Fender Princeton Vibrato Pulsing Noise

[misterc57]

I have a 1974-ish SF Princeton (Non Reverb). The vibrato makes a pulsing noise at idle, adjustable by the vibrato knobs, even with the amp volume set at zero and no guitar plugged in.

Schematic attached. Any suggestions greatly appreciated.

Thank you. MarkO

Fender-Princeton-AA964-Schematic.pdf

[Helmholtz]

Are your power tubes well balanced?

[misterc57]

Are your power tubes well balanced?

Thank you Helmholtz. That must have been the problem, I swapped output tubes and problem gone.

MarkO

[Helmholtz]

The LF tremolo signal is applied to both power tube grids. It modulates the bias and thus the gain of the tubes (as gain varies with bias).
The tremolo signal itself does not appear at the OT secondary - but only if both tubes have the same gain.