The PM2DX was introduced in 1928 and is described by Mullard as being for detector and general purpose applications within battery broadcast receivers.
The envelope is smaller than the PM2 and the electrodes are obscured by the gettering over the top of the dome, but the pinch is clearly visible.
The identification lettering is on the top of the envelope in gold. Both the filament volts and the maximum anode voltage are printed.
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.
The balloon envelope is 40 mm in diameter and excluding the B4 base pins, is 78 mm tall.
Reference: Data-sheet. Type PM2DX was first introduced in 1928. See also 1928 adverts.