On the early amps, the ones with a varistor in them (pop riveted to the case) the case must be grounded. The later ones W/O varistor still had the label, but in fact did not not need to be grounded. The saftey over voltage function was contained in the newer out put transistor,,,,the one you can not get anymore and therefore caused the end of production.
It will be a bit tougher to get it going on 6V.
12V will work with 2-6v coils in series like late commando,