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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
nmmusicman
276 posts
 
Hey Tony, I have the same amp and have had the very same problem you have. The 4 12AT7's provide the bias to each KT88 output tube thereby if one 12AT7 goes bad, then the corresponding KT88 will glow cherry red. The Mac's are built rugged and there is usually no need to investigate further but to replace the bad 12AT7 and get rid of it or at least mark it so it won't be mistakenly reused. I would also replace the KT88 as the Genalex KT88 that went cherry red several months ago on me finally gave up the ghost in a bright display of arcing a few days ago upon power up which blew the mains fuse. Replaced the fuse and the KT88 and all is well.

Cheers
tonydec OP
23 posts
 
Thanks. I had the tubes tested, and the one that was in the number 2 spot tested poorly, which I think corresponds with the number 2 KT88. I did plan on replacing it because I figured it severly shortened it's life. I'm going to have them all tested just for curiosity to see the differences in numbers.

Tony
jedinite24's avatar
jedinite24
1,503 posts
 
Hi Tonydec

What was the outcome of your checks? Were the KT88s damaged by the faulty 12AT7 tube?

Thanks
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