Hi, A couple of questions. Can any of you give me an explanation of what the ground switch does on an amp and do I need have one? My 1969/1970 Marshall doesn’t have one but I see pics of other amps like mine that have three rocker switches on the front panel; is one of those the ground switch? Also my Fender Vibrosonic reverb doesn’t have one either but looking at the Fender specs it says it should have a 3 position ground switch. What’s the deal with a three position ground switch? I could go and google this but I want to hear it from the workshop tecs first for a ‘sound’ explanation.