Rob, I agree with your conclusions. The sensitivity analisys around Idss and Vp for Rds is conclusive. Thanks for taking the time to point this.
Considering what you mention, a simpler, efficient and effective way to go would be to use a 1Mohm resistor instead of the 10 Mohm, and omit the OpAmp entirely and measure directly with a decent DVM. This greatly simplifies the matching fixture to: one resistor, one 9V battery, and a DVM.
As additional information, last night I ended up using a 6 Mohm resistor on my matching fixture (5 x 1.2 Mohm, since I didn't have anything higher available at home). The voltage difference in Vp when measuring directly across the resistor string or the OpAmp output was consistently 20 mV (like an offset). So in practice this won't affect matching since it is identical for all devices tested. This value should be even lower when using a 1 Mohm resistor. Vor Vp=-3V, the current through the JFET will be 3V/1Mohm = 3 uA.
On another matter, it is really hard to set the LFO offset, since tiny variations on the lower (most negative) voltage greatly affect the sound. The resistance relation of the JFET is highly nonlinear, so I'm moving to study LDR's for now.
Another important hing to notice is that according to this new light, there are two additional matched pairs in my batch: 8 & 9, and 10 & 12. It is sad to see there are no good candidates for a triplet, not to say a quad. o building a Phase 90 would require actually 50 or more JFETS to choose from to get a good matched set.
Regards,
STM