The POVT39 is the M-OV LS5A fitted with a Shaw base. This bayonet base was much used in early telephone repeater systems. In general, the LS series were used by the Post Office for telephone repeater amplifiers. See alsoPOVT25.
The valve itself is a dull emitter valve designed for audio amplification. It was introduced in 1924 and has a lower impedance than the original LS5, and so could produce more output power.
In the mid 1920s the term 'dull emitter' was often used to cover a variety of production processes, all of which produced low temperature, hence dull, emission surfaces.
The etched lettering is feint but GPO (General Post Office) and VT No. 39 can be read. With several VT ranges of valves it is now accepted that the GPO valves are termed POVT.
Here is the GEC identification. Inside the anode is seen from the side.
The grid is wound on a single support with a round end to form two side supports. Here the filament is broken but the tension spring and hook are clear to see.
Face view: the grid supports are spot welded in two places to the struts held in the pinch.
The top of the grid winding and the supports for the grid assembly are taken to the side and held in a glass rod. The anode has thin wire spot welded to the side that is in turn spot welded to the support struts rising from the pinch.
The UV base pins with the copper wires soldered within grooves. The locating pin can be seen on the shell and the holes in the shell provide fixing points for the Bakelite insulator.
The balloon envelope is 56 mm in diameter and, excluding the UV base pins, is 118 mm tall.
References: Data-sheet, Data & 1005. Type POVT39 was first introduced in 1924. See also1924 adverts.
