Plate Current Increase

In a lightly loaded amplifier the frequency characteristic stays flat at high frequencies, even with a droop in load impedance, but the plate current rises in inverse proportion to the impedance. If the plate current can rise enough to maintain constant output voltage, this plate current rise may be objectionable from the standpoint of tube heating or plate supply regulation. Values of plate current rise calculated on the basis of constant output for low and high frequencies are shown in Figs. 124 and 125.

Fig. 124. Rise in plate current due to transformer impedance change at low frequencies.

Fig. 125. Rise in plate current due to transformer impedance change at high frequencies.

Many satisfactory audio amplifiers have plate currents which would be excessive at the extremes of the range if high or low notes were amplified continuously. They are not damaged because these tones are of short duration.