The PM1LF was part of the first set of Philips-Mullard valves. The design of the PM1LF AF triode, dates from 1926. The box anode is mounted horizontally in the upper part of the envelope.
The basic operating conditions for the valve are given by the paper label on the base and the information etched in to the dome of the envelope.
Printed in red on the label is the message that virtually no glow is visible from the valve. Even dull emitter thoriated tungsten filaments would glow visibly, but the PM1LF uses an oxide coated filament that operated at an even lower temperature. The low consumption of the filament was a very important consideration when accumulators had to be taken to be recharged at regular intervals. Normally each week.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 80 mm tall.
References: 1931 data-sheet & 1043. Type PM1LF was first introduced in 1926.