The Mullard PM2, introduced in 1929, is a 150 mW audio output triode for use in battery driven receivers before the days of indirectly heated cathodes. The PM2 has an oxide coated filament made by the reduction of Barium Azide during production. This example is complete with its paper label on the base.
The logo of aerial, coil and earth is on the logo as well as the BVA logo.
The envelope also has the Mullard registered trade mark.
The electrode system is mounted horizontally. Adverts of the time show the construction clearly and make much of the flexibility of the filament. Early tungsten wire was very brittle and the filaments would easily shatter.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 92 mm tall.
Reference: 1043. Type PM2 was first introduced in 1926.