The AZ31 directly heated full wave rectifier and is fitted with an IO base unlike the earlier AZ1 that had the continental Ct8 base. The AZ31 has identical electrical characteristics to the AZ1.
It has a 4 Volt filament rated at 1.1 Amp. The maximum anode RMS voltage is 500 Volts. At this anode Voltage the DC current could be up to 60 mA. At 400 Volts on the anode the output could rise to 75 mA and with 300 Volts on the anodes the output current could be 100 mA. The maximum reservoir capacitor had to be no more than 60 µF.
The oxide coated filament is a ribbon made as an inverted V within the rounded anode. The overall construction is different to our AZ1 indicating more than a base change.
The top mica fitted into the shoulder of the envelope bulb. The tops of the filament are held in spring cups to maintain tension when hot.
The classic envelope is 45 mm in diameter and, excluding the IO base pins, is 97 mm tall.
References: Datasheet, 1043 & 3002. Type AZ31 was first introduced in 1938.