The PM2DX was introduced in 1928 and is described by Mullard as being for detector and general purpose applications within battery broadcast receivers.
The filament is for accumulator use and the anode is designed for dry battery operation with a maximum anode voltage of 150 Volts.
The PM2DX was also popular as a driver valve for quiescent push-pull output stages such as the PM2B.
With the gettering confined to the dome, the construction can be seen. The box anode has a rectangular grid with a V shaped filament within.
The identification is etched into the dome.
The anode is rippled for strength. The coated filament centre support can be seen to have no wire attached at the pinch.
The grid extends beyond the anode box.
The V shaped filament.
The balloon envelope is 45 mm in diameter and excluding the B4 base pins, is 88 mm tall.
Reference: Data-sheet. Type PM2DX was first introduced in 1928. See also 1928 adverts.