Diagnosing oscillation in Engl Savage 50

[Choan]

Dealing with some brutal parasitic osciallation in this English Savage 50.

It occurs on clean and distorted in high gain, and distorted low gain when everything is cranked. A brutal frequency!
Oscillation seems to be happening on the preamp board. The only thing that makes it go away on the scope is pulling v2. It ceases to exist on all tubes and speaker output. I’ve measured all resistors in v1 and v2, measured electrolytics with ESR meter. Replaced a shorted cap, and there are a few others showing low resistance. Voltages on pre and power tubes seem to be fine, oscillation throws the power tubes into red plating. Hovering my hand  v2 makes it disappear! I made a faraday cage with aluminum tape since the chassis didn’t have any. Installed a 47k grid stopper on v1 for kicks, nothing changed.

Aside from replace the low resistance caps… where should I be looking? The pf value caps? I’m dying on thing thing! Total pain in the arse as there are no markings on pcb for components.

Schematic is here

https://el34world.com/charts/Schematics/files/Engl/Engl_screamer_50_sch.pdf

[trobbins]

How many tubes did you try out in V2 position, given there are 4x 12AX7 in that amp?

[Choan]

The client had tried a new pair, my boss tried a new set, and I brought a known good 12ax7 in and tried that too. Chop sticking is revealing nothing physically altering the feedback, no obviously microphonic tubes/caps. We tried new power tubes, as well.

[trobbins]

Can you confirm each of the control relay contacts work?


What is the oscillation frequency?  Does it change in frequency/magnitude with tone pot changes, or lead or clean input gain pot levels, or reverb tank disconnected?

[Choan]

Control relays haven’t been tested with a meter, but audible clicking and channel/mode switching seems normal. From what I can recall the tone stays the same but volume of osc is effected by the volume and gain. Switching to high gain lead I believe shifts it slight up in pitch but I’d need to confirm. Interestingly the oscillation gets quieter in volume on high gain lead, much louder on clean/lead high gain.

[trobbins]

Perhaps probe signal on each side of a relay contact being exercised.


Try and identify where the signal originates, or exclude where it doesn't originate.

[Latole]

I will check for leaking capacitors from the input to V2
I admit it's just a hunch