The M-OV Type S215 was (we think) the very first 'upright' screen-grid valve to reach the British market. It was introduced a year or so later than the first screen grid valve, the M-OV S625. It may also have been a World First to achieve quantity production. Close on its heels were the essentially similar S610, S410 and Spoint8 types, differing only in filament ratings. This exhibit looks an early example and is complete with the screw top cap.
Although most domestic radio manufacturers didn't bother, professional users liked to mount these valves so that they poked through holes in the metal chassis in order to isolate the anode and grid circuits at the level of the internal metal 'skirt' connected to the screen grid. This skirt can just be seen in the photo. The wish to match external screening to the level of this skirt explains the slimline base cap and the characteristic bulb shape of these early screen grid types.
The azide process worked best with horizontal electrode assemblies and was not really suited to screen grid valves. Because of this the S215, although widely acclaimed and used, was not completely successful and was replaced by improved but otherwise similar oxide-coated types (eg, S23) once M-OV had developed the necessary processes.
The Osram label, the consumer brand of M-OV.
The screw top cap. This style of top cap was replaced by the metal 'top hat' in the mid 1930's. The American metal valves used the 'top hat' from the outset.
The balloon envelope is 43 mm in diameter and, excluding the B4 base pins, is 116 mm tall.
References: 1005 Type S215 was first introduced in 1929.