Vacuum tubes ran the gamut in size: from behemoths used to power broadcasting transmitters to miniatures used in early hearing aids. Whether bulky or tiny, they could amplify sound and that gave AT&T engineers the idea to use them in the first PA systems. By the 1930s, AT&T was using vacuum tube amplifiers to pump up the music as well as the human voice, and by the 1950s they'd found a home in guitar amps. Without vacuum tube amplifiers we would never have had the hard-edged growl of the amplified electric guitar -- and the first 30 years of live rock and roll simply wouldn't have been possible.

  AT&T also pioneered the vacuum tube's replacement: the transistor.