The CV3986 is a special quality sub miniature double triode. The commercial Type designation is the 6021 and this also appears on the envelope.
The KB/D identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the D identifies the maker as Mullard Radio Valve Co., Whyteleafe.
The twin triode sections side by side in a tube with an outside diameter of 9 mm.
The side of one anode shows the heater entering the cathode tube.
The thin glass tube envelope is 9 mm in diameter, and excluding the B8D/F base pins is 34 mm tall.
Reference: Data-sheet. Type CV3986 was first introduced in 1957. See also1957 adverts.
