Re: Diesel Diefet - the quest: Weird tone stack response

Posted by Frank_NH on Apr 26, 2015; 1:17pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/Diesel-Diefet-the-quest-Weird-tone-stack-response-tp19847p19890.html

You can substitute different jfets but be aware that because the Vp and Idss are quite different between the j201 and the pn4393, the gains will be different as well.  This particular amp sim is basically a bunch of common source jfet stages in series with some post gain tone shaping.  Using my jfet spreadsheet, with a j201, the first stage has a gain of about 27!  (Using a pn4393 cuts this gain in half to about 15).  There are two more stages after the gain control, so as you can imagine, it doesn't take long before the signal is clipped - a lot.  When you hear a "farty" low end, my theory is that the bass frequencies are getting too much gain and are clipping.  One way to control this is to reduce the source bypass cap values - the three 1 uF caps at the bottom of the layout.  You could even eliminate them, but the that would reduce the gain/stage by a factor of four.  What has worked for me in an experiment I did a while back is to remove the source bypass cap in the first stage only (in this case the 1 uF cap on bottom, left side).  This reduces the gain going into the subsequent stages (where there is some tone shaping going on), and so the tendency to have a "farty" low end.