For bit of nostalgia I had a rummage and managed to find my old FET tester. It looks an ugly mess but this thing is old... I made this in my student days as a quick and dirty tester for J310, 2N3819, 40673 and BF981 devices.
The tester contains a 7.68MHz crystal oscillator and the idea is that the FET under test is a buffer amplifier after the oscillator and the tester measure the Gm at 7.68MHz using a diode detector.
I can't remember how I calibrated it but it was probably with my old Tek 585 scope. With a 4 pin socket it could also test the classic 40673 MOSFET and a BF981 could be tested by placing/pressing it on top of the 4 test pins that are around the test socket. Not the best arrangement but it was quick and effective.
It was at its best when selecting J310 or 2N3819 JFETs for similar (or highest) Gm and it could also spot any duds really easily. I haven't used it for many years but it still has the little piece of paper inside with the FET pinouts.
It looks a mess but this was made in a hurry on a shoestring using salvaged parts from an old CB radio and an old tobacco tin
