This battery triode was designed in 1926 for the HF stages in broadcast receivers. The filament would be run from a 2.0 Volt accumulator.
The electrodes lie horizontally in the balloon envelope.
One side has the letters HF and on the other is the aerial, coil and earth logo surmounted by the Mullard name and the Identification PM1.
The horizontal anode with the letters HF on the outside of the glass.
The etched PM1 photographed with the microscope.
Another image from the microscope. This shows the top of the filament held at the end of the tension spring. The grid wire is much thicker than the filament.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 90 mm tall.
References: 1931 data-sheet, Data & 1043. Type PM1HF was first introduced in 1926. See also1926 adverts.
