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Is JTM30 Fixed Bias????????


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10/29/1999 6:33 AM
David Bates
Is JTM30 Fixed Bias????????
Help!!! I just took my Marshall JTM30 to a local amp tech.....not the guy who usually does my work....had some new Philips 6L6 WGB's put in....he said it was a fixed bias amp so no re-biasing was necessary (originally uses 5881's)....do any of you guys know the real dope on this???  
Thanks for any advice  
David
 
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10/29/1999 12:56 PM
dr

There's might be something lost in the translation of what he told you. Fixed bias amps should always have the bias checked when changing tubes, even if the new tubes are the same type. He may just be saying that he didn't need to adjust the bias after installing the new tubes. Ask him if he checked the tube current after he installed the new tubes. If he says he didn't check tube current after replacing the tubes, find a new tech.  
 
 
10/29/1999 1:47 PM
MJ Harnish

Either there was some miscommunication or your tech doesn't understand the concept...  
 
Fixed bias just means that the bias is set at a fixed point (voltage), whether the circuit uses a set resistor value (ala Mesa Boogie) or uses a pot to dial in the value (ala Fender). It does mean not that it's self-biasing (ala Vox or Matchless). The whole terminology is a little misleading since you a Fender is technically an "adjustable fixed bias" while a Mesa Boogie is a "fixed fixed bias."  
 
As I recall, the JTM30 does not have a bias pot so properly biasing the tubes requires changing a resistor value. The Fender blues series (deluxe and deville) were the same way; I think Fender finally started added a pot on the HR series.  
 
It's possible that the tech meant to communicate that the tubes were biased just fine as is, but I would clarify this since it also could mean that he looked in there, didn't see a pot, and decided that the tubes must bias themselves (which they doesn't). If that's the case you've found someone who doesn't know his/her trade very well and it's a crap shoot whether your tubes are anywhere near a "good" bias point. However, if the amp sounds good and the plates don't glow red when the amp is on it's *probably* just fine.  
 
MJ
 
10/29/1999 3:27 PM
GFR

The bias on the JTM30 is fixed at -39,1V with a string of zeners (15V, 15V, 9V1). So you can't change the bias changing a resistor - you would need to change one of the zeners.
 
10/29/1999 9:04 PM
MJ Harnish

Yikes, what a headache....  
 
MJ
 
10/30/1999 2:19 AM
David Bates

Many thanks for all the input....the amp sounds OK and none of the tube plates a "glowing"...my regular tech is due back from vacation in about a week so I'll get him to give it a check-up as soon as he's back...again thank all of you guys for your input...  
David
 
10/30/1999 4:29 AM
Bruce

It's actually not that hard because you can swap known zener diodes for the values in the amp to increase or decrease the actual bias voltage.  
Some old VOX amps have a couple zeners in the bias supply too.  
Bruce
 

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