The CV1664 (commercially the 3A/110A is a directly heated triode on the UV base that is called BC4 by the GPO and was designed for telephone repeater service.
The prototype was the 4022AR. The old GPO code is POVT88 and the Army code is AR13.
The KB/FE identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the FE identifies the maker as STC, Oldway (Additional to Paignton factory 'F').
The Commercial Type designation and the STC name.
As a low power valve the box anode is left bright.
Looking down on the valve the geometry and the filament tension springs are visible.
Face view of the top of the anode.
The classic envelope is 44 mm in diameter, and excluding the UV base pins is 103 mm tall.
