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previous: J Fletcher You are right...I was wrong...I mad... -- 1/26/1998 11:54 AM

Re: Right bias for JCM 800?

1/26/1998 12:44 PM
GVB	Re: Right bias for JCM 800? I use a "generic" method of setting the bias in an amp because the scope method is not reliable on it's own. By this I mean that you may set the tubes to hot if you only look at the notch, and not measure the current. If for instance you are not using matched tubes, the notch may not disappear until one tube is running to hot etc... Setting the bias point to 1/3 to 2/3 of the dissipation rating is strictly empirical. I have read a few articles over the years, and these suggested that beyond 1/2 there was not much of an improvement in sound, but there was shortened tube life. You can check if it sounds OK at 1/2, and adjust from there to suit your needs. I forgot to mention though, that these settings should be done with no input, all the volumes at zero, and should supposedly be done with no load (i.e.. no speaker). I really don't see this last point, because there will be no power/signal transfer in the output transformer because there is no signal, only DC, and therefore a load should make no difference to the tube or OT for that matter. In most of the amps I've biased, I find that the bias does need to be just a little hotter than 1/2 for my tastes, but not that much more. Beyond that, the article is right - I did not notice an appreciable improvement in tone, and I'm REALLY picky about tone. I've found that most of the articles that say to set your bias point to _ current at _ voltage with _ type of tube, do indeed go just slighly hotter than 1/2 PA. GVB

1/26/1998 12:44 PM

GVB

Re: Right bias for JCM 800?

I use a "generic" method of setting the bias in an amp because the scope method is not reliable on it's own. By this I mean that you may set the tubes to hot if you only look at the notch, and not measure the current. If for instance you are not using matched tubes, the notch may not disappear until one tube is running to hot etc... Setting the bias point to 1/3 to 2/3 of the dissipation rating is strictly empirical. I have read a few articles over the years, and these suggested that beyond 1/2 there was not much of an improvement in sound, but there was shortened tube life. You can check if it sounds OK at 1/2, and adjust from there to suit your needs.

I forgot to mention though, that these settings should be done with no input, all the volumes at zero, and should supposedly be done with no load (i.e.. no speaker). I really don't see this last point, because there will be no power/signal transfer in the output transformer because there is no signal, only DC, and therefore a load should make no difference to the tube or OT for that matter.

In most of the amps I've biased, I find that the bias does need to be just a little hotter than 1/2 for my tastes, but not that much more. Beyond that, the article is right - I did not notice an appreciable improvement in tone, and I'm REALLY picky about tone. I've found that most of the articles that say to set your bias point to _ current at _ voltage with _ type of tube, do indeed go just slighly hotter than 1/2 PA.

GVB

Replies:

Doc ".....supposedly done with no load ... -- 1/26/1998 2:13 PM
J Fletcher Well...just to expand my answer a b... -- 1/26/1998 2:30 PM