Parasitic oscillation problem untraceable

[JMPGuitars]

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.

[Bieworm]

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?

[TriodeLuvr]

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

[Bieworm]

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?

[JMPGuitars]

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.

[TriodeLuvr]

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

That wasn't it, and I'm not sure I should remind you. Seriously, it's the issue of bias changing with power output in self-biased AB1 stages. The more current flows through the tubes, the higher the voltage across the cathode RC becomes. I'm surprised this isn't better known in the guitar amp world, but maybe the audible effect is mostly hidden because the tubes are already in overdrive when it happens. Anyway, this is why I suggested that he measure the cathode voltage when he's playing through the amp. If there's a significant increase, he might need to lower the R (hotter bias) or substantially increase the C, as you suggested.

Bieworm, my other suggestion is that you copy the PI from the JCM800. I don't have time at the moment to run a sim on the configuration you posted, but it's not correct. Just copy the Marshall design. No reason to use a cathodyne or anything else. This could very well be the problem.

Jack

[JMPGuitars]

If it were in front of me, I would correct the LTP PI, raise the power tube cathode capacitor substantially, and possibly add a snubber if needed.

Bieworm, Make your PI match this schematic but don't mismatch the plate resistors: https://mhuss.com/Plexi6V6/Plexi6V6.gif

[TriodeLuvr]

Bieworm, Make your PI match this schematic but don't mismatch the plate resistors: https://mhuss.com/Plexi6V6/Plexi6V6.gif

I'm wondering why you suggested this.

Jack

[crgfrench]

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?

If your issue is too much gain, you shouldn't switch to a cathodyne type.
The typical cathodyne setup provides twice the gain of an LTP.

[crgfrench]

I'm wondering why you suggested this.

Some people believe that a slightly unbalanced PI (by not compensating one of the load resistors) provides a sweeter sound.

[crgfrench]

Sorry for the late reply here, since you alleviated the problem by changing from para to LTP already, but if you decide to try again with the paraphase one thing I would do is separate the Ks on U5a and U5b: Change R71 from 820R to 1k5 (keeping it on the u5a K) and add a new "R71b" also 1k5 on the U5b K. The shared K bias resistor induces positive feedback which can lead to instability. Maybe separating them would have helped.

[Bieworm]

Sorry for the late reply here, since you alleviated the problem by changing from para to LTP already, but if you decide to try again with the paraphase one thing I would do is separate the Ks on U5a and U5b: Change R71 from 820R to 1k5 (keeping it on the u5a K) and add a new "R71b" also 1k5 on the U5b K. The shared K bias resistor induces positive feedback which can lead to instability. Maybe separating them would have helped.

Does that mean that with the LPT, which has a shared K resistor too, can lead to positive feedback too?
I don't remember what schem I followed for the LPT, but there is an 820R cathode resistor on mine. Maybe the 470R is better for a hotter bias. Foolishly I added an extra tail resistor of 1.5K in series with the 10k.. can't remember why, but I probably put it there to add a NFB loop in case I wanted to...
Here's what actually is in the amp now. Notice the 1k2 screen resistors on the 6v6, the extra 1000pf 3kV snubber cap and on the PI grids there are 470k grid stoppers ( coming from 0 to 1k5 to 10k to 470k)

16097452412112507165341271126422.jpg

[Bieworm]

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?

If your issue is too much gain, you shouldn't switch to a cathodyne type.
The typical cathodyne setup provides twice the gain of an LTP.

I thought the cathodyne provided the least gain of the tree PI types?
In ascending order:
1 cathodyne
2 LPT
3 paraphase

Or am I wrong?

[Bieworm]

Bieworm, Make your PI match this schematic but don't mismatch the plate resistors: https://mhuss.com/Plexi6V6/Plexi6V6.gif

I'm wondering why you suggested this.

Jack

I tried the 82k + 100k already. But since there is such nasty fizz on the OD there is not much to notice about the extra sweetness... put it back to 100+100k

[Bieworm]

If it were in front of me, I would correct the LTP PI, raise the power tube cathode capacitor substantially, and possibly add a snubber if needed.

Bieworm, Make your PI match this schematic but don't mismatch the plate resistors: https://mhuss.com/Plexi6V6/Plexi6V6.gif

I will try the huss plexi power section you suggested. I intended to do that in the first place, but eventually didn't for some reason.
I only have a 1000uf and a 4700uf 63V electrolytic cap .. should I try the latter?

