This valve is marked both CV430 and STC 29C1. It is a directly heated diode with an inverted 'W' single strand filament.
The KB/F identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the F identifies the maker as STC, Paignton.
John Rehwinkel has supplied data on this exhibit. It is a temperature saturated diode and it was used to measure and regulate voltages. The method was to place the voltage to be measured across the filament and monitoring the anode current. Tektronix used a 2AS15 for the regulation of filament current in some oscilloscopes.
The CV identification and KB/F reference.
Side view of the anode.
The two helical filament tension springs each have a hook holding the top of a filament loop. The filament looks to be uncoated and passes four times through the anode.
Another example with blue lettering.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins, is 57 mm tall.
