The PM22A is a battery audio output valve for use in domestic broadcast receivers. It was introduced in 1932 with a B4 base a a side terminal, then in 1936 it was re-issued on a B5 base. It is listed 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.
The move away from the messy Azide process allowed makers to show clear glass to the world again. The painted envelope has almost gone just a painted envelope dome remains to carry the identification details.
The classic envelope is 38 mm in diameter and, excluding the B5 base pins, is 94 mm tall.
References: Data-sheet, 1040 & 1043. Type PM22A was first introduced in 1931. See also 1931 adverts.