[morzh]

I received 7 tubes from an eBay guy with a perfect feedback who mostly sells tubes.
All are 12AT7. In my Mac 275 (a modern version, 2009 YOM) the tubes are weak, I bought it used, so they don't sound that good.
So. I get the tubes, all claimed to test close to NOS.


I test all 7 on my Hickok 600A.
5 do indeed test fine, 2 test weak, on the English scale - Replace, one is truly superweak, under a 1000 uMho (minimum 4000 for a new).
OK. So I put 3 of them in the amp, having forgotten to put the 4th one (there are 4 AT7). It plays fine for a few minutes, then I turn it off to put the 4th tube in, turn it on again, wait a bit, and hear a motorboat from the channel. Then one of the KT88 glows red. I turn it off, replace the tube with the 5th one (the last that tested goid). Turn on, everything fine. Play it for 2 minutes. Then - a really LOUD POP, and I see a red glowing KT88 in the other channel. Take all the AT7 tubes out, replace the original Mac labelled tubes back. Turn on....not working. The loud pop was blown fuse. Replace the fuse, then it works. With all original tubes, worked for an hour...no problem.





Now, don't know what to make of it. The seller suspects I am trying to stiff him, I guess, but....the tubes that are trying to destroy the amp test fine and I believe the guy did test them in an amp circuit as he said he did. Maybe the tube in Mac is stressed more? There are DC coupled curcuits there, if the bias goes to crap because of a tube, who knows what might happen. The driver tubes work as Cathoe followers driving the output tubes DC coupled.