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Maths is worth the wait, especially as a complex modulation source for the Rainmaker. Maybe poke around on eBay, or the FS listings here or on Muff Wiggler.

Yeah, the Double Helix is actually a good way to get into Buchla-esque territory. Like most complex VCOs, it's got a bunch of useful functions behind the panel, and ample points on front to mess with it. I also like the gate-like setup on the right side, where you can alter dynamics along with timbre via envelopes or just plain pulse signals.