While I was rummaging around in some boxes of junk recently i found an uncessesful version of a diode based VCF that i built on veroboard sometime in the early 80's. The date on the opamp is 1979 but it wouldn't have been that long after that! It never worked and there were a lot of mistakes in the translation to veroboard. It was based on a design in the magazine Practical Electronics. In turn that was based fairly heavily on the EMS version of the VCF that was in the VCS family of synthesizers. Forbin will have a version of this filter from Yusynth - the PCB is pretty much populated and now i just need to try and design a front panel for it!