The NR15A is a British Navy triode receiving valve. Introduced in 1924 it was the first naval valve to have an oxide coated filament. Our other NR15A was made by the Azide process this clear envelope shows that the oxide coating on the filament was made by the modern paste coated technique.
With a little help from Photoshop the lettering is revealed in false colour within the M-OV ellipse.
The attached paper label identifies the valve as a PM40X. We have no information on such a Mullard valve.
The older construction of using a glass rod to hold the tension springs remains although without the Barium splashes of the Azide process mica discs could have been used.
A closer view.
Hybrid construction with a mica sheet holding the top supports of the anode and control grid.
The balloon envelope is 40 mm in diameter and, excluding the B4 base pins, is 85 mm tall.
References: Data-sheet, 1043 & private communication. Type NR15A was first introduced in 1924. See also1924 adverts.
