Been working on this for a couple of days, got fairly decent sounds going this morning!
NE5532 triangle wave LFO goes into a VCA based on Ray Wilson’s design from Make: Analog Synthesizers (TL072 and LM13700).
I have some more tweaks to go, the LFO speed or rather, range can be tweaked a lot by replacing a capacitor. In a build, I could use a capacitor to switch between ranges – in the video we can hear pretty slow modulation going on, but weird effects could be generated by switching to a small capacitor and modulating a high frequencies. The LFO is a simple switch away from being a VCO as well 😀