# Quick Question: Resistor value for Fender bias adjustment...



## StevieMac (Mar 4, 2006)

In an AB763 type amp, I tried a 22K resistor grounded to a 10K bias pot and the bias range with a particular set of 6L6GC tubes was -50 to -80 V. Tried 33K resistor and it dropped from -45 to -65V and a 53K resistor got me from -37 to 50K. So...what do I need to do in order to get the bias range from about -20 to -60? Perhaps I should be using a 50K pot instead? As you can tell...I only know enough to not kill myself when trying to figure this stuff out!


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## jb welder (Sep 14, 2010)

Sounds like something is wired funny. As the series resistor is increased, the voltage range should end up more negative.
Is the resistor in series between one of the outside pot legs and ground? The other outside leg of the pot goes to the diode, and the wiper goes to the resistors in the power tube grid circuits.


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## StevieMac (Mar 4, 2006)

jb welder said:


> Sounds like something is wired funny. As the series resistor is increased, the voltage range should end up more negative.
> Is the resistor in series between one of the outside pot legs and ground? The other outside leg of the pot goes to the diode, and the wiper goes to the resistors in the power tube grid circuits.


I have it wired up much like this (see below), except the 27K resistor is a 53K (grounded to pot) and the pot is then grounded to the chassis. The only other change is that the left lug (looking from the front) on the intensity pot goes to a .01uF cap on the board...


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## jb welder (Sep 14, 2010)

Can you repeat your voltage measurements checking at the wiper of the bias pot. The higher value series resistor should give the most negative voltage readings. (Example, 22K resistor, -40 to -50V, 50K resistor -50 to -80V sweep).
Pics would also help.


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