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Setting tube bias

I have a TAD 60 tube amplifier and I alternately run EL-34 (both regular bias and single ended class A bias) and KT88 tubes. I tried to switch out the EL-34 to the KT 88 and I can't set the bias adjustment low enough to keep them from going very high very quickly. No matter how many turns I go to the left, the bias keeps going up. This has never happened before and I'm not sure what is going on.

I went back to the EL-34 tubes and had no problem setting the bias. This is just happening with the KT-88 tubes. They are the same tubes I've run before so it isn't a new tubes issue.

Any help would be great. Thanks.
gmsasso
rockadanny
1,037 posts
 
Bad tube(s)!
bdgregory
1,580 posts
 
what they said. No reason a TAD 60 won't bias KT-88s - I had one and switched between EL34s and KT88s now and then without problems, except for when I had a tube go bad.
atmasphere's avatar
atmasphere
7,068 posts
 
I would get the KT-88s tested. If they are OK, its possible that the bias supply in the amplifier is having problems.
gmsasso OP
31 posts
 
Thanks all. It would be strange for all 4 KT-88s to go bad at the same time. If the bias supply in the amp is having problems, wouldn't it be having problems also with the EL-34s. This is strange indeed. I'm gonna try to bias the KTs again and see if I have better luck.
gmsasso OP
31 posts
 
Bdgregory: You had a TAD 60 and ran KTs. Even when I could bias them at 550 each, when I tried to bias two of them lower to run single ended class A, I couldn't get them to go low enough. I'm thinking there is something going on with the TAD. Thanks.
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