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OT: Is there an amp biasing how-to somewheres?


squareking

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There are two main types of biasing used in tube amps, one is "fixed bias" and the other is "cathode bias." Oddly enough, fixed bias amps have a variable resistor to adjust bias, while cathode biased amps are considered "self-biasing" but in some cases need to be tested and adjusted by swapping out the bias resistor. The Blues Jr is an example of this, cathode biased and from the factory tends to run the tubes too hot.

 

If you have a fixed bias amp, it's relative simple to check and adjust the bias BUT THERE IS SOME VERY HIGH VOLTAGE PRESENT so you must take proper precautions. Sometimes fixed bias amps need to have the bias resistor replaced when the range won't go low or high enough, this happens at times due to swapping types of tubes.

 

You can buy one of the socket type bias probe kits, this makes things easier and safer. This page shows how to build one as well as use one:

 

http://www.el34world.com/charts/biascurrent.htm

 

The bias pot is usually on the circuit/turret/tag board, so involves removing the chassis. Bias current is a negative voltage, usually the pots are oriented so that turning it counter-clockwise increases the negative voltage. So it's a little counter-intuitive, when the tubes are running too hot you turn it "up" instead of "down" as you typically use a potentiometer.

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