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Automotive vacuum tube amplifier.
Many utilized "vibrator" type power supplies, in which a relay or other mechanical switching element generated AC voltage which was subsequently transformed and rectified to yield the high DC voltages required to operate vacuum tubes.
Vacuum tube radios were used in cars until being displaced by transistorized radios for reasons of cost, heat, size, and efficiency (but not sound quality). Note that vacuum tube amplifiers are still renowned by musicians for their subjective warmth and natural sound quality.
Curiously, transistors themselves were ultimately utilized as the switching element (to replace mechanical relays) with the advent of "switch mode power supplies" (SMPS, trademarked by Motorola). This allowed the now familiar, more efficient and much higher power car audio amplifiers, including the world's first modern automotive vacuum tube amplifier that uses vacuum tubes for all audio amplification and transistors in the power supply, produced since 1986 by Milbert Amplifiers website (http://milbert.com)
Several other companies have since tried to produce "tube car amps," including Blade (of Canada) and Manley (of pro audio fame), and some others produce "hybrid" designs in which "token tubes" are used for voltage amplification and/or feedback but power audio amplification is still performed by bipolar or MOSFET transistors. (see butler audio, trutechnology, others).
Also notable in pro audio is Sear Sound Studios website (http://searsound.com).
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