Tube issues, 12at7 affect KT88?

Check it out as I described. I'm interested to see if that caused your tube to run away. Someone recently gave me a dilapidated Dynaco ST70 that worked, though not well. The glass on the power tubes were separated from their bases but still functioned as the vacuum was maintained. Basically the tubes were held on by their wires. I replaced the power tubes and as soon as I powered the unit up, one tube glowed bright red and it's cathode biasing resistor blew out, actually splitting in half, thankfully protecting the output transformer. I haven't had the time to figure out what happened yet, whether it was a short, an open, a lousy tube socket or whatever. In fact, I plan on simply overhauling the amp. The power tubes checked out ok and are in another vintage amp. Personally, I suspect a bad socket connection for the affected tube. Reading your situation reminded me of mine. Good luck

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?