<Bzzzt!> Incorrect! We were looking for the vintage device.
Using a mono-block amplifier as mains power for another amplifier - Page 2
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Actually, a DC-300 and an HP sinewave oscillator was a pretty common method. There was another device, though, made for the purpose.
Pacific Recorders Multisync MDA-1B. All in one capstan speed controller, worked with most synchronous capstan motors of the day. Sorry, couldn't find a picture. I think there was also a competing product, but those memory cells haven't been exercised in a long time.