The Marconi branded M-OV DET12 is a VHF transmitting triode. The data-sheet is of the similar 834.
The anode dissipation was 50 Watts.
The graphite anode connects to one top pin and the control grid to the other. The paper sticker identifies the valve as having been part of BBC stock in 1957 and that it was supplied on 23/07/1943.
The large Marconi logo. The milky colour of the getter tells us that the vacuum has failed.
Looking into the base of the graphite anode the helical filament and control grid can be seen.
A different view of the filament and control grid. The bright filament is thoriated tungsten wire. The grid wire is corroded.
The top connections. The filament top support can be seen and the helical filament attached to it.
The balloon envelope is 64 mm in diameter and, excluding the B4 base pins, is 140 mm tall.