variable bias versus fixed bias guitar amp

If the resistor was 15k we select a 10k resistor and a 10k trimpot, so we can adjust from 10k to 20k, and the old value is in the center of our adjustable range. Since we want to be able to vary the bias voltage up and down, we want a resistance that is adjustable to less and more than the resistor we replaced. We will remove the resistor and replace it with an adjustable resistance. This resistor, together with the trimpot resistance is a voltage divider network setting the bias voltage. In some amplifiers the trimpot will be mounted on the back panel, labeled O.T. MATCHING, but the procedure will be the same, we only replace the grounding resistor. In some circuits the grounding resistor will have a different value, often 33kΩ (orange, orange, orange, silver), some Fender amplifiers have a trimpot without tap and two additional resistors instead. I have marked the 15kΩ grounding resistor (brown, green, orange, silver) mounted on the tapped adjust trimpot with a red arrow. These images show the bias balance pot of a typical circuit. Do not confuse the bias balance pot with the hum balance pot! It is a pity the bias setting option was dropped by CBS with that change.īut even so, it is a much easier, simpler and less invasive change to add a bias adjust pot to the existing balance setup and keep the balance, instead of replacing it with the earlier bias adjust: one only needs to replace the grounding resistor on that pot with another trimpot. I have never heard of anything more stupid: that was the one good idea the CBS engineers introduced, allowing the owner to use output tubes mismatched in plate current, or to rematch them for drift after years of use. While some circuit changes introduced by the CBS corporation who now owned Fender were not for the better and a particular amp may benefit from returning to the earlier part values used, often also the Output Tubes Matching bias balance adjustment is ripped out and replaced with the earlier style bias setting adjustment. In recent years it has become the fashion to 'blackface' the so-called Silverface Fender amplifiers made between late 1967 and the mid 1980s. Adding a bias adjust trimpot to a Fender bias balance Introduction

Variable bias versus fixed bias guitar amp