Filmosound - Guitar - Ground

[atmars]

First, let me say I have some experience modding amps, but my technical knowledge is limited, so forgive anything I say that seems ridiculous.

I have a Bell and Howell Filmosound amp that I am using for guitar. It is transformerless. I have acquired a isolation transformer for the sake of safety, but I have read that I should ground (attach chassis to third wire of mains plug) the chassis for an additional measure of safety. I gave this idea a try, but the result is a lot of hum. I removed the ground wire from the chassis and all is good again. Is there a way to ground the chassis that does not result in a world of hum?

Hers is a link to the schematic:

http://s294.photobucket.com/albums/mm109/atmars/?action=view&current=filmosoundL-2.jpg

Thanks!