Tube issues, 12at7 affect KT88?

Well whatever the problem with that tube is it's causing that KT-88 to run away. Usually, if the DC voltage operating points are upset, that will upset the balance of the amp and over drive the output. Or for some reason the bias on that KT-88 is being affected and your loosing negative bias voltage on that tube. I believe in the vintage 275 the 12BH7's are the driver tubes. Is this a new 275 or a vintage 275?

Tony what's the last tube in the chain before the KT-88s?

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..

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.

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.

Well you do have to draw the line somewhere. When I do a mod on someones preamp or amp I take a long hard look at what was the intent was of the designer of the circuit. In the case of Dynaco, there were always cost considerations. This was the stereo for people who couldn't afford McIntosh or Marantz, or wanted to build it themselves. Heath kits were also popular. Obviously with the ST-70 or MKIII I would never replace the transformer unless it was damaged. BTW on the MKIII's I had heard that the older gray painted transformers were better then the newer blacks ones but I was unable to prove if that was true or not. There are other sites with other circuit board replacements. I would check them all out. I have absolutely no problem with an all triode tube input and driver section. If it's something like a 12AU7 coupled to a 12BH7 that works just fine.