What is Cathode bias (aka) Auto Bias versus fixed bias?
Cathode bias is an early method of biasing tubes where a resistor is placed in the cathode. Current draw through this resistor makes the grid potential negative with respect to the cathode. And, changes in tube conduction are automatically compensated by changes in bias so it’s also referred to Auto Bias. In Fixed Bias a negative voltage is applied directly to the grids of the output tubes. It should be noted that preamp tube are almost always cathode biased whereas output tubes may be either or both. Without getting into the technical issues of the difference between a fixed bias/cathode bias we will talk about the tonal qualities and how they differ in the real world.
