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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
 
Atmasphere, if one tube has a short, couldn't it pull the minus bias voltage for the other three down with it? Also, possibly send positive voltage on the bias from one shorted tube? If this is the case, a tube tester may not show the defective tube either. A few years back, I had a new tube glow orange and show good on two testers, a Hickok and Eico 667. They exchanged it and put it back on the shelf. I ordered more, and got that same tube back. I returned it, they put it in an amp they had, and it glowed orange. The testers( theirs and mine) would not give enough voltage to show the defect.
hifihvn
943 posts
 
I guess another thing that would come into play, would be how much current the bias can supply in the amp. It is not something to play with since it can put to much of a load on the transformers too.
gmsasso OP
31 posts
 
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.
bdgregory
1,580 posts
 
have you sent an email to Paul describing this problem? Whenever I corresponded with him I received a reply with "probable cause" explanation from him within hours. I ended up sending min in to him once when it was blowing tubes. He fixed it and added some updates to it for a reasonable charge (less than <$100 as I recall including return shipping)
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.
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