Cold Cathode Valves and Air-Cooled Kenotrons


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                   Kenotrons are vacuum rectifying hot-cathode valves of the diode type, used until the Seventies in the high tension circuits of x-ray tubes. The first Kenotron was introduced by Saul Dushman in 1914 (the K3 of General Electric, put on the market  years later). 


                  Early kenotrons were of a remarkably large size in order to ascertain adequate cooling in the ambient air. Later makes were fitted, like x-ray tubes, with some sort of metal heat radiator fixed to the anode end of the tube. But finally, like x-ray tubes, kenotrons became smaller in their external size and were oil-immersed in the high-tension transformer tank of the x-ray unit.


                  In the late sixties and during the seventies solid state semiconductor rectifiers replaced kenotrons in high tension generators. Most modern x-ray units are presently equipped with electronically controlled high frequency generators.


                  Prior to the invention of the kenotron in 1914, rectifier valves were of the cold-cathode (filament-less) type. Four of them are presented in this series.



A Prophecy :  In his book “Textbook of Radiolgy”, 1918, p.62 , in the chapter about valves, Edward Reginald Morton wrote: “…. It will, of course, be understood that if we had a purely unidirectional supply of current for our tube, no rectifying device would be necessary……..Such a supply is not only possible but more or less easily available, and in all probability valve tubes and such devices will be relegated to the museum of X-ray antiquities”.


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