MARCONI ROUND 'CA' VALVE, 1913

First produced in 1913 the grid and anode connection are on the side, the tube at the top contains asbestos which when heated drive a gas into the body of the valve and thus the vacuum could be brought to a proper degree of softness and thereby restore the characteristic curve to it's normal shape. The soft vacuum three electrode valve intended as an amplifier was much superior to any which had preceded it. The third electrode was introduced by Lieben- Reisz (Germany) and Dr. Lee de Forest (USA).
The electrode consist of a nickel cylinder, nickel grid, and hairpin platinum filament coated with a mixture of barium and calcium oxides.

Nortel Collection

A1309