2266c noise from reverb

[jab996]

So I have this annoying hum coming from the reverb.
The hum persists regardless if reverb is on or off.
The hum seems quietest with the reverb knob around the 1 o'clock position.
Turn the knob either direction, and the hum intensifies.
Nothing else seems to affect the noise.
Any fix it ideas ?
Sometimes I suspect it could be some kind of grounding issue.