External grid
External grid
Pre-1920 DIY external-grid triode from a car light bulb and a mustard-pot.
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Early external grid tubes
Probably the very early external grid vacuum tube is the one described in the Marconi and Round British Patent 13247/14, May 29, 1914. Here is a drawing of the tube:
Even more weird is the arrangement of electrodes proposed by Western Electric for some vacuum tube amplifiers. Here the grid is the inner cylinder and the filament is wound around it, insulated by a layer of nickelous oxide, as shown in the figure below.
Source: Thermionic Tubes in Radio Telegraphy and Telephony by John Scott-Taggart, The Wireless Press Ltd., 1921.
Regards, Emilio
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