Bassman 5F6A / Concert AB763 amp oscillation

[chocopower]

Hi,

I´m working in Fender Concert. The owner only use the Normal channel and ask me to mod the Vibrato channel to a 5F6A bassman preamp.

Work is donne, and bassman channel work well except when i´m using the Bright channel and dial at max the unnused Normal channel volumen.
The oscilation is not progesive. It happen only just in the last point of the pot, signal decrese a lot and oscilation start. Suddenly bias current rise from 33mA to 60mA.

posible cullprits:
I´m not using shielded wire at any point.
47pf PI snubing cap not installed.

Any other idea?

as soon as i can, i´ll post a schematic and layout.

edited with layout:

[darkbluemurder]

Hi David,

The lead dress is the culprit. The input wires from the Bassman channel go underneath the tone stack of that channel. Since the tone stack signal was amplified two times by inverting gain stages the tone stack signal has the same polarity than the input signal which causes the oscillation. The best solution would be to move the tone stack components into the empty area on the board (additional eyelets will be required). Not only would this separate the tone stack signal from the input wires, it would also allow you to shorten the wires between the tone stack and the pots. If this is not feasible for you then the next best solution would be to use shielded wire for the inputs and mount the 33k grid stopper resistors directly to pins 2 and 7 of V2.

Cheers Stephan