Help me identify an amp

Here’s a thread that contains photos of the original documentation. They would be worth downloading and saving should the amp need service:

https://www.diyaudio.com/community/threads/st-70-gsi-mod.324845/

It is probably in good working condition, but as others have said, it should be powered up slowly with a variac over a couple of days to allow the electrolytic capacitors to reform. If you’re not comfortable with this, you can probably find a local tech or even an experienced tube hobbyist who could do this and generally check the amp out. For further, more technical advice, Audiokarma is a good place to post that will allow you to communication more easily with someone who might be able to offer help.

Thanks again to everybody for all the advice. I will look into acquiring spare tubes and study how to power on safely with a variac. It's possible there's one among my father's things, as he was very fond of tube hi fi amps, as well as vintage guitar and bass amps. As for the schematics, I really appreciate that, but I read that there were multiple versions of the GSI mod, so I'll have to check to make sure that matches the circuit in the amp. It seems that Andy Fuchs, the guy behind GSI, is still building amps in New Jersey, so maybe I'll contact him with any problems.

Sadly I'm not at the old house that often, so I may not be able to hook up the amp or test it for a few months. I am looking forward to trying out the entire rig, though. Apart from the Dyna power amp, there's an Audio Research pre, a Linn turntable, and some homemade woofers that will be a pleasure to try out.

Sounds like you have a solid plan in mind.  Good luck!

Sorry for your loss.  But at least you have his gear as a memento.  I don't think my pops has anything to leave behind.  He was a car guy but got rid of everything some time ago.  Best of luck!

Oy showing my age.  Looks like an old Dynaco ST70.  Four tube

input circuit is a little different though.