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Issue with tube bias drift in my integrated amplifier!

Posted by dandaroy (A) on December 5, 2021 at 11:59:02

I have a Cayin tube integrated tube amp. Details of my system can be found at the link below. For power tubes, I have a quartet of matched Mullard reissue KT88s that I bought from VIVA tubes only 5 months back and has no more than 200-300 hrs. After the tubes settled in, I did not check bias for a while until recently when I started checking it every few days or a week or so. Every time I check, I notice appreciable bias drift. When I have noticed it, I have set the bias correctly. Today, I let the amp warm up for 30 min and checked bias and noticed that it was slightly outside the recommended range on the low side. I set the bias dead on and played music for 3 hrs and then checked bias. The drift was not outside the recommended range, but it has come down. The funny thing is all the 4 tubes show almost the same amount of drift (either high or low) every time I check it.

I apologize if some of my questions are elementary or stupid, but I have a fairly rudimentary knowledge of vaccuum tubes or how a tube amplifier works. I know nothing about what bias setting is doing:

1. Is this the tubes or could be it due to the amplifier? The reason I say this is because the drift is farly same on all tubes.

2. If it is the amp, can anything in the small signal tubes affect the power tube bias?

3. I have Herbie's tube dampers on all the tubes. Could that be contributing to this?

4. Even with my previous tube amp, I have noticed some bias drift more often than my liking. Is there anything in the power supplied to the amp that could be causing this?

Finally, if you can tell me how to troubleshoot this, I would really appreciate it. My amp has a easy bias system, but obviously not auto-bias. Is it possible to install an auto bias system?

P.S. Despite the bias drift, I do not notice any appreciable change in sound quality. Today, as the bias drifted from beginning to end of listening session, I was paying attention to this. The sound of my system is pretty good.

TIA.
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I. Dandaroy

• My system   (Open in New Window)

Edits: 12/06/21 12/07/21

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