The PM14 is a screen grid (tetrode) valve and the design dates from 1927-8. It is on a B4 base and the anode connection is taken to a top cap. In our exhibit the top cap is missing and the pip is broken and the vacuum looks ruined.
Courtesy of strong sunlight it was possible to see into the envelope. The filament and control grid seem to be as any small signal triode of the period. The screen grid is of very narrow pitch and encased in a bright rectangular box. This closes off the sides and top of the grid. The active faces are open to reveal the fine grid wires.
During the 1920s battery valves had various filament voltages and the Mullard PM12, PM14 and PM16 were all the same basic valve with two, four and six Volt filaments respectively. The Mazda 215SG shows an equivalent of the PM12 but with the top cap in place.
The envelope has no markings visible. The paper label has the details.
The rest of the label text.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 101 mm tall.
Reference: 1043. Type PM14 was first introduced in 1927.