Oscillation in 70's Fender Quad Reverb

[telecaster]

Problem Solved! Thanks to everyone for your suggestions and input.
I spent a lot of time reworking the two long wires to the reverb control and finally settled on shielded wire for each, both grounded at the potentiometer end only. This improved the symptom, now the reverb control had to be turned almost all the way to 10 to start the feedback. I then replaced V4's common cathode bias cap and the feedback completely disappeared. Along the way I did retension the tube socket and reverified all the grounds with an ohmmeter.

Chuck H's photo is right on the money, these mid-70s Fenders with Master Volume are a tangled mass of wire behind the control pots.

[SmartAlex]

Cool! So, was this one of the Fender Amps that has a pull switch on the master volume? The pull switch connects some of the reverb circuit to make distortion. You can have problems when the switch gets sticky.

Problem Solved! Thanks to everyone for your suggestions and input.
I spent a lot of time reworking the two long wires to the reverb control and finally settled on shielded wire for each, both grounded at the potentiometer end only. This improved the symptom, now the reverb control had to be turned almost all the way to 10 to start the feedback. I then replaced V4's common cathode bias cap and the feedback completely disappeared. Along the way I did retension the tube socket and reverified all the grounds with an ohmmeter.

Chuck H's photo is right on the money, these mid-70s Fenders with Master Volume are a tangled mass of wire behind the control pots.

[Chuck H]

In fairness, replacing that bypass cap may have fixed the problem without the need to employ shielded leads. But I'm glad it's fixed.