Tube issues, 12at7 affect KT88?

Was the power tube's socket "stretched out" and not making good contact with all of the tube's pins? That's the first thing I would have checked. You should rule out the simple stuff first.

I'd pick up a decent tube tester from ebay. The price you'll pay will be about the price of ONE good tube, so it will pay for itself as soon as it catches the first bad tube. Plus, it helps take the guesswork out of the equation. I'm sorry, I don't have any answers for you.
I had a bad tube or socket that lit up a power tube and split a resistor when I rolled tubes. That's when I decided I needed more test equipment to understand what was going on. In my opinion, the only downside to owning a tube tester is how big they are. But I own every other kind of test equipment, so I figured if I was going to mess with tubes, I needed to tool up. The plus side of owning your own tube tester is how many new friends you will make. It's like owning a truck or a station wagon! Good luck.

If you have a DMM, you might find a short or open on your suspect tube by comparing it to a known good tube. Look through the tube, see where the wires are going and check those pins for opens or shorts. I learned that trick from a friend last year. It won't tell you the tube's quality, but it will identify an open or short. You can use this method while you're looking for a tube tester.

DMM stands for Digital Multi Meter. Before the display's on the meters went digital, they were called VOMs (volt ohm meters).
They are relatively inexpensive starting around $12.95 at Sears on sale. They measure volage, current, resistance and continuity.
What I was getting at by suggesting you test yout suspect tube with a DMM would be similar to testing a light bulb, the filament is either connected (some resistance), open (no continuity) or shorted ( no resistance). Other uses include setting an amp's bias and testing batteries.

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

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.

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?

Curcio Audio Engineering has too many (well 3) options to go into on this thread. Basically, it's stick with the 7199's if you are restoring, an all triode model fully direct coupled, or all triode CCDA. Non of which makes any sense to me at this juncture in time. I assumed the other versions came about because the 7199's went out of production and became scarce and drove up the price ( I know an adapter is available). Some threads I've read prefer that ol' Dynaco sound, others, well you get the picture. I planned on researching and learning this craft a bit more before I began the project (I'm more of a solid state guy right now).
If it's not to much trouble, could you explain the differences mentioned above or do you know of a site with that info? I've been torn between restoring or rebuilding, but if it's a MKIII rather than the original 70, I'd probably lean more towards a rebuild using state of the art components for a more controlled sound.
Thanks

I agree with you about that line, which is another reason why I haven't bothered with the ST70 yet. No rush, I love the sound of my Aragon 2004, and only want to get a tube amp running well enough to see what all the fuss is about.