Jfet Clipping Fail

[Chris in Idaho]

Hello Smart People,
I recently (as of 4am yesterday morning) completed a TS808 type pedal using the PCB from GGG with sockets for the clipping diodes.  It fired right up and works great so I decided to start experimenting...  I searched and found a few threads about using Fets as clipping diodes so I pulled a couple of MPF102's out of the bag from smallbear and soldered the Gate and Source together and plugged them into the sockets.  Then I donned headphones (using the guitarport tonight) plugged in the paul and... it's really clean and quiet.  With all the controls maxed the pedal produces considerably less than unity gain.  I've tried reversing the polarity, I've tried running the Jfet in one position and the 1n914 in the other and nothing changes my quiet lifeless cleanness. 
So, I read back over the threads that I read before and most of them seem to be running a diode in series with the Jfet as one big happy clipper.  Do I need to do that to make the Jfet turn "on" because of some voltage thing or am I completely barking up the wrong tree with this whole idea?

PS Green pedals are pretty.

[twabelljr]

Did you find this thread?: http://www.diystompboxes.com/smfforum/index.php?topic=76337.0
If not, I hope it helps. Have fun!
The last poster mentions tying the Drain and Source together. Try it with jumper wires and test them with meter on diode test to see what the forward voltage is. Measure the ones you made too for comparison.

[Chris in Idaho]

HHmmm...

I re read a big long thread about doing this with a MOSFET and I picked up something I missed before:

R.G.:
There is a problem here; the reverse/substrate diode prevents this "diode" from blocking in the reverse direction. In one direction it electronically looks like the amplified-diode MOSFET you want, and in the other it's an ordinary silicon diode. You can use an external diode in series with the drain or source to block the substrate diode from conducting, or if you're using two MOSFETs, you can connect them so the substrate diodes themselves are what allow current flow in the "right" directions each way.

I think maybe my Jfets don't block current in the reverse direction so instead of current going through one and clipping it goes backward through the other... and doesn't clip.

I'll try putting a regular diode in series with the Jfet and if that doesn't work I'll go get a couple MOSFETs as a last resort... and if that doesn't work I'll put the screamer on ebay lol.

[Chris in Idaho]

HHmm I also just found this: http://webcache.googleusercontent.com/search?q=cache:10soRz0ri7gJ:www.nhn.ou.edu/~bumm/ELAB/Lect_Notes/BJT_FET_transitors_v1_1.html+jfet+as+diode&cd=1&hl=en&ct=clnk&gl=us&client=firefox-a&source=www.google.com

Which has this:



And says this:

JFET Diode.  The JET pn gate junction can be used as a diode by connecting the source and the drain terminals.  This is done if very low reverse leakage currents are required.  The leakage current is very low because the reverse leakage current scales with the gate area.  Small gate areas are designed into JFETs because it decreases the gate-source and the gate-drain capacitances