AT40 (Army Transmitting) balloon Marconi-Osram pip top – AT40X long bulb Marconi Osram helix grid circa 1920s

Description

Marconi Osram AT40 balloon valves you see on the left stand  for Army Transmitting valve with typical filament of 7 Volts .

The early stage of technology saw most of these valves based on the “R” type, especially the helix grid of the pair AT40X you see on the right with pip/horns. Both pair of valves are dated from 1921 onward.

Tested and working