The VT51 battery audio output pentode was capable of 1.1 Watts at 7% distortion. Originally designed as audio output pentodes these economical valves found numerous uses in portable battery-powered military transmitters.
This specimen is characteristically Cossor shaped and will be a Cossor Type 220PT. The Old Stores code is also on the glass. The VT indicates an RAF transmitting valve.
The label over the A M and Crown of the Air Ministry clearly states not for airborne use.
The Cossor name on the dome.
The main top mica has fingers to grip the glass and above it is a smaller shelf holding the helical filament tension springs. Our other Cossor VT51 has a cylindrical anode.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 96 mm tall.
References: Data-sheet & 1043. Type VT51 was first introduced in 1933.