In order to utilize even 25 percent of the theoretical amplification factor of high mu tubes, it is necessary to use a plate load resistance and plate potential too high to be practical. By the use of a dummy tube in the plate circuit, however, one can obtain a resistance characteristic which makes it possible to attain the full amplification of 1500–2500 per stage for the ``57'' tube using only 300–600 volts plate supply. Output voltages as great as 450 volts are attainable with negligible distortion.
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Otto H. Schmitt (1933) studied this question.