replacing a MOSFET gain stage with a JFET

[mordechai]

I am thinking of toying with a circuit that currently has a MOSFET pushing a JFET to produce the overdrive, ROG's excellent "Peppermill" circuit.  I am going to put a modified booster in front of the circuit as an initial gain stage, so I want to replace the MOSFET with a JFET, as I think that the additional gain stage will be sufficient to use a JFET/JFET combination and still get a decent overdrive.  What additional changes to do you recommend to help facilitate this modification and the additional gain stage at the front of the circuit?  Do the two 1M and single 1K resistors surrounding the MOSFET in the original circuit require alteration?  Is there still a need for the red LED connecting the 1M/1K to ground? 

[Meanderthal]

 I would replace the whole mosfet stage with a fetzer circuit or Tillman preamp or some kind of gain stage designed specifically for jfet, because it won't bias up if you just drop it right in place of the mosfet. Different animal. 3 jfet gain stages will overdrive nicely...

[Meanderthal]

Have a look at this one, might be helpful:

http://www.runoffgroove.com/odie.html

[mordechai]

I see what you mean (re: the Odie example).  Would I be better served, then, by taking something like the Odie, taking off the first JFET stage and replacing it with the booster instead of putting the booster in front of the Peppermill and replacing the MOSFET with a JFET?

[Meanderthal]

 The only thing I'd change on that formula is to add a bias pot to the last stage of the Odie circuit so you can dial in the sweet spot like the 2nd stage of the peppermill. Oughtta make a nice versatile overdrive then!

The ability to bias a jfet is helpful because jfets are very inconsistent in performance, even the same manufacturer/etc.. They're actually tricky to bias sometimes.  But, it's worth it when you find the sweet spot, they sound great.

[mordechai]

Well, here's what I've cooked up, theoretically, thus far.  It's probably got a million things wrong with it but you will see easily what I have in mind: a treble booster at the front end of the Odie circuit driving the next two JFET stages.  I wonder if it need to be a treble booster, though, in order to have the signal slam the next two JFETs into overdrive.  I would appreciate any comments or suggestions, or points where you see problems that can be solved.  I'm trying below to insert an image from photobucket of what I drew up.  I don't know if it will work.  If it doesn't, I'll just add another post and include a link:

[mordechai]

Damn....didn't work.  Here's the link, please take a look.  And can someone please let me know how I can upload an image??

http://s1130.photobucket.com/albums/m533/mordechaibenzev/?action=view&current=mutantoverdriveproject.png

[B Tremblay]

Just a reminder that I'd appreciate it if builders did not use runoffgroove.com's schematic symbols for other circuits to prevent confusion: http://www.diystompboxes.com/smfforum/index.php?topic=44909.0

[mordechai]

Oops...my apologies, absolutely did not have any malicious intent!  I'll refrain from that error in future posts.

[Meanderthal]

 When you're in reply mode, there are 2 rows of icons up top. 2nd row, 3rd icon from the left is the insert image function. Just insert url of the pic and bingo.

No, it doesn't have to be a treble booster to work. You can raise the value of the coupling caps to let more bass thru if you want, I'd say .1uf for all oughtta do it(for guitar). Looks good!