The BTH B3 is a dull emitter triode designed for general purpose applications. The electrode assembly is vertically mounted and the filament is a single strand of thoriated tungsten wire. Our exhibit has the filament intact and powers up to a bright orange glow.
The single strand filament seen passing through the centre of the grid spiral. The top end is sprung to maintain tension when operating.
The dome carries the identification lettering and logos etched into the glass.
The BBC logo is not of the early stamp variety and this suggests that this exhibit dates from no later than late 1927.
The electrode assembly. The pinch is on the left with red platinium seals and the grid is attached one side to a support rod. The arm at the top of the image is the support for the filament tension spring. The latter can be seen attached to the top of the support.
In this image the filament can be seen passing through the grid.
The balloon envelope is 37 mm in diameter, and excluding the B4 base pins is 88 mm tall.
References: 1003 & 1004. Type B3 was first introduced in 1924.