The PM1HF is 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.
The gettering and remains of the reduction of the filament coating to an active coating covers the top of the envelope.
Looking up from the base the bright anode can be seen to lie horizontally. No mica could be used as it would have been covered in the Azide residue.
The glass has the lettering 1HF and the BBC stamp below. As this third variant stamp was little used after 1927 we can assume that this exhibit dates to between 1926 and 1927.The balloon envelope is 40 mm in diameter and, excluding the B4 base pins, is 74 mm tall.References: 1931 data-sheet & 1043. Type PM1HF was first introduced in 1926. See also 1926 adverts. |
