The 3A/107B is a directly heated triode based on the 4019B. The old GPO code is POVT75 and the Army code was AR11 the valve was later specified as the CV1655 for the UK Government.
The 3A/107B was designed for use in telephone repeaters.
Above the box anode are the three filament tensioning springs. The filament passes through the grid six times.
Top view of the springs.
Here the oxide coated filament and control grid windings can be seen.
The classic envelope is 44 mm in diameter, and excluding the B4 base pins is 101 mm tall.
Reference: Data-sheet. Type 3A/107B was first introduced in 1947. See also1947 adverts.
