Re: A few definitions _ Overdrive, Boost, Fuzz, Distortion

Posted by Frank_NH on Oct 12, 2014; 1:40am
URL: http://guitar-fx-layouts.238.s1.nabble.com/A-few-definitions-Overdrive-Boost-Fuzz-Distortion-tp13960p14238.html

Paul,

You have your switch label backwards, I believe.  Swap it around and you should be good to go.

Here's how to use it.

(1) Insert the JFET, minding the pinout.
(2) Set up your DMM with the clips and turn it on.  Set it to read DC volts.
(3) Switch the tester to the Idss mode.
(4) Turn on the power, and look at your DMM.  It should read a voltage like 0.0656 V (or 65.6 mV) for a J201 (it will be higher for other JFETs).  Multiply this voltage number by 10 and that is you Idss in mA.  In the example we would have 0.0656 x 10 = 0.656 mA.  Record this value (spreadsheets are great for this!).
(5) Flip the switch to the Vp mode.
(6) Read the voltage.  Let say this is 0.93 V.  This is the Vgs(off) value (though the Fetzer article still uses Vp and I use that designation too) - it is really a negative number for biasing applications, so I record this as -0.93 V.  Again this voltage will be higher for other JFETs.  To get a feel for the numbers, look at the end of the Fetzer article, where they list several JFET types and the typical ranges of Idss and Vp.

Go ahead try some JFETs and let us know what numbers you get.  Good luck.