A lot of vintage amps have a two-pronged power cord. Here is my position on this: If you bring in an amp to me with a two-prong cord I am going to insist that you have me install a properly grounded three prong cord. Here’s why: In the case of a two-prong AC cord, in the advent that the 120V AC wire gets loose inside the amp and rests against the metal chassis, it will make the chassis of the amp possibly lethal (and at the very least increbibly painful and injurious) to touch. With a properly wired three-prong cord the above scenario (the “live” wire getting loose inside the amp and touching the chassis) would short the 120V AC wire to ground, and blow the amplifiers main fuse. Fuse blown, nobody hurt. That’s because the chassis (due to the new three prong cord) now has a direct connection to ground (earth) provided through the third prong. I take keeping my fellow musicians safe seriously!