The LS5A from M-OV was a dull emitter valve designed for audio amplification. The LS series were much used by the Post Office for telephone repeater amplifiers. SeePOVT25.
The LS5A was introduced in 1924 and has a lower impedance than the original LS5, and so could produce more output power. Later versions of the LS5A moved to a Bakelite base with the seal concealed within.
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.
An almost perfect etched type one BBC stamp. This dates the valve to between November 1922 and September 1924.
The Marconi Osram Valve logo etched into the other side of the envelope.
The nickel base cap with the A for anode stamped in it. The filament tension spring is seen on the right.
Unlike the LS5 the grid is shaped to pass flatter across the filament. Sadly the filament inverted V has broken.
The pinch end with another view of the shaped grid winding.
The balloon envelope is 56 mm in diameter and, excluding the B4 base pins, is 107 mm tall.
References: Data-sheet & 1005. Type LS5A was first introduced in 1924.