Tube issues, 12at7 affect KT88?

Heyraz...Check out the coupling caps in the ST-70. They do go bad. Also if it's really old and the bias supply uses the selenium rectifier, replace it. Those rectifiers leak voltage and the bias supply suffers. Caps in all power supplies as well..

Hifigeek1-
Thanks. My ST-70 was kit built by someone else with no pride in his workmanship. I mean unbelievably messy in there. I plan to pretty much rip it back to the transformers and replace everything neatly. The only decisions I haven't made yet are whether to use the 7199's (they're in great shape) on a new drive board, or to replace the drive board altogether with a mod board. I also haven't decided if I want to use a cap board or replace the can. That project is on the back burner for a little while. Thanks for the advice.

Heyraz. May I suggest you replace the board in it and there by replacing the 7199 as well. Both the 7199 and the 6AN8 used in the MKIII are not very reliable. That tube is a medium mu triode/sharp cut off pentode. Although a good idea at the time of the design, it's hardly enough tube for the job. In fact after about 6 months or a years worth of wear on the tube and one half of the output waveform will prematurely clip. You will never get full output and distortion products will go up from the premature clipping. Also, hum is a distinct possibility and the culprit is that 7199/6AN8! Since the amp was designed around the RCA version of that tube, it's the only one that seems to work the best and those are very hard to come by. There are a number of people making upgrade circuit boards and power supplies. I would however keep the GZ-34/5AR4 tube and not replace that with a solid state device. Also, be careful with the circuit board upgrades as you can run the risk of overheating the power transformer attempting to power too many tubes.

The Amp is stuffed away in the box so I'm really not sure what version it is, but if I'm reading into your message correctly, the DynaKIT is based on the DynaCO MKIII. Is that correct? I never realized there was a difference until now. The 7199's say Dynaco Made in USA. Also, thanks for the advice regarding overheating the of power transformer, that's the first time I've ever heard that.
What driver board would you recommend?

That's a good question. I haven't really looked into them. Find out what your options are and let me know. Dynakit's were available for most Dynaco products including the FM3 tuner. If you use more tubes on a new mother board, depending on the tubes used to replace the 7199/6AN8, the filaments of those tubes will draw more heater current. Hopefully it's close to what the 7199 drew. Search the internet for circuit board updates. I believe Joe Curcio makes one. He designed some of the circuits for Sonic Frontiers.