The Audion Triode Amplifier

Lee de Forest
The Triode amplifier was invented in 1906 by Lee de Forest. It improved on earlier designs like the Fleming valve by adding a grid with a charge in addition to the two diodes. By 1912 the amplifying properties of the triode had been fully fleshed out and triodes and various iterations of them, became ubiquitous (All About Circuits: The Triode).
The way the triode worked was by manipulating the current flowing between the cathode and anode. It did this by adjusting the voltage of the grid between the ends of the diode. As electrons flowed from the negatively charged cathode to the positively charged anode, their speed could be adjusted by either running a negative or positive current through the grid. In audio terminology this allowed for the amplification of an audio "signal" to several times it’s original. In order for this to work efficiently the triode had to be evacuated of all air to allow for the flow of electrons mostly unimpeded. These “vacuum tubes” dominated nearly all audio applications in recording, playback, and performance until they were later replaced by the transistor in the 1970’s (Sarkar, 2006).
The combination of the ribbon microphone and an active amplifier like the triode revolutionized the recording process. Earlier, purely analog recording suffered from a number of problems, namely the quality and strength of the audio signal. However with the ability to vary the amplification (gain) of an electric signal during the recording process, recordings could capture a much broader and far more dynamic range of sounds (Sterne, 2003).
Sarkar, Tapan. History of Wireless. Hoboken, N.J.: Wiley-Interscience, 2006. 355.

"The Triode." : Electron Tubes. Accessed December 3, 2015.

Sterne, Jonathan. The Audible Past: Cultural Origins of Sound Reproduction. Durham: Duke University Press, 2003.