/03/21 6:41 am
Here's a sound clip. Reverb tubes removed.. made no difference
That doesn't sound like a PO. It sounds like crossover distortion or blocking distortion.

I'm assuming your cathode resistor is 250Ω. In that case, you're biased too hot, which would cause that. 134% dissipation, pushing 16W on 12W max tubes. You should start by rebiasing to get your dissipation to 85% or less.

Thanks,
Josh
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

There is a 400R cathode resistor. So its 83%
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Do you have any screen resistors on the power tubes? If not, add 1K screen resistors. I don't know what your target voltages are, but see if raising the PI plate voltages helps at all.

You might also want to raise those 1.6K grid resistors. Try 5.6k, 8.2k, or a little higher depending on what you have available.
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Sun 01/03/21 9:43 am
Do you have any screen resistors on the power tubes? If not, add 1K screen resistors. I don't know what your target voltages are, but see if raising the PI plate voltages helps at all.

You might also want to raise those 1.6K grid resistors. Try 5.6k, 8.2k, or a little higher depending on what you have available.
I have put 1k2 screen resistors on the 6V6's.
The 6V6 grid stoppers are already 10k.
I have 470k grid stoppers on the PI.
Also replaced the CC 100k PI plate resistors with CF 100k, just in case...
I even tried moving the plates of the PI to node 3. No change in any of those attempts.

Is there any hail to be found in lowering the B+? I guess not.. only lower volume with the same problem me thinks...
Big chance it's power stage related because the clean channel has it too when cranked.. exactly the same sound.
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