Twin reverb boost oscillation

[drewl]

Agh these f-ing...
Anyway, Fender Twin reverb with the master volume boost is oscillating at V4b, when you turn the reverb pot up to 3 or more with or without reverb tank hooked up, with or without reverb footswitch turning reverb off probably from the sloppy wiring picking up the signal and amplifying it.

Osc. frequency varies with pull boost on.

I know I've fixed this before, any ideas?

[pdf64]

Likely to be a bad cathode bypass cap, shared between both section of V4.
Pete

[drewl]

That's what I thought but it's not the case.
I think that fixed the same problem before on a different amp but I tried it with no change.

[drewl]

It's gotta' be the two wires going to the reverb pot picking up a signal and amplifying it, I'll try a sheilded coax for it and see if it makes a difference, but there's got to be a a bad part somewhere that is causing this.
Hmmmm...

[drewl]

Well after much f-ing around, the only thing that killed this oscillation was splitting up the shared cathode circuit of V4.
Instead of a shared 820ohm/22uf each cathode got it's own 1.5k/22uf and BAM problem solved.
Maybe this will help someone else in the future.

[pdf64]

Sorry, I missed your previous posts; just out of interest, did you try a different tube in V4, as an odd few can exhibit various types and degrees of coupling between the 2 sections?
Pete

[drewl]

Yes, I tried a few different types of 12AX7.