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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
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.
hifihvn
943 posts
 
05-17-11: Gmsasso
Thanks all. It would be strange for all 4 KT-88s to go bad at the same time.

No one said all four have gone bad at once. You need to test all four to see if any of them are bad. In most cases, that's the easiest, and cheapest method. If all the tubes are good, then get the amp checked for a bias problem.
atmasphere's avatar
atmasphere
7,068 posts
 
I agree with Gmsasso- it *is* strange for all 4 tubes to be too 'hot' all at the same time. Really seems to point to the amp not having enough bias voltage in its bias supply.
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