The exhibit is marked as RAF VT.1A and also Royal 'Ediswan' Valve 'A'. The reference is 1120 and the serial number is 1458. These transmitting valves were made for the RAF by both Ediswan and Mullard.
This is an early transmitting valve and is based on the R Type design. The valve electrodes are built on the Ediswan four point pinch seal and the bright metal base shell indicates an early production version of the design. Later versions employed Bakelite bases.
We have no operating details on this valve but a good guess is a six volt filament and 400 to 1,000 volts on the anode. The anode dissipation looks to be limited to a few Watts.
The plated base has the A embossed to denote the anode on the opposite side of the G as seen in the main picture.
The construction is like the R type with the exception of the filament. The bright emitter filament is a Tungsten wire that is corrugated along its length. This effectively increases the length of the filament and consequently the emissive area.
The grid within the anode cylinder is a helix but this example has a pronounced sag.
The concentric electrodes are firmly fixed into the four point glass pinch.
A close-up of the inner structure. The filament is much thicker wire than the receiving types. This is to allow for greater heating current and thus greater anode current. These early transmitting valves employed anode voltages up to 1,000 Volts.
This image has been enhanced to reveal the lettering etched into the envelope.
This view shows the top of the filament in the centre and on the right, within the anode, can be seen the grid helix fixed to the two horizontal supports by a wire that binds the grid to the supports. The supports themselves are held to the main support by being clamped.
A close-up view of the grid and filament.
Here the sinusoidal shape of the filament is revealed. The anode supports are welded to the anode where the folded ends of the anode sheet meet.
The balloon envelope is 50 mm in diameter, and excluding the B4 base pins is 93 mm tall.
Reference: 1003. Type VT.1A was first introduced in 1919.