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Can't bias tubes on amplifier

Hello all:
I've been here before regarding an issue I am having with a Tube Audio Design TAD-60 amplifier. It is designed to take EL-34, 6CA7, or KT-88 tubes. The problem I am having is that while I can easily set the bias for EL-34 tubes, the KT-88 tubes heat up way past 600 very quickly and no matter how far down I turn the bias adjustment screws, I can't get the tubes to stop overheating.

I've ordered new KT-88s thinking that it may have been the tubes, but they still overheat. The guy who built the amp, Paul Grzybek, has passed away. Any thoughts would be welcome, as well as someone who is good at fixing problems like this. Thanks in advance.
gmsasso

Showing 2 responses by gmsasso

gmsasso OP
33 posts
 
Thanks for the advice, Newbee and Hifigeek. I have been able to bias KT-88s in the past with this amp at he 500-550 mV settings, but have never been able to bias tubes 1 and 3 at 200 mV to achieve a single ended class A operation as Paul describes. Now I can't get any of the KT-88s to stay below 600 mV.

I can stay with the EL-34 tubes, which sound very good, and I can achieve the single ended bias with them. But one of the things I like about this amp is that you can use a number of different types of tubes and get a different sound.
gmsasso OP
33 posts
 
Haven't tried 6550's although they are approved for the amp. I'm reluctant to put more money into tubes if I am having the bias problem with KT's.
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