Well, first off the 50L6 isn't a 50V 6L6 although you might be able to get a 6L6 to work with the right heater voltage. In fact only the original 12L6 is a direct 6L6 variant (and only the original 19W version) - all the other __L6s are lower power dissipation tubes (25, 35, 50) and these fall into two "families." And there should be a voltage dropping resistor in the series heater string to make up for the difference between the 97 volts of tube heaters and the 120 VAC power line.
But the real problem is that even if you add a heater tranny and change the tubes one side of the chassis is still connected to the power line and you can get the shit, or perhaps life, shocked out of you if the plug is inserted incorrectly.
What you actually need is an isolation transformer - a 1:1 ration tranny that will power the amp while isolating the chassis from direct power line connection. This is what you should order if you're ordering transformers - but you can also make one by using two identical "salvage" transformers hooking the "secondaries" to each other and driving one "primary" from the AC line and deriving an isolated 120 VAC from the other "primary" - one of the best uses for old monitor, etc. trannies.
These amps were cheaply made but shouldn't hum to an horribly objectionable level - but perhaps even now you've got the plug reversed and the chassis is hot to the surrounding house wiring - nice way to get some hum!
Hope this helps.