The AV33 is tungsten filament control diode. It is operated in saturation mode so that the anode current is a function of filament voltage. This provides a control voltage for use in AC regulators where the control element is a saturable reactor.
Typical operation would run the filament at 3.7 Volts and with 100 Volts on the anode the anode current would be 3.0 mA. The filament would be drawing 0.96 A. The minimum anode voltage for 80% of saturated current is 20 Volts.
In the first Theramin musical instruments the amplitude was controlled by a saturated diode with the filament run from the output of an oscillator. The oscillator output being controlled by the hand capacitance of the operator.
The anode side view.
The anode is ribbed fro dimensional stability with thin nickel sheet.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 79 mm tall.