Hello:
I'll take a shot at it. I converted my Supro amps to grounded plugs.
1. In the old days of two prong plugs the polarity switch allowed you to make sure that the (hum reducing) cap connected to the chassis and the power cord was on the neutral. This was done two ways. The switch either changed where the cap was connected or the cap connection was fixed and the switch reversed the power cord leads.
2. When changing to a grounded chassis this cap must not be connected. It will put ac voltage potential into the ground which can shock you at best and make the PA, other instrument amps, hum or take them down with fried ground traces on their pc boards. I have seen it happen and the only gear still running was the tube amps.
3. I found, however, that I could not count on club owners hiring qualified electricians to wire the stage plugs so.......I installed a three position polarity swith (center off) so when a ground was not present (amp humming uncontrollably) I could then switch the polarity cap in circuit. My situation was a little different in that I was running two amps in stereo so I also included a ground lift (second 3 pos switch in the power hole) so I could float one amp and cure ground loops (especially when using a DI).
Don T.
