The 3V4 audio output pentode was designed for use in battery portable radio receivers. It features a centre tapped filament, and was designed for operation with equal voltages on screen grid and anode.
The bright nickel anode box is rectangular in cross section and obscures the grids. The filament is a pair of single strand loops and thereby passes through the grids four times. Tension for the filament is maintained by a spring at the top of the bulb and this is also the centre tap connection. At the base end two filament strands are connected to each connection.
The filament supports are held on the bar that connects the two halves of the beam plates.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040. Type 3V4 was first introduced in 1946. See also1946 adverts.