Tube issues, 12at7 affect KT88?

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.

Got it, one of those I have.

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.