I’ve heard of people getting shocked by a vintage tube amplifier that was ungrounded and the 2-prong chord looked like it had seen better days.   So I deicided to add a new 3-prong grounded plug.    I used an existing ground location which was a bolt for the transformer.   You can see it in the lower left of this photo.

The only real difficulty was that the new wire diameter was slightly thicker than the old wire and the gromet that holds the wire in the case was almost impossible to press back in.    My solution was to take a hot soldering iron and melt the inside of the gromet to create a little larger opening and a bit more space for the wire.  It took a couple of attempts but eventually worked and clipped into place.