File:Audion tube.jpg

English: A De Forest audion tube, the first electron tube that could amplify. About 2 in. (5 cm) diameter. The first triode, or tube with three electrodes, a filament, grid, and plate, the audion was invented in 1906 by American engineer Lee De Forest. This tube originally had two filaments located in the center of the tube, so that when one burned out the "spare" could be used, but in this tube both are burned out; only the ends of the filament wires are left. The two connected grids (zigzag wires) are visible on either side of the filament, and the two plates are outside the grids. The filament was connected to the screw terminal at bottom, while the grid and plate connections were brought out to wires at the top. Manufactured for a few years beginning about 1910, the residual air left in these early audion tubes due to inadequate evacuation created ions that caused the tube to function erratically, and shortened the life of the filament. This is evidenced by the dark deposit of material on the inside of the glass envelope, sputtered off the filament by ion bombardment. This version, which was given the trade name "ultraudion", was designed for use in regenerative receivers.