The HL2 is a dull emitter battery triode. It was designed as a detector in broadcast receivers, in the early 1930s. This exhibit is made by Lissen and the gettering obscures part of the construction.
The close spaced control grid can be seen at the base of the anode. At the top of the anode the filament top can be seen together with the helical tension spring and integral hook.
The identification is printed on the dome together with the Lissen logo and Made in England.
The silver gettering acting as a mirror and reflecting the camera lens entering the light tent.
The balloon envelope is 40 mm in diameter and, excluding the B4 base pins, is 92 mm tall.
References: Data-sheet & 1040 Type HL2 was first introduced in 1931. See also1931 adverts.
