Q About RG's JFET Matcher/Tester

[Paul Marossy]

I built this a few days ago, have it set up to test JFETs with the two common pinouts. Anyhow, I popped some J201s in there just for fun, and my DMM was reading voltages in the millivolt range (my readings were like .171V, .235V, etc.), not the 2-3 volts mentioned on RG's page. Did I do something wrong, or did I miss something on that page?

[Mark F]

When I first breadboarded the JFET Matcher I was getting the same sort of readings you mention. The problem with mine was the 10-47uF cap in the voltage divider was bad. I replaced it with a good cap and it worked fine. That solved my problem. Also, as I'm sure you know, JFETS vary all over the place. That being the reason for the circuit. Try some different series of JFETS and see if your readings vary in any meaningful way. Hope this helps

[brett]

Sounds ok.
J201s have very low Vgs (and very high transconductance).  Your typical 2N5458s etc will test more like 1 to 2V.  MPF102s often test 2 to 4V.

[Paul Marossy]

A bad cap, huh? Well, I guess I could try replacing it and see how it works. I guess I should try a few different kinds of JFETs, too.  :oops:

[Paul Marossy]

Thanks Brett. Maybe it works after all. I'll try it with some other FETs and see what I read.

[R.G.]

Nota Bene: J201's are specified with Vgsoff of as little as -0.10V. Those could be good readings for a J201.

J201's are not necessarily what you want for variable resistance duty, because the Vgs off is so small. Any LFO driving them has to have a max size less than the Vgsoff of the device.

[Paul Marossy]

Hmm... that's interesting. That confirms my suspicion that it had to do with the J201s. I'm going to try some other FETs out tomorrow evening.

Incidentally, I have some J201s in my Phase 45 and it seems to sound just fine. Maybe I got lucky? I suppose I ought to stick some other FETs in there and see if there is any difference in the way it sounds...

[Lonestarjohnny]

Paul, i've been just swapping a few jfet's around and rebaising them and sometime's i stumble on a nice combo of fet's that really sound good together, funny thing is i have'nt found any bad sounding, but some do seem to work better.
johnny

[Paul Marossy]

Well, yeah, they all sound good, but as far as LFOs go...
I think I just got lucky on the Phase 45.