You certainly need to get an earth ground hookup in these old two wire amps, but you don’t have to ruin the resale value of the amp to do it.

Here is how I do it, I take a small plastic project box and stick a 2 prong receptacle on one end, a three prong (IEC) receptacle on the other end and I pull out the earth ground wire and solder it to the PT chassis ground.

Here is a dumb diagram :

Doing this means you can leave the old plug in place, assuming it is not frayed and dangerous. You still get the benefits of 3 Prong safety, assuming you do a good job of soldering the earth ground (the half round part of the 3 prong plug) to the chassis. I did this at the ground wire of the PT. Don’t go alligator clipping this thing on or anything stupid like that. If it gets loose, you have a good chance of blowing something.

Now you can use a standard 3 prong computer power cable, and just plug the two prong plug into the other end.

This and removing the death cap will make your amp much safer, but it does have a drawback. You will now discover how well your amp is grounded. My GA-5 Crestline is pretty well star grounded but I still get some popping when I touch the strings.