Help me identify an amp

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

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.

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.

Looks like an old Dynaco ST-70K kit.

Good testing 8417's are still available for $400-$500/quad, though I would advise picking up spares/matched quads in a timely manner as they are considerably more rare than good vintage EL34's (which also run around $400-$500+/quad).

DeKay

8417 are out of production and unavailable. Quicksilver used to offer an EL34 mod for their 8417 monos.

Man, this should be in the HI-FI hall of fame.  I would love to hear what it can do after you get it in working order.

Tubes for Hifi makes the kit similar to these amps called the VTA ST-70. Also have a ST-120 version. All their stuff are modifications to the original Dynaco gear. I’m sure they could help you source any parts you need to replace and also help diagnose any issues you are having.

@dungen 

I bet it will sound great.  People are still making mods for the ST 70.

let us know.

+1 variac - to bring up power slowly/safely to see if working, especially for tube type amps.  

connect some speakers, any load, when you power up tube amps

+1 on the variac and nice and slow you don't want to fry the amp especially transformers which are the best part.

Carlsbad2 - Absolutely Not!!! Never ever do that with vintage tube gear unless you want to risk replacing an expensive and hard to find transformer.

Bring up the voltage slowly on a variac is the right thing to do.

so it may be a GSI modified Dyna Stereo 70 that uses 8417 output tubes

Mystery solved: the PCB says GSI on it; however there appear to be different versions, so I will keep digging around. It’s likely my dad saved the documentation somewhere.

I will look up Will Vincent. Thanks for the recommendation. 

Jerry, thanks for giving me a chuckle. Everything is shoved in different parts of the house ever since Hurricane Sandy. I’ll get around to hooking it up and testing it out one of these visits!

Thanks again to the community  

I'd start by plugging it in and seeing if it makes music.  If it doesn't, the smoke will help you identify the parts that need replacement.

Jerry

The best person to ask would be Will Vincent. Search the name on USAM to see some of his modified amps and to get his contact info.

While I’m at it, I may as well ask my next question: what is it gonna need to get up and running again? It certainly hasn’t been used since 2012, probably even longer.  I know electrolytics fail with time. Given the usefulness (not to mention sentimental value!) of the gear, I will definitely want to take it to an amp tech for restoration. What should I anticipate needing to do?

The ST70 was sold as a kit and also fully assembled.  The tape labels suggest DIY.

The PCB is not a Dyna but may offer a clue.

Fantastic, thanks to everyone for helping! He often mentioned Dyna to me (we were also both interested in Sunn bass amps, which are based on a Dynakit design). Is the ST70 something that was sold as a kit? He may have assembled this himself if so. I will try to open up the chassis later and see if the PCB has any marks on it. 

Wow. Now that is one old amplifier. 

The chassis, layout, transformers are from a Dyna ST70 but one that has been significantly modified in many way.  I suggest a search for Dyna mods with 8417s to see if you can find a match.

Dynaco ST70 that's been modded.

It could be that it was made by an individual on a blank chassis. There are a lot of amps like that around.