The Thorpe K.1 (K1) is a bright emitter triode from 1934. The feature of the Thorpe valves was the helical wire anode rather than a solid cylinder.
The Lettering is within an ellipse and has Thorpe Radio Valve at the top, Thorpe K.1 in the centre and London at the bottom.
The BBC stamp is the first one with the words Type Approved By Postmaster General. This stamp was used from 1st November 1922 until September 1924 which fits with the valve's introduction in 1924.
The horizontal electrodes above the pinch with the lettering on either side of the bulb.
The filament is intact and the three electrodes are mounted concentrically. The B4 base pins are of the simple spit pin variety.
A close-up of the electrodes. The filament is corrugated and there is no additional tensioning spring.
Here the far end of the grid is in focus and the filament is hidden as it runs straight between the supports.
The balloon envelope is 50 mm in diameter and, excluding the B4 base pins, is 100 mm tall.
Reference: Adverts & Data. Type K.1 was first introduced in 1924. See also1924 adverts.
