English: Some of the first AM vacuum tube radio transmitters using the first amplifying vacuum tube, the triode or Audion, built around 1916 by the inventor of the Audion, Lee De Forest. Before the Audion, spark gap transmitters dominated radio, which couldn't transmit sound but instead transmitted information by wireless telegraphy, spelling out text messages in Morse code. The Audion, invented in 1906, was the first simple device which could produce continuous waves which could be amplitude modulated to carry sound. The amplifying and oscillation ability of the Audion was not recognized until 1913, so these are some of the earliest AM transmitters. De Forest used the trade name "Oscillion" for the larger tubes like these used for transmitters.

The Audion tubes are visible mounted on the front of the units (in the center unit they are inside) so that the operator can see if the filament is glowing and adjust the filament current by brightness. Audions were always mounted upside down, with the delicate wire filament loop inside the tube hanging down, so when it got hot it wouldn't sag and touch the grid. The lefthand set is equipped with the standard telephone style carbon microphone which was used in those days. The multiposition switches visible on the front panels adjusted the plate voltage.

Caption: "Recent wireless telephone apparatus developed by Dr. Lee De Forest, employing the Oscillion or oscillating-valve type of generator"