The PM2B is a double triode audio output valve from the early 1930s. Like similar valves this was designed to offer reasonable volume at minimum consumption from dry cell HT batteries by being operated as a Class B push-pull stage. Pure Class B suffers from cross-over distortion as the initial conduction region of the valves operating slope is non linear.
The input voltage is quoted as 40 Volts peak to peak into a 4,000 Ohm impedance. The anode load is given as 14,000 Ohms. To drive the PM2B more power was required than could be supplied from the PM1HL and a suitable valve for use as driver was the PM2DX.
The dome of the balloon envelope carries the details of the valve. This is probably an early example of the PM2B as one of our other PM2B exhibits is encased in the later classic envelope.
The pair of separate electrode assemblies can be seen firmly held by the mica supports. The filaments are probably spray oxide coated and not the earlier Azide process. The earlier process was messy and required glass support rods outside the anode cavity to prevent the oxides contaminating other structures.
The balloon envelope is 46 mm in diameter, and excluding the B7 base pins, is 96 mm tall.
References: Data-sheet & 1043. Type PM2B was first introduced in 1933. See also 1933 adverts.