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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
hifihvn
943 posts
 

05-18-11: Gmsasso
Thanks everyone. I tried the KTs again, one at a time, and it is still a problem. I'm taking the amp in for a check up.
Gmsasso (Threads | Answers | This Thread)

Sounds like you have a local shop to make it easy. If something is wrong with the bias, it might keep getting worse even with the EL34 tubes in it. You could be in the other room if the tubes started to overheat and cause damage to the transformers, or something else. It could get expensive, plus the parts might not be available anymore. Please let us know what was wrong. Thanks.
hifihvn
943 posts
 
I re-read (after more coffee) and caught the one at a time. That does sound like a bias problem.
atmasphere's avatar
atmasphere
7,068 posts
 
Hifihvn, just for the record, yes, if a tube had a short then it could mess with the other tubes, depending on the circuit of the amp. Not in this case though- something is wrong with the amp.
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