The Dual BiasTester-MPD™ 7591 is designed specifically for the 7591 power tube and has the plate voltage output capability. Two switches on the A/B Control and you have the bias current and plate voltage for either tube.
From biasing tables supplied in the instructions, the bias range (upper and lower limits) can be determined from the plate voltage measurement.
It works with any musical instrument or hi-fidelity audio amplifier that uses a 7591. For other tubes please inquire.
This probe measures the cathode current through a 1-Ohm 1% tolerance power shunt resistor on pin 5 of the tube. A precision high voltage divider in the probe is used so the same voltage range on your multimeter measures the plate voltage and tube current.