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Fender Bias system conversions


How to convert a bias balance system to a common bias system.
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 Here are 3 typical later style Fender bias balance systems. The drawing on the left is a AB568 Vibrolux Reverb amp. The upper right drawing is a AA1069 Bandmaster Reverb amp. The lower right is a AA270 Twin Reverb amp.
 Usually there are one or more resistors to the right of the bias pot on the way to the circuit board. The wires that leave the bias pot and go to the circuit board are attached to eyelets on the circuit board. Where they attach to the board does not matter since we are going to undo everything and make each system a simple bias adjust system like on older fender amps.
 The only parts you may need are two 220K 1/2 watt resistors. You can use the existing bias pot and the resistor that is attached to the back of the bias pot. If the bias capacitor on the bias board is old, you should change this to a new 47uf/100 volt electrolytic capacitor.

image This is the Vibrolux reverb after the mod. Notice all the resistors to the right of the pot are gone. There is a wire from the center of the pot that goes to the circuit board. This eyelet is the center point of two 220k 1/2 watt resistors. Each 220k resistor goes to an eyelet where a .1 capacitor is soldered.
There was a 100k resistor on the right hand eyelet, but it was removed and replaced by a 220k resistor.
You can leave the 15k resistor on the back of the pot alone. Older fenders had a 22k resistor in this position but the 15k will work just fine.

Note: You are going to do the same exact thing to the circuit board on all of the bias mods. Whatever arrangement is there now, remove it and make it look like the diagram on the left. You should have a wire that goes from the center of the bias pot to the junction of two 220k resistors. These two resistors then split and each one goes to a .1 capacitor. There should be an under the board wire from each of these 220k/.1 capacitor junctions. Each of these wires goes to pin one on one of the power tubes.

Note: Ignore all the extra crap on the power tubes. This is the subject of another mod.


image Here is the stock AA1069 Bandmaster Reverb bias system. Notice that it has a bias pot with a tap point on the rear left side of the pot. You can use this pot as a regular bias pot by just not using the rear tap point. Also notice that the resistor on the body of the bias pot is soldered to the center lug of the bias pot. The bias pot has two wires that leave the pot from the two outside legs of the pot.
image Here is the AA1069 Bandmaster Reverb bias system after the conversion. The wire that leaves the bias board, now goes to the lower right tab on the bias pot. The resistor on the back of the pot is now soldered to the upper right tab of the bias pot. The center right tab of the bias pot now goes to the center of the two 220k resistors on the circuit board. See the circuit board mod section above to see how the circuit board is altered.

image Here is the stock AA270 Twin Reverb bias system. It has a bias pot with a tap point on the rear left side of the pot. You can use this pot as a regular bias pot by just not using the rear tap point. Also notice that the resistor on the body of the bias pot is soldered to the center lug of the bias pot. The bias pot has two wires that leave the pot from the two outside legs of the pot. It is a bit different from the Bandmaster diagram above. There is a resistor that leaves the bias board and goes to the bias pot tap point. This bias pot tap point also has a filter cap. There are two filter caps in this bias system. One is on the bias board and one is connected to the boas pot tap point.
image Here is the AA270 Twin Reverb bias system after the conversion. A wire has replaced the resistor from the bias board to the bias pot. This wire goes to the lower right tab on the bias pot. The resistor on the back of the pot is now soldered to the upper right tab of the bias pot. The center right tab of the bias pot now goes to the center of the two 220k resistors on the circuit board. See the circuit board mod section above to see how the circuit board is altered.



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