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Tube issues, 12at7 affect KT88?

Hello. I have a McIntosh 275V. I just tube rolled my 12AT7's. Started listening, and one of my power tubes, a Penta Labs KT88 went bright cherry red. I shut it down quick, let it cool down and swapped back. Looks like it's back to normal. Letting it cool down again and I'll swap tubes again and see what happens.

If it goes cherry again, what would make the 12at7 affect the power tube like that?

Thanks in advance,

Tony
tonydec
heyraz
177 posts
 
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.
hifigeek1
770 posts
 
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?
tonydec OP
23 posts
 
Version V, newest version.

Heyraz, you are absolutly right. I am considering purchasing a tube tester.

HiFigeek, the weird thing is, only one tube runs away. I don't have the wiring schematic, but I'm guessing the first 2 AT's run the first 2 KT's, and the next 2 AT's run the next 2 KT's, with the 12AX's leading? That is what makes me think I have it narrowed down to 2 of the 4 AT's.

Tony
hifigeek1
770 posts
 
Tony what's the last tube in the chain before the KT-88s?
heyraz
177 posts
 
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.
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