The 3A/108A is a telephone repeater triode. The design dates from 1947 and it was made for replacement purposes. The 3A/108B is the same valve but fitted with a B5 base cap.
The valve is also marked as a CV1653.
The anode is bright as this is a low power device. The cathode is an oxide coated filament.
The top loops of the filament are held taught by the top mounted springs.
Looking down through the top mica to the twin hairpins of the filament and close wound control grid.
The classic envelope is 44 mm in diameter, and excluding the American 4-pin bayonet base pins is 105 mm tall.
Reference: Data-sheet. Type 3A/108A was first introduced in 1947. See also1947 adverts.
