• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Biasing a Class A amp information

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Biasing moves the quescent point up or down on the load line. It's easier to envision what happens if you have the characteristic curves of the tube in front of you. The load line is from supply voltage (V+) to max plate current (V+/RL).

Normal Class A operation is to bias the tube such that plate current is 1/2 of V+/RL. That gives the most output voltage swing. As you move the quescent point either way it takes less input voltage to reach the non-linear regions of the curve (output will clip sooner).
 
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