I have a '70's Silverface Bassman and there's this toggle switch on the back panel labeled "Ground." You push it in and you can adjust it to three different positions- left, right, or middle. I am completely lost as to what it does.
It does exactly what it says, it selects the right ground for the amp. If you've got the wrong one selected you should notice a hum coming from the amp, and you might even get a little zap if you touch something that is grounded while you're playing.
My Gear:
American Deluxe HSS Strat
Crate GT212
Vintage '66 Fender Champ Amp
Dunlop Crybaby 535Q
Jekyll & Hyde Ultimate Overdrive
Vintage '74 MXR Script Phase 90
Vintage '77 MXR Flanger
Orange Squeezer (clone)