What is "bias" and do I have to bias my amplifier?

Bias refers to the steady-state operating point of the tubes at a certain intended level of current. Some amplifiers are designed using a circuit with "fixed bias" which means the voltage difference between the grid and cathode of the tube must be set at a fixed amount so that the right operating point is maintained. As the tubes age or other conditions vary, the amplifier can move away from its normal bias and need adjustment or maintenance.

My amplifiers all use another technique called self-bias, cathode bias, or "automatic bias" where a resistor is used on the cathode of the tube, and if current through the tube ever alters to be higher or lower, the resistor automatically adjusts the voltage on the cathode due to Ohm's Law, bringing the tube back to the expected level of bias. So no maintenance or adjustment for bias is ever needed.