#1
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.
#2
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)