Cathode bias

In electronics cathode bias, also called self-bias, is a circuit used with amplifying vacuum tubes (valves) such as triodes, tetrodes and pentodes to provide a steady negative DC bias voltage on the control grid electrode of the tube, to set the operating point. It consists of a resistor connected between the tube's cathode and ground, in parallel with a bypass capacitor. Used with indirectly-heated tubes, it is one of the most widely used biasing circuits in vacuum tube electronic equipment.