JTM Bluesbreaker RI V1 Parasitic oscillation

[ziggy007]

Gday all,
Brains trust time,
Stumped with this one to the point it has to be something obvious I'm missing.

Amp was in for blowing fuses, fixed that, but noticed when testing that the Bright channel has a parasitic osillation all thru the waveform out of the vol pot, "caused' by the 100pf bright cap.
1khz 200mv test signal, is as clean as, on Ch2 at same point with same cap fitted.
Clean signal out of the valve plate thru the coupling cap.
Not the valve, even swapped the Grid/plate inputs to v1, so not the input network. Ran a shielded wire with 38kR direct to valve pin to no avail.
Not the tremolo network as far as i can see. (I disconnected input to V1)
Not the plate resistors.
Not the following stage.
I've fiddled with lead dress as much as I can without ripping it all apart.
Bottom line is that the amp works, although the out put is not as "nice" as I'd like it to be, pointing towards that issue.
Out of shear frustration i thought I'd ask here if any one had had similar issues.
Ive worked on tons of these circuits and never had an oscillation like this, at this stage of the amp.
cheers
Gav

[g1]

Is it a ceramic cap? They can be microphonic so if you haven't tried replacing it, it's worth a shot. Or swap with the one for Ch.2.

[ziggy007]

Thx mate.yeah tried that as well. Even tried a 1000pf film cap. Same. Tried diff test freq as well.

[Helmholtz]

Please post scope pics (with scope settings) of the oscillation.

[ziggy007]

Here's a few Phone photos.
(And I'd also replaced the pot prev as well)

72: Output ch1
73: output ch2
73: Output ch1, Yellow is input to Pot.
74: Output Ch1, yellow is output of pot Wiper (Ch1)
75: Output ch2, yellow is output of pot wiper (Ch2)

[Helmholtz]

The dirt looks like random HF noise but could be traces of HF oscillation as well.
HF oscillation typically is a nice sine but at low level on a digitizing scope I can't be sure.
Also the noise pattern looks repetitive.

In any case the bright cap would strongly increase any HF content at lower vol settings, so what you saw doesn't mean much.
Not sure did you try a different input tube?

The JTM45 power amp likes to oscillate between 50kHz and 150kHz at low presence settings.
There is a possibilty that power amp oscillation somehow couples to the input tube.
To exclude such effect either turn presence fully up or pull the PI tube.

You mentioned the output not being "nice". What does that mean? Scope pics?

[pdf64]

I wonder if lifting C6 the mixer resistor bypass cap, would help? That looks to be the main difference between the channels.
Also maybe try adding grid stoppers to the output valve control grids.

I agree that it would be a good idea to eliminate it just being noise. Removing the valve in V2 break any preamp positive feedback loops.

[ziggy007]

Thx all.
Yes , had tried new valve, isolating that stage completely, lifting C6, the output valves have 5.6k grid stoppers, and I tried some snubber caps across the plate resistor and from plate to grid/Gnd. Still same.
after consultation with owner, he is happy to take it. I’m happy for it to go! Haha.
thx again for suggestions.
cheers Gav