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In Reply to: GE 6CA7 - Can't bias tube low enough posted by Capt. Z on April 12, 2007 at 04:56:47:
It appears one tube is simply too "hot" to fall within the bias adjustment range of the amp, that's all. There is likely nothing "wrong" with the tube, it just needs a bit more negative voltage on its grid to bring the bias current into spec.You have a couple choices - either make the changes needed to provide the required bias voltage or take that tube out. With those fat bottles being so rare, I'd look seriously at the bias, but that's up to you.
DO NOT throw the tube out or any such thing. It tested well, it behaves well in the amp, it's just a bit too hot for the amp's bias circuitry. Tubes like that pop up all the time. Even if you don't use it in this application you may have use for it in others.
Follow Ups:
I defenently will not throw these tubes away.So far I prefer these tubes by far over my JJ EL34L. Actually, I would like to keep these tubes in my amp if possible.
Usually when I turn the bias all the way down on this tube the LED tries to go off, so the off set should only be minimal.
Also, I will only live 4 more weeks here in Spain, before I am coming back to the USA. Maybe after I re-convert the amp to 120 Volts the bias setting be different.
What would happen, if the tube would get to much bias over a long period of time?
Too much bias current can speed up the wear on the tube. But the current may still be okay as far as the tube goes, just not okay with your amp. And it does sound like you are close to the proper bias so it's probably not a big deal.The line voltage will likely have an effect - but what that effect is remains to be seen. In general higher line voltages mean more negative voltage is needed for bias. What the secondary voltages are to the bias winding and the B+ winding will be is dependent on a lot of factors - AC volts at the wall, the trafo output voltage with the lower voltage primary, etc.
And I'm with you - I'd try my best to keep those in the amp too!!
Thanks for yoru info. It has been very helpful in understanding what's going on in this case.
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