The DE2 is a dull emitter triode. Rubon were a London based manufacturer that was active only during 1924. This exhibit carries the type two BBC stamp that was in use from September 1924. And thus this valve dates to the last quarter of 1924.
The type two BBC stamp. The outer words are Rubon Puratone Valve. The operating conditions are carried on the box and on the paper label around the composition base cap.
The gettering obscures the inner electrodes.
With the filament energised the internal structure can be seen through the silver gettering.
In close-up the anode can be seen as a wire mesh. It is flattened on one side and this is where it attaches to the support. The grid helix is mounted quite close to the anode mesh. The filament is a single strand of thoriated tungsten wire.
The base pins are of simple split pin construction. The red box has the letter M within.
The wide glass tube envelope is 29 mm in diameter, and excluding the B4 base pins is 90 mm tall.
Reference: 1003. Type DE2 was first introduced in 1924.