The PM5B is a six volt battery triode for use in resistance/capacity coupled circuits. Both the original and improved versions were released in 1927. The improved version has a filament current draw of 0.075 Amp.
The blackened interior of the bulb is characteristic of the Azide process for forming an oxide coated filament.
The Type designation and the aerial, coil and ground logo.
The Mullard logo and Made in Great Britain.
The B4 base pins.
Thanks to Mark Mallison for sending the images.
The classic envelope is mm in diameter and, excluding the B7 base pins, is mm tall.
References: Data-sheet & Data. Type PM5B was first introduced in 1927. See also1927 adverts.
