Abstract: An amplifier is described including a pair of cathode-coupled pentode vacuum tubes. A first pentode of the pair includes a control grid configured to receive an input signal. A second pentode of the pair includes a control grid configured to be coupled to an input reference signal. A cascaded voltage regulator includes a first stage configured to generate a plate reference voltage for providing voltage to a plate of each pentode. The regulator includes a second stage configured to generate a screen reference voltage based on the plate reference voltage for providing voltage to a screen grid of each pentode. The amplifier can also include a push-pull output stage having a servo circuit configured to adjust the control bias signal for one of a pair of vacuum tubes included in the stage until the plate currents in the pair of tubes are substantially equal.

Abstract: An amplifier is described including a pair of cathode-coupled pentode vacuum tubes. A first pentode of the pair includes a control grid configured to receive an input signal. A second pentode of the pair includes a control grid configured to be coupled to an input reference signal. A cascaded voltage regulator includes a first stage configured to generate a plate reference voltage for providing voltage to a plate of each pentode. The regulator includes a second stage configured to generate a screen reference voltage based on the plate reference voltage for providing voltage to a screen grid of each pentode. The amplifier can also include a push-pull output stage having a servo circuit configured to adjust the control bias signal for one of a pair of vacuum tubes included in the stage until the plate currents in the pair of tubes are substantially equal.