Dynatron

"For the brand, see Dynatron Radio Ltd"

The dedicated dynatron vacuum tube was invented by Albert Hull in 1918 [http://ieee.cincinnati.fuse.net/newsletters/200401.pdf] . It has three electrodes: a thermionic cathode, a perforated anode, and a supplementary anode or plate, and its characteristic curves have a region exhibiting negative resistance, which is the property desired.

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.

Operation

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.