The PM22A is a battery audio output valve for use in domestic broadcast receivers. It was made during the 1930s and features in the 1934-5 Mullard catalogue. The power output is low at 350 mW and so efficient loudspeakers were required to achieve an adequate listening volume. The output is lower than the PM22 but the filament power consumption was also less.
For correct operation the anode load should be 19,000 ohms.
This exhibit has the older balloon envelope later production moved to the shouldered or classic envelope.
The internal silver gettering is peeling away and some of the inner structure is visible but not clearly in this image. The anode lies horizontally and the grid extends beyond the bright anode.
The side of the anode box.
The balloon envelope is 50 mm in diameter and, excluding the B5 base pins, is 90 mm tall.
References: Data-sheet, 1040, 4024 & 1043. Type PM22A was first introduced in 1931.