About Jack's suggestion on lowering the 6V6 cathode resistor and fire up the bias to 100% ...makes sense in a way to avoid blocking distortion or is it just going to make things worse? This amp is so damn loud already. I even added a 1/4 power switch to be able to play it at home

[TriodeLuvr]

I thought the cathodyne provided the least gain of the tree PI types?
In ascending order:
1 cathodyne
2 LPT
3 paraphase

The cathodyne is the least only if we don't consider the other half of the tube. If we think about the 12AX7 driver as a whole, then the cathodyne PI will have one half of the tube with a gain of roughly 50 driving the other half (the actual cathodyne stage) with a gain of about unity. The LTP exhibits gain of about half that amount or a little more.

Jack

[TriodeLuvr]

I only have a 1000uf and a 4700uf 63V electrolytic cap .. should I try the latter?

Those are BIG caps for this purpose. What you gain with larger caps is a longer time constant. If this is a bias problem, that means a sustained overdrive will take longer to create the fizz. However, as the signal falls away, the unwanted distortion will stay with you longer because bias will take longer to normalize.

About Jack's suggestion on lowering the 6V6 cathode resistor and fire up the bias to 100% ...makes sense in a way to avoid blocking distortion or is it just going to make things worse? This amp is so damn loud already. I even added a 1/4 power switch to be able to play it at home

Just so we're on the same page regarding terminology, blocking distortion is what happens when a too-large signal draws grid current on positive peaks, causing the grid of the power tubes to move negative. That can have a similar effect, but it's a totally different cause, and the smallish coupling caps that are typical in guitar amps tend to minimize it as a problem.

What I was referring to earlier is cathode bias shift, and it's specific to self-biased Class AB1 amplifiers. At idle, the DC voltage on the cathodes is determined by the current through the tubes and the value of the cathode resistor. However, when the tubes are driven with a signal, they draw more current, because this is AB1 operation. That additional current creates more positive DC voltage at the cathodes, pushing the tubes in the direction of cutoff (the grids become more negative, relative to the cathodes). Depending on the specific combination of resistor value, signal level, RC time constant and signal duration, the result can be significant crossover distortion. The tubes will operate closer to Class B than AB1, and they will continue to do so until average current falls and the cathode capacitor discharges.

I want to stress that this might or might not have anything to do with the problem you're having. It's just one of the things to nail down, given the nature of what you're experiencing. This is why I suggested monitoring the DC voltage at the cathodes while you listen to the problem. If you have a DVM, clip lead it to the cathodes and ground, and watch what happens when you smack the amp. You might find that certain cathode voltage levels correspond to what you're hearing.

Not sure how practical this might be for you, but another approach would be to temporarily ground the cathodes and apply fixed grid bias. In a pinch, you could use three 9V batteries in series and a 100K pot. The batteries will last many hours in that configuration, much longer than you would need to do a listening test.

Jack

[JMPGuitars]

If it were in front of me, I would correct the LTP PI, raise the power tube cathode capacitor substantially, and possibly add a snubber if needed.

Bieworm, Make your PI match this schematic but don't mismatch the plate resistors: https://mhuss.com/Plexi6V6/Plexi6V6.gif

I will try the huss plexi power section you suggested. I intended to do that in the first place, but eventually didn't for some reason.
I only have a 1000uf and a 4700uf 63V electrolytic cap .. should I try the latter?

About Jack's suggestion on lowering the 6V6 cathode resistor and fire up the bias to 100% ...makes sense in a way to avoid blocking distortion or is it just going to make things worse? This amp is so damn loud already. I even added a 1/4 power switch to be able to play it at home

I would use the 1000µF cap. The higher one will make the amp way too stiff.

You're not that far off from Huss' PI. Your cathode resistor is too high, and that could be part of the problem. You might also need to raise the tail resistor. You can try 15k to 27k and see what happens.

BTW- the PI sweetness is what you currently have setup. 100K on both plates is what gives you the desired imbalance. 82K + 100K is an attempt at balancing (even though the values aren't perfect).

[JMPGuitars]

I'm wondering why you suggested this.

Some people believe that a slightly unbalanced PI (by not compensating one of the load resistors) provides a sweeter sound.

Yup. The (almost) perfectly balanced PI is more for hifi than guitar.

[Bieworm]

You're not that far off from Huss' PI. Your cathode resistor is too high, and that could be part of the problem. You might also need to raise the tail resistor. You can try 15k to 27k and see what happens.

You mean lower the cathode resistor from the PI, no? I'll go for the 470R then...
Do I need to swap the coupling caps too from .01 to .022? Is there a problem leaving the .1uf cap to ground as is?
The amp has plenty of bass, so maybe leaving the .01uf caps in isn't a bad idea? Unless I'm missing something...
And about the cathode resistor of the 6V6's.. leave it at 83% or check if the near 100% is better?