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
Valve transmitters circuit. This circuit design has been used at 1296 MHz using disk seal triode valves such as the 2C39A. The grid is kept at ground, the drive is applied to the cathode through a capacitor. The heater supply must be isolated with great care from the cathode as unlike the other designs the cathode is not connected to RF ground. The cathode may be at the same DC potential as the grid if a valve such as the 811A (zero bias triode) is used, otherwise the cathode must be positive with respect to the grid to provide proper bias. This