Please dont go cheap and quick with ac current! replace 2 prong plugs asap! especially with tube amps. Cheap and quick is never safe with electricity.
The last time I used a 2 prong plug on an amp, I nearly killed myself. I unplugged the cable at the end of a session with a friend, and dropped the cable to the floor like I had so many times before...
The cable hit a mic stand base, and created an arc that put a hole right through the plug on the end of the cable! Clean through! A hole 1/4" in diameter!
This means that I had the hot end of the plate potential (voltage) going through the tip of the plug. I was barefoot, drinking an iced coffee dripping with condensation, and touching metal equipment that was all turned on. I honestly have no idea how I did not electrocute myself.
Even if you try it, and feel a slight tingle right after plugging in something; with time that unreleased energy can build up to stupidly high levels. It just sits there waiting to find a way to ground to be discharged.
That amp I wrote of above was a 5-8w Silvertone. A 50 watt Marshall has voltages 4-5 times higher on the plates.
With Mark's power supply question, we were writing of 9v dc with 200-300mA. With that amp, were talking 400-500 VOLTS AC internal voltage and in the power section theres a matter of 10AMPS of current. Be careful.
There are many reasons why guitar players are the only people stupid enough to keep using thermionic technology (I say lovingly.) Efficiency and safety are the big two reasons why everybody else was in a rush to move on.
