The Six Sixty ss210HF was made by Mullard and is electrically identical the the PM1HF. The difference is in the lettering on the bulb.
The PM1HF was part of the first series of Philips-Mullard valves. The filament has an oxide coating applied by the Azide process. The PM1HF was introduced in 1926 as an HF amplifier and also for use as a detector. The µ or amplification factor is 15.
No inter electrode capacitances are quoted for this battery triode valve. The filament would be run from a 2.0 Volt wet accumulator.
The identification and basic operating details are etched into the dome.
Inside the triode is mounted horizontally. The grid wires extend to the outside of the bright box anode.
One filament support with the wire trapped in a fold and the one side of the grid support welded to the rods attached to the pinch.
The V of the filament is held in a hook fashioned from a bend in the support rod.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 77 mm tall.
References: 1931 data-sheet, Data & 1040. Type SS210HF was first introduced in 1928. See also1928 adverts.
