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Adding Bias Trim Pot

2/18/1999 6:03 PM
Adding Bias Trim Pot
To All DIY's;  
I can not find a tech to do even  
the most simple job on my 59 Fender  
Bassman re-issue because of the printed  
board. At last I must do it myself.  
How do I do it? What type and value  
trim pot do I buy? Exactly where do  
I place it. Is there an additional  
resistor I put with it? How do I  
avoid damaging the board, (how much  
wattage on the iron. I can do the solder  
I'm sure, but locating the correct  
resistor to replace and the steps  
is what I do not know. Please state  
the location in relation to something  
or two. I can identify and read resistors  
I am replacing all my tubes and I  
need to bias them. Thanks Walter
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2/19/1999 1:27 PM
T. Pruitt

If you can get your hands on Gerald Weber's book "Tube Amp Talk for the Guitarist and Tech" there is a chapter in there on adding an adjustable bias to the Fender Bassman reissue. He tells you everything including the resistor number thats written on the circuit board so you know exactly which resistor to replace. Someone out there may know the resistor number or have the schematic handy.  
I don't remember the resistor number but for Gerald's approach all you need is a 50k cermet potentiomter, a small amount of hookup wire, and some silicone glue. You replace the resistor with the pot, use the wire to make the connection and a nice drop of silicone glue to hold it in place on the board.  
Then you can plug in a tube rectifier and bias the tubes accordingly and get that "sag" effect.
2/19/1999 9:36 PM

I did the bassman RI mod based on the description in GW's book with the exception of adding a resistor in series with the pot. It's a pain in the butt to get at but it's worth it. Mark

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