Establishing Cathode Bias
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.
This scheme inherently introduces dynamic, even harmonic distortion. As the input signal becomes more positive, cathode current increases, increasing bias and reducing gain. As the input signal becomes more negative, cathode current decreases, decreasing bias and increasing gain. The result is a plate signal with positive excursions greater than the negative input and negative excursions smaller than the positive input. It must be borne in mind that the input and output signals are ideally exactly out of phase. Since cathode bias is normally employed at audio or very low radio frequencies, issues such as transit time and interelectrode capacitance may be disregarded and the ideal assumed.
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