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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 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.

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?

Oscillations can be difficult to diagnose. Do you have a scope or other test gear?

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..

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?

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

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

Thanks Josh for pointing these things out. I will look into those..

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?

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.

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

https://www.dropbox.com/s/iaifnpishg09c ... 3.m4a?dl=0

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

There is a 400R cathode resistor. So its 83%

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.

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.

Bieworm wrote: ↑

Sun 01/03/21 9:56 am

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.

Don't lower B+. I said to try and raise the plate voltage on the PI. That means lower the PI plate resistor values. Try 82K.

JMPGuitars wrote: ↑

Sun 01/03/21 12:24 pm

Bieworm wrote: ↑

Sun 01/03/21 9:56 am

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.

Don't lower B+. I said to try and raise the plate voltage on the PI. That means lower the PI plate resistor values. Try 82K.

Tried that already too, by clipping 2 470k resistors in parallel with the 100k's.
Did the same on every 100k preamp plate load resistor.. to no avail.
I also tried swapping the 820R cathode resistor with a 1.5k on the 1st preamp stage. I kinda liked that sound though. But still fizzzzz.
Also tried the removal of the 500pf bright cap on the OD channel. It took away the highs, but did not remove any fizz.
Should I be trying to tweak coupling caps?

Just a couple observations/questions... Is that phase inverter design being used by others without an issue? There's no tail resistor, and the two sections appear to be unbalanced. Also, if the outputs are biased too cold, you might be experiencing the exact problem I mentioned in a different post that got Josh fired up. At higher volumes, DC voltage across the bias network goes up, and if it goes high enough, the tubes enter Class B and you get crossover distortion. It would be worth checking the cathode voltage with a voltmeter while you're playing, in order to rule this out.

Jack

I changed this paraphase phase inverter into a LPT PI. It sounded immediately better with the LPT. Volume is more controlable.
The paraphase added too much gain.
Should I try cathodyne instead?

TriodeLuvr wrote: ↑

Sun 01/03/21 3:14 pm

Just a couple observations/questions... Is that phase inverter design being used by others without an issue? There's no tail resistor, and the two sections appear to be unbalanced. Also, if the outputs are biased too cold, you might be experiencing the exact problem I mentioned in a different post that got Josh fired up. At higher volumes, DC voltage across the bias network goes up, and if it goes high enough, the tubes enter Class B and you get crossover distortion. It would be worth checking the cathode voltage with a voltmeter while you're playing, in order to rule this out.

Jack

lol, if you're referring to the screen resistors, we were basically arguing two different points. I was right about that though.

I think the PI thing does apply here. His PI plate voltages seem low to me. He needs to correct that.

I'd probably also increase the power tube cathode cap value. A lot.

A snubber circuit on the output tubes might also help.