![]() ![]() This additional control grid modulates the current that flows between cathode and anode. In 1906 Lee De Forest added a third electrode and invented the first electronic amplifying device, the triode, which he named the Audion. The diode conducted electricity in one direction only and was used as a radio detector and a rectifier. The simplest valve (named diode because it had two electrodes) was invented by John Ambrose Fleming while working for the Marconi Company in London in 1904. Until the invention of the transistor in 1947, most practical high-frequency electronic amplifiers were made using thermionic valves. Valve amplifiers can be used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, high quality stereo amplifiers, military applications (such as radar) and very high power radio and UHF television transmitters. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers in the 1960s and 1970s.
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