Correcting 2 previous definitions of electron tube used in guitar amps and high end audio components: The basic elements of electron tubes are correctly described in #6, but the writer is somewhat mistaken in saying that heat is not converted into electricity. In fact the role of the heater is to heat the cathode to incandescence so that under sufficient local field strength (voltage, e.g.) it emits electrons which travel to the plate. The control element(s) (grids) between cathode and plate control the flow of electrons by increasing or decreasing the potential near the cathode. The grids are porous so electrons can pass thru them on the way to the plate, which is maintained at a high positive potential (100-300 volts or more) relative to the cathode so as to attract the electrons. The signal to be amplified is applied to one of the grids, such that its small current variations are greatly amplified by its effect on the emission of electrons from the cathode. Elements other than the cathode are not intentionally heated, their temperature is not critical to the proper operation of the tube.
by Dale Meyn April 6, 2007
Get the Tubemug. 
by brandabur August 27, 2016
Get the tubesmug. by Jewmangi August 11, 2008
Get the Tubingmug. by skd2488 November 7, 2007
Get the tubemug.