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Biasing the Jr. (Read 871 times)
Samsdad
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Posts: 9
Biasing the Jr.
04/30/17 at 18:30:37
 
The owners manual states that you should always bias any tube you install to 40mA/.825 VDC (and just above that directive, it states that the .825 figure is an ideal, and that you should try to obtain a reading of between .600 and .900 volts).

So I just put some Gold Lion KT66s in my new Jr. and with a 5V4 sitting in the rectifier spot I measured about 1.25 V with the bias trim turned all the way down. I replaced the OA3s with OD3s and the reading didn't change. Then I put in the big Russian 5U4 equivalent, and the measured bias voltage dropped to 1.05 volts. Finally, I put an RCA 5Y3 in and obtained a .840 millivolt measurement on the bias.

If I exceed the bias of .900 volts will I hurt the amp (e.g. be running the transformers too hot) or will I just be shortening the life of the output tubes? The latter is acceptable, the former not so much.

According to the online Weber Bias Calculator, a KT66 running in Class A at a 330 plate voltage can be biased up to 68.1mA. This appears to be ~ 90% max dissipation. The 1.25V that I was at initially with the 5V4 in place appears from a rough ratio calculation to have been a 60mA-ish bias (~ 80% dissipation according to the Rob Robinette bias calculator, which is supposedly considered an "average" bias point, rather than "cool" or "hot").

By the way, I never measured the bias after changing out the stock Valve Arts 274B (and new-production Svetlana EL34s) to the 5V4.

I'll get to giving a listening impression here in the next few weeks for those who are interested in a comparison between the Jr. to a Super Zen CKC that is also in my possession.

Thanks in advance.
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Samsdad
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Re: Biasing the Jr.
Reply #1 - 04/30/17 at 19:01:07
 
UPDATE: Well, answering my own question to an extent, having a 5v4 in the rectifier and running (at least this quad) of KT66s won't work as I just had to turn it off due to redplating.
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Archie
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Re: Biasing the Jr.
Reply #2 - 04/30/17 at 19:14:06
 
Redplating?  Did something get red-hot!?

Steve told me once that running higher bias on the ZMA gets the output transformers too hot and shortens their life.  He did not recommend it.  In fact, he reduced the original recommended bias for the ZMA from 60ma to 50ma.
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