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.

Early experimenters and manufacturers used a battery to provide this bias. This battery, called the "C" or bias battery provided voltage but almost never was called upon to deliver current. Thus, such batteries lasted nearly as long in service as they would have on a shelf. In 1985, The Department of Engineering and Technology at Cuesta College in San Luis Obispo, California was presented with a "C" battery date stamped 1927. Department Chairman W.E. English and Instructor W.T. Hanley conducted experiments which demonstrated that the battery still performed satisfactorily in its originally intended role more than 50 years after its manufacture.

Battery bias, however, is not self-adjusting, and does not accommodate differences between a new tube and one that has aged, differences between various tubes of the same type, or substitutions that may be made in tube type by repair technicians. Cathode bias automatically accounts for all these possibilities. It is inherent in the technique that the bias level is set by the operation of each individual tube.

To establish cathode bias, a resistor is placed between the emitting element, or cathode and the negative return of the "B" or HT supply. Current drawn through this resistor by tube conduction places the cathode slightly more positive than the negative return. The grid input is returned directly to the negative supply, causing it to be negative with respect to the cathode. Thus, changes in tube conduction are automatically compensated by changes in bias.

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