Power meter for biasing an audio amplifier

An improved diagnostic meter, of the type used to measure the conduction of a vacuum tube in an audio power amplifier, which provides a display of the average value of instantaneous power dissipated by the tube. A probe containing a current sense resistor and a voltage attenuator interfaces between the tube and the amplifier to sample current through and voltage across the tube. A multiplier circuit computes a product of the sensed current and voltage, and a low-pass filter computes an average of the product. A display represents the average as a numerical value of Watts dissipated by the tube. A further improvement to the meter incorporates a DC-DC converter into the meter's power supply circuitry which allows the meter to be powered from the AC or DC filament supply provided by the amplifier.