[audin]

So when restoring a transformer radio, is it a good idea or bad idea to add a three prong power cord and tie ground to the chassis?

[codefox1]

I vote yes. You can lose the "Death capacitor" which can afflict appropriate pain if it fails. On the other side (the dark side,) you can lose all protection if somebody uses one of the adaptors to defeat the grounding, or if the wall outlet is wired incorrectly. I would add a fuse as well for good measure. Transformer or not, results can be shocking!

[Ron Ramirez]

It's not a bad idea, as long as your wiring is correct and up to code. Of course the cord will not look authentic, but if you're looking to have an easy ground for your transformer-operated radio, this would do the trick (again, assuming your house wiring is up to code).

Do not add a three wire grounded AC cord to a transformerless(AC-DC) set. Even if the B- is floating (not connected directly to the chassis), don't do it.

[AvoJim]

Here is some cloth covered three conductor wire which might help with the look.

http://www.sundialwire.com/3-conductor-1.aspx
Jim