Early Techniques
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.
Read more about this topic: Cathode Bias
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