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
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
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.
Hifigeek, looking at it from the side, back row is 4 KT88's.

Front row is 2 12AX, 2 12AT, 1 12AX, 2 12AT. I remember reading the first 12AX is only needed when run in stereo and not mono blocked. So I am assuming 2nd 12AX and 2 12 AT for first 2 KT's, 3rd 12AX and 3rd and 4th 12AT for last 2 KT's. But that's just a guess. It is the 2nd KT that gets hotter, which makes me think it's one of the first 2 12AT, probably the 2nd if my guess is correct.

Heyrazz, what is a DMM? Eyeballing the tubes, it appears everything looks connected.

Continued thanks,

Tony
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.