Subbstitute Mosfet for Jfet questions

[screamersusa]

There are a ton of useful J201 jfet circuits out there to play with   I am wonder how should I approach or treat
using BS170 instead of J201?  I'm thinking in terms of a Basic BS170 circuit as a block (like a tube) substituting what would be considered the J201 "Block" in some of the existing circuits. For example, Fet eq and Diefet or Engl Mod2. Some circuits will obviously not work.

What would be considered "the building block J201" vs the Bs170 Mosfet.

[R.G.]

The simplistic answer is that it is very difficult to do this because the enhancement MOSFETs are so different from JFETