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Post subject: Silverface Twin Reverb Bias question
Posted: Tue May 21, 2013 8:00 am
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I have silverface TR that I put new JJ 6L6's in and am biasing her up. Here's my issue/question.
I am using transformer shunt method to get close and then just going by ear.
One side is measuring 44ma per tube and the other side is measuring 12.5ma per tube!!!!! It's a matched set and I have tried other matched quads with similar readings.
Is my OT shot???? whimper............
What could cause such a problem???

Thanks in advance?


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Post subject: Re: Silverface Twin Reverb Bias question
Posted: Tue May 21, 2013 9:55 am
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Transformer shunt is very dangerous . Better to put 1 ohms cathode resistors on each tube.

Bias pot is not a bias adjustment, it is a balance pot . You know that ?

Lets say , if you send more negative bias voltage to one side , the other side will receive less negative bias voltage .

In other words , if you want a cooller bias one side , you will do a hot bias on the other side


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Post subject: Re: Silverface Twin Reverb Bias question
Posted: Tue May 21, 2013 10:22 am
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I am aware of the shunt method being the dangerous method. It was just a quick assessment of current draw.
It is a fixed bias pot in the amp, I am adjusting. Not the Bias Balance that is on the exterior of some amps. So are you saying that the internal pot off the bias supply is a Balance??????? I was not aware of this!!!!!!! Are we getting our lines crossed here?
This amp does not have a any balance pots on it. Filament or otherwise!
Thanks for your time.
Any other thoughts?


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Post subject: Re: Silverface Twin Reverb Bias question
Posted: Tue May 21, 2013 10:42 am
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If this amp is a original Silverface , shure the bias pot inside IS a balance pot not a bias adjustment.

Look drawing here , the layout and see if it is your amp .


ampwares.com/schematics/twin_reverb_aa270.pdf


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Post subject: Re: Silverface Twin Reverb Bias question
Posted: Tue May 21, 2013 11:14 am
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All Twin Reverbs after the original AB763 circuit revision use a bias-balance arrangement. That's every silverface made after May of 1968.

Arjay

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Post subject: Re: Silverface Twin Reverb Bias question
Posted: Tue May 21, 2013 12:10 pm
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I will have a look when I get home! Thank you so much!
I have a '63 AB763 in my roster. This amp is a friends twin. Master Vol No Push Pull!


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Post subject: Re: Silverface Twin Reverb Bias question
Posted: Tue May 21, 2013 3:28 pm
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Looks like a bias balance pot to me.

http://ampwares.com/schematics/100W_Mas ... in_Rev.pdf

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Post subject: Re: Silverface Twin Reverb Bias question
Posted: Wed May 22, 2013 1:12 am
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goldstache , you can "easily" modify this balance pot to an adjustment pot if you can solder electronic .


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