The 3A/107A is a telephone repeater triode released in 1947 as a replacement valve for use in older equipment. It was not intended for use in new equipment.
The bright anode with the control grid extending above it. The coated filament wires are held in hooks at the end of the tension springs that appear at the top.
A closer view of the oxide coating on the filament wires and also the shape of the hooks. On the glass is the CV Type designation of CV249.
The classic envelope is 44 mm in diameter, and excluding the UX4 base pins is 100 mm tall.
Reference: Data-sheet. Type 3A/107A was first introduced in 1947. See also1947 adverts.
