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Mysterious power tube behavior

On my Willsenton R8, I was using JJ 6L6GCs. One of them got a bit noisy, and then soon after startup the bias would go to zero; bias in the other positions remained steady. I switched it to different sockets, same behavior. I threw in a set of known good KT77s, all 4 positions bias fine. Put the 6L6s back in, that one tube still caused bias to go to zero.

The mystery is that sound quality, other than the noise, was never affected. And the misbehaving 6L6 tested fine on my recently calibrated Hickok 600; the reading were exactly the same as the other 3 tubes.

Anybody got any idea how a tube with zero bias could act perfectly normal? Visual inspection of the tube didn't show any differences either. I've been using tubes a long time and I've never seen this.

armstrod
ellajeanelle's avatar
ellajeanelle
386 posts
 

I am no tube expert but did have what I thought was a tube that hummed. It’s on a secondary system in my office and not loud enough to annoy me or be heard from my desk, but the audio was fine.

After reading your post I just walked over the amp and moved my ear around the whole thing and realized that the noise was originating from the transformer and not from any of the 8 tubes!

czarivey
5,701 posts
 

That tube probably have bias circuit open. Therefore no current you can read there. The reason that tube works is simply because for push-pull amps most tubes are "cold biased" to like 50mV anyways. 

armstrod OP
602 posts
 

@czarivey,

So the tube has an internal bias circuit that could be malfunctioning and the tube would still test good? The bad tube shows no bias in any of the 4 sockets, and a known good tube biases fine in all 4 sockets.

czarivey
5,701 posts
 

Only if it's in PushPull setup or "cold bias". It won't work single-ended. 

I would test an input resistor usually 1k and it's between ground and pin5 on bad tube. In case with KT77 it should show value, but if in case there's no value in 6l6, then you know that control grid is compromised there within the vacuum body. I'm just hesitant on why tube tester scores a good tube in that case. 

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