The dynatron was a specialized vacuum tube invented in 1918 by Albert Hull at General Electric laboratories for use as an electronic oscillator to produce radio waves. It has three electrodes: a thermionic cathode consisting of a heated filament, an anode consisting of a perforated plate, and a supplementary anode or plate. Due to the phenomenon of secondary emission of electrons from the plate, its characteristic curves have a region exhibiting negative resistance, and it was designed to exploit this property to excite oscillations in a resonant circuit, a mechanism called a dynatron oscillator.
Other early vacuum tubes with four or more electrodes not designed for the purpose had significant secondary emission from the anode, and, when operated with the anode at a lower voltage than another electrode, exhibited negative resistance and could be used as oscillators or for other functions. Later tubes had anodes treated to reduce secondary emission, normally an unwanted phenomenon, and were not suitable as dynatron devices.
In dynatron operation the supplementary anode is maintained at a lower positive voltage than the perforated anode. The secondary emission of electrons from the supplementary anode makes the dynatron behave as a true negative resistance, allowing it to be used to generate oscillations over a wide range of frequencies or as an amplifier. When a control grid was added between the cathode and the perforated anode, the device was called a "pliodynatron", with similar properties to a tetrode or pentode tube with significant secondary emission operated in dynatron mode.
Operation with a tube not designed specifically as a dynatron is described in the article on the dynatron oscillator.
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