Grid tubes
Microwave Electron Tubes – Technologies and gridded tubes

The triode concept – the prototype of the grid tube – is an old one. It was in January 1907 that Lee de Forest, an independent American researcher, took out the first patent. He named his device Audion, at a time when the amplification of sound signals for telephony was mobilizing the industry. The patent was assigned to AT & T (Western Electric) in 1920, who shared the rights with GE (RCA) for the development of the TSF and telephone, and de Forest reinvested his fortune ($400,000) to develop (with mixed success as far as he was concerned) talking pictures