Triode were invented, Philipp Lenard used the principle of grid control while conducting photoelectric experiments in 1902. The first vacuum tube used in radio was the thermionic diode or Fleming valve, invented by John Ambrose Fleming in 1904 as a detector for radio receivers. It was an evacuated glass bulb containing two electrodes, a heated filament and a plate (anode). Triodes came about in 1906 when American engineer Lee De Forest and Austrian physicist Robert von Lieben independently patented tubes that added a third electrode, a grid, between the filament and plate to control current. Von Lieben's partially-evacuated three-element tube, patented
European Triode Festival European Triode Festival Since 2000 each year the "European Triode Festival" (ETF) has been held in various venues in Europe (the first three events were called "Aarhus Triode Festival" aka "ATF"). The ETF focuses on do-it-yourself audio valve amplifiers using triode vacuum tubes, and brings together high end vacuum tube audiophiles from all over the world (from as far away as Finland and New Zealand). At the festival, participants demonstrate their (all home-built) equipment (analogue/digital/amplifiers/speakers/etc.) and discuss aspects of high-quality vacuum tube sound reproduction. The event is a private venture of in total 300 audio lovers, of which some 120