The HL2 is a dull emitter battery triode. It was designed as a detector in broadcast receivers, in the early 1930s.
The box dates this exhibit to 1947 and it looks to be a replacement type, The HL2 in 1947 would not be used for new equipment.
The bright anode end elevation. The single folded sheet can be seen at the point where the ends meet.
The thin sheet anode is ribbed for greater mechanical rigidity. The coated filament is an inverted V and the filament tension spring can be seen above the top mica.
A diagram of the construction.
The classic style envelope is 28 mm in diameter and, excluding the B4 base pins, is 71 mm tall.
References: Data-sheet & 1040 Type HL2 was first introduced in 1931. See also1931 adverts.
