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Just got some slightly used GE 6CA7 and they do sound wonderfull in my CJ CAV 50. Only problem I have is that one tube can't be biased low enough. The biasing on the CAV 50 is done by turning the bias up, until a red LED comes on, than turn it down again, until it goes off. However, I have the bias settings of all 6CA7 much lower than on EL34's. And one of the 6CA7 actually can not be biased low enough for the LED to go off. Though sometimes after warm up it may go off, properly also depending on the slight ups and downs of the mains voltage.Any opions or suggestion?
Follow Ups:
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.
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.
Capt
Are these tubes new pretty much? If they are its probably just a bad one but i think you said it follows no matter which socket you put it in right? If not check the neg bias voltage at pin 5.You should be able to vary it from neg 27 to neg 50v as I recall on this amp.A leaky coupling can cause your problem to but its probably the tube.
Hi Michael;How can I check the negative voltage? I do have a DMM. Pin 5 and Pin???
Hi:
Are these the large bottle 6CA7's, about the size of a 6L6GC or larger?If so, and the amp was designed for the skinny bottle 6CA7/EL34 type, that might explain it.
The large bottle type needs more negative bias then the EL34. In some amps, you can change a resistor in the bias circuit to change the operating range of the bias adjustment.
Hope this helps! Al
"The large bottle type needs more negative bias then the EL34."I'm not sure I agree. The tube specs don't indicate that should be the case. And my experience has shown that the variance from batch to batch and tube to tube of either type (6CA7 or EL34) is the biggest difference in bias required. I have plenty of hotter EL34s that need more negative voltage than the cooler 6CA7s.
Hi Jim:I agree, the tube specs don't indicate that. But in my experience, generally, the 6CA7's have been hotter, requiring more neg bias than EL34's, regardless of brand or batch.
Please correct me if I'm wrong, but is not the 6CA7 a beam power tube, and the EL34 a true pentode?
Perhaps that would explain the difference?
Al
The GE 6CA7 are shorter and fatter than the EL34, they are also called "Fat boys".
Just checked the LED after one hour or so after warm up and the LED light has gone off.Still wondering, why it takes so long.
Checked the LED again a few times and I has been on again the last few hours. Switching the tubes to a different position only means, that the bias 'problem' follows the tube.
I would scrap it...you don't want to mess with marginal power tubes.
Not meant as a personal question -- really! If the tube is a little gassy, after it warms up thoroughly, it may be adsorbing enough gas to bias correctly. You should be able to use a tube tester to find out. If the tube is gassy, it's finished as far as I know.
Tube tester shows NO short or Gas
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