Fender Bassman"Tremoloing"

[gary sanders]

I have a Fender Bassman AB165, ( modded to AA864 ),it sounded great for a while but its got some kind of oscillation going on.Sounds like a bias problem does and its like it has a tremolo effect but as you know bassmans dont have tremolo.All filter caps and diodes were replaced earlier,except for the bias supply cap.Could this capacitor cause this or would it be something else?

[Leo_Gnardo]

All filter caps and diodes were replaced earlier,except for the bias supply cap.Could this capacitor cause this or would it be something else?

Yes, for a part that costs what $2 maybe, change the bias filter cap. I ALWAYS change that one. Cheap insurance. Just be careful & don't do what I have sometimes done. Positive end of the bias filter goes to ground.

Unplanned "vibrato" could be motorboating, a circuit's reaction to insufficient filtering. If any of the filter caps are failing or disconnected, instability AKA motorboating can be expected.

[gary sanders]

Thanks man.I hope its the bias cap.All of the filter caps are new.

[M Fowler]

I hope that bias cap does the trick because to me what your describing is an effect that can occur bad filter caps. Well more of an out of tune sound.