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Post subject: Bias Question
Posted: Wed Aug 18, 2010 3:21 pm
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Aspiring Musician
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I just popped in a new set of JJ's 6l6 the other day.I read the bias as 38mA and rising so i lowered it down.They the bias started going down.So basically the bias has not been to stable.So after a hot of just letting the tubes run they have finally stabilized ,So i am wondering does temperature of my place effect the bias ,yesterday it was 68 in my place and reading 36mA today its 72 and reading 37mA.


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Posted: Wed Aug 18, 2010 3:51 pm
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What amp and what method of biasing?

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Posted: Wed Aug 18, 2010 3:52 pm
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Ambient temperature would have to swing much more dramatically than four degrees to have any effect on the bias, if it were to have any affect it at all (which is very unlikely).

New power tubes generally require a "burn-in" period (perhaps an hour or so) after the initial installation and warm-up to set the idle bias accurately.

Arjay

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Post subject: Bias
Posted: Wed Aug 18, 2010 4:56 pm
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Agree with Arjay, the tubes have to warm up before checking the bias. The idle current will be higher before the tubes warm up. Over time there will be a little drift, which is normal. A change from 36mA to 37mA is OK. You might recheck your bias after using the amp for awhile, letting the tubes settle in. For 6L6 tubes, at 430V plate voltage have a recommended plate current of about 27.8 mA to 33.1 mA, and a 485V plate voltage has a recommended plate current of 23.7 mA. Of course you can adjust to suit your own needs. :)

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Posted: Wed Aug 18, 2010 5:22 pm
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Sorry i for got to mention i was using a twin reverb.So i figured they are 25 watt tubes and my plate voltage is 460 at 70 percent dissipation i come up with 38mA. I have noticed that it is starting to settle, but it has been longer then it normally is.I usually just let them run for and hour a day for a couple of days.and recheck the bias after a week.i'll let you know in a few days


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Posted: Wed Aug 18, 2010 6:07 pm
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38mA is a skosh high IMO. I bias my TRRI at 37mA and my 135-watt UL TR at 35mA (both amps equipped with NOS blackplate RCA's). But if your ear says it's okay and the bottles aren't red-plating then run with it.

Arjay

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Posted: Wed Aug 18, 2010 7:42 pm
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Retroverbial wrote:
38mA is a skosh high IMO. I bias my TRRI at 37mA and my 135-watt UL TR at 35mA (both amps equipped with NOS blackplate RCA's). But if your ear says it's okay and the bottles aren't red-plating then run with it.

Arjay


Arjay,
Dumb question for you...on a Champion 600 where you have one 12AX7 and one 6V6 do you need to bias when you replace tubes if you are using the exact same tubes??
ABS :D


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Posted: Wed Aug 18, 2010 8:55 pm
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The Champ 600 is a single-ended class A amp AFAIK, with fixed cathode biasing. You need only replace the tubes with an equivalent type of suitable quality.

Arjay

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Posted: Thu Aug 19, 2010 5:51 am
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I had a NOS RCA 6V6 one time and you could set the bias and after the tube started getting hot it would start "running away",the reading on the DVM would rise steadily until the plates would start getting cherry red.
It was that one tube,I had bought it from a guy who sold NOS tubes and called him,he didn't believe me since he had "tested" all the tubes he sold,but I sent it to him and he said "sure enough,it's defective."
I know this is not really related to the question,but threw it in for what it's worth.
Tubes are really interesting,ain't they? :D


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Posted: Thu Aug 19, 2010 5:55 am
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Retroverbial wrote:
38mA is a skosh high IMO. I bias my TRRI at 37mA and my 135-watt UL TR at 35mA (both amps equipped with NOS blackplate RCA's). But if your ear says it's okay and the bottles aren't red-plating then run with it.

Arjay


I agree with Arjay. Which Twin? 85, 100 or 135 watt? I usually like to run them a touch warm and trust my ears and like Arjay says watch for red plating.

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