Adding a grounded plug, need help

[Warped]

My friend has a old accordion amp that was recently used at a gig (for harp) and the guy got a bad electrical shock. he'd like to change it from a 2 prong plugin to a grounded 3 prong. What's involved to do this?
He sent me this in an email:
"That old accordion amp that I've got has no nameplate with model, serial # etc. on it. Andy did some searching and apparently Giuliette didn't make amps, just badged some. Glen Woods would work on it for me if I had a schematic. He thinks that a capacitor on the instrument ground jack could be the problem. Also he has tubes. Any idea how I could find a schematic? It stands about 3' tall, is massively heavy and has about a 14" speaker in it."

I've searched the net for a schematic, but this is an old amp and not likely that one is around. Isn't this a simple thing to do? Don't you just connect the ground to the chassis and you're good to go? Any help would be appreciated a lot.

[dai h.]

if the amp is the old kind of amp with no power transformer, you need an isolation transformer to use it safely with an instrument plugged in (and person touching instrument)--not just an added 3-prong power cord. If you don't understand anything about this, then it might be a good idea to take it to a professional. I certainly wouldn't want to die for an accordion amp.