The 50A5 is an output beam tetrode that has an output of up to 4.3 Watts. It was designed for use on a series heater chain of 150 mA and consequently for low anode voltages.
The design is American but this is a UK product. The Brimar name stands for British American Range.
The Loctal bas showing the key on the central spigot. Originally in the UK Loctal was designated as B8G but later changed to B8B. The precise specifications may vary but the valves are effectively the same.
Thanks to Mark Mallison for sending the images.
The wide glass tube envelope is mm in diameter, and excluding the B8B base pins is mm tall.
References: Data-sheet, Data, 1040 & 1043. Type 50A5 was first introduced in 1947. See also1947 adverts.
