The Marshall from Hell.

[Randall]

I have straightened out a few different issues with this Darn 2204. But I still have a perplexing oscillation. I have corrected some pot mis-wiring, replaced the jacks and lead dress. But it will still squeal or just go nearly silent with the gain pot cranked. It is a moog synth with the cable unpluged at the guitar end. I installed sheilded leads on V1 pin 2 and to the 68K for pin7. I swapped pre tubes. With a tone injected, it doesn't do it if my scope probe is touching eiher plate on V1 or the first grid of V2. PLaying guitar through it, it sounds raspy and bad on the Hi jack, the Lo jack sounds OK. I also installed a non existing 120pF cap across plate and cathode drawn on V1.

What could be causing this? I had a Marshall in a couple of years ago that sort of sounded like this, and it turned out to be the OT. But OT's don't cause oscillation do they?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[The Dude]

An OT shouldn' cause oscillation unless it's primary is wired backwards. Has the OT been replaced? At any rate, it sounds like your oscillation is in the early preamp stages and not the output. For clarification, you say touching your scope probe to V1 stops the oscillation. Is that V1a, V1b or either?

[Randall]

I don't think the OT has been changed or reversed, those wires are bundled like original. And sorry, I meant both plate resistors of V1.

[The Dude]

I would try temporarily clipping in a filter/decoupling cap on the V1 PS node. You might have a decoupling issue causing oscillation (bad cap).

[Randall]

No change with a 33uF cap clipped across first the V1 side and then the V2 side of that 50/50uF cap (that is under the board).

[Randall]

Sometimes a couple of glasses of wine and a midnight re-visit can do the trick. I found the 470pF cap coming off the preamp volume pot was actually a .0047uF, which is ten times larger. I'm not exactly sure why this fixed the oscillation and ripping sound, or if was a combination of everything I did, but it seems to be settled down and stable now. I CAN still drive it to pulse a little with my signal generator and cranking the gain all the way up, but with guitar it stopped doing it. I am so ready to be rid of this.

Ah, the joys of cleaning up after a hack. Thanks for the help as always.

[pdf64]

It’s worth checking, has R6, V1b’s 10k cathode resistor been bypassed, eg as a gain boosting mod?

[Jon Snell]

This is what generally helps with HF take off on the first stage;

[Randall]

"It’s worth checking, has R6, V1b’s 10k cathode resistor been bypassed, eg as a gain boosting mod?"

It has not.

"This is what generally helps with HF take off on the first stage"

Nice, I will try this.

edit: I did try this mod, and it does seem to help. the amp sounds and functions much better after all I've done. I don't understand how this mod works? I'm not grasping how bypassing a plate resistor tames HF?

[pdf64]

A small value cap across the plate resistor (or between plate and cathode) will roll off the stage’s gain as frequency increases. Also shifts the phase response, hopefully in a way that increases stability.

[Helmholtz]

I don't understand how this mod works? I'm not grasping how bypassing a plate resistor tames HF?

Remember that B+ is AC ground. So capacitively bypassing the plate resistor means an additional load whose impedance drops with increasing frequency.
This rolls-off HF in the plate signal.