Sine wave scope question

[Chuck H]

I have both variable FB and presence control. Transformers are at the opposite sides of the chassis so I'm running a shielded cable from the 16Ohm tap to the variable FB (it's close to the PI) and then to the PI).
Maybe I forgot to mention that I'm experimenting with power scaling using the "screen voltage method" by adjusting the screen voltage, bias and FB simultaneously as in Marshall AFD/YJM100. FB varies from 29 to 7k (22k pot). This effect is more pronounced at lower power levels at their max non clipping output. I also noticed that when I dial the presence a bit those edges disappear but then the output goes into clipping.

Well then... The issue is complicated. I'll admit that the info available is insufficient for me to draw a bead. If I had this on the bench,.. Maybe? The circumstances are dynamic. Watching the occurrence in real time would be my only hope. Other than to say that it is almost surely the peripheral circuits in the closed loop (their lead dress, shielding, phase shifts and/or grounding) that are causing the oscillation. Experimenting with lead dress, shielding and ground terminations for these circuits could stop theoscillation, but not necessarily. And it would be exhaustive. But this is where to look for solutions other than top end bleeders.

IHHO if you have it to where a small bleeder stabilizes the amp when you have such circuits in the closed feedback loop, you are already doing pretty well. Especially if you don't hear any detriment to the tone with the top end bleeders.

Do try a 100pf for the cross plate PI circuit as suggested. Increase the value only if the 100pf is ineffective at stopping the oscillation.

[GainFreak]

Do try a 100pf for the cross plate PI circuit as suggested. Increase the value only if the 100pf is ineffective at stopping the oscillation.

I tried 100pf and it had some effect on those "edges" but a 250pf cap seemed to cure the problem completely.

[TimmyP1955]

Are the grid stoppers on the sockets or on the board? If the latter, move them to the sockets.

If your computer is a laptop, see if running on battery has any effect. If it's a desktop, try disconnecting the ground of your scope probe.

[GainFreak]

The power tubes sockets are mounted on a PCB and the stoppers are right at the respective pins. If that was a power tubes issue the cap wouldn't affect that would it?
I'm running a desktop and my scope's ground is not isolated from the PC.

[Chuck H]

The power tubes sockets are mounted on a PCB and the stoppers are right at the respective pins. If that was a power tubes issue the cap wouldn't affect that would it?
I'm running a desktop and my scope's ground is not isolated from the PC.

The cap on the PI is within the negative feedback loop so it would affect everything in the loop. ie: With the cap in place top end from the PI no longer reaches the power tubes. So the power tubes no longer feed that signal back to the tail of the PI for re amplification to send to the power tubes. Round and round she goes. That isn't to say this is the root of your problem, just that it's plausible. Any distortions created within the loop appear as a new signal at the PI tail and are summarily re amplified. This is why I think lead dress and cross talk for your in loop circuits is probably the cause.

[GainFreak]

This time I'll go the easy way and leave that cap there. Thanks for all your comments and suggestions.