The history of vacuum tubes in synthesizers goes all the way back to 1939: the Hammond Novachord, one of the first polyphonic synths ever commercially manufactured, relied on an array of 163 vacuum tubes to produce sound.
Tubes are expensive, power-hungry and unreliable in comparison to solid-state electronics, so their use in synthesizers never quite caught on, despite the warmth, character and organic texture that's characteristic of tube-based gear. In the modern era, only a handful of synths have incorporated vacuum tubes into their (monophonic) design: namely the Knif Audio Knifonium, the Erica Synths Fusion System II and Korg Volca Nubass.
