The CV5331 is a high slope VHF double triode. There is no maker's identification but as it was made in Holland and has the font used by Mullard we can be sure of the originator.
The low operating voltage suggests that this valve was expected to operate in cascode.
See alsoECC189 for the directly equivalent commercial version.
The anode has a deep depression at the centre as the working face is a small rectangle and the rest is heat sinking.
Looking into the anode pressing. The cathode tube has a flat face and rounded ends. The heater is formed as a hairpin of insulated coiled wire.
The central screen.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins, is 48 mm tall.
References: Data-sheet & 1040. Type CV5331 was first introduced in 1960. See also1960 adverts.
