I suggest you buy an elementary treatise on how vacuum tubes operate, and then read it. In the accepted model for a vacuum tube, electrons are emitted from the cathode when it is heated. The cathode accumulates electrons and hence is thought of as having a negative charge. Because the anode is positively charged, or relatively deficient in electrons, they flow cathode to anode. If this is all that happened then vacuum tubes would be like light bulbs; the cathode would give up electrons in an uncontrolled manner, and they would flow to the anode. When this process reaches a state of equilibrium, the tube would be burned out and that would be the end of it. But instead there is a "grid". The grid is situated in the path of the electrons on their way to the anode. In a functioning tube, most of the time, the grid carries a net negative charge with respect to the cathode, and thus retards the rate at which electrons get all the way to the anode. That is, when no music signal is present, the grid has a DC voltage on it that is negative with respect to the DC voltage on the cathode. When a music signal is present, think of that as an AC voltage, a voltage that varies with frequency and in magnitude and represents music. So, music enters via the grid which always has a constant DC bias voltage on it, in simplest terms. The music signal modulates the grid bias voltage, and in another story, that’s how we get amplification of the signal across a resistor that connects the anode to the PS voltage. When amplifiers talk about "bias", they are usually referring to this steady state DC voltage on the grid. That grid bias voltage also results in a steady state current (electrons) passing from cathode to anode when there is no music signal. This DC current is referred to as "bias current". The grid bias voltage, the "plate resistance" of the tube, and the power supply voltage between anode and cathode, also called the "plate voltage" together determine the bias current. (Ohm’s Law.) For any power tube, or any tube at all, you can usually find a complex graph depicting "plate curves". This single graph will show current on the Y axis and plate voltage on the X axis, and a set of curves depicting what that tube will do at different acceptable grid bias voltages for different changes in plate voltage or current. And....
