Amplifier by their physical placement in the signal chain; a preamplifier may precede other signal processing stages, for example. The first practical electrical device which could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912. Today most amplifiers use transistors. The first practical device that could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912. Vacuum tubes were used in almost all amplifiers until the 1960s–1970s when the transistor, invented in 1947, replaced them. Today, most amplifiers
Valve audio amplifier pentodes or beam tetrodes, 6L6 or EL34 but occasionally KT88, 6550, or the lower-power EL84 in "Ultra-Linear" connection. The output stage is preceded by a voltage amplification stage (pentode or twin triode) and a phase-splitter (twin triode). Twin triodes with two identical sections in one envelope are used, usually the noval types 12AT7, 12AU7, or 12AX7 or equivalents, less usually the octal 6SN7. The earliest mass usage of valve audio amplifiers was for telephony. Valve amplifiers were critical in development of long-distance telephone circuits and submarine telephone cables. Radio applications followed soon after, where valves were used for both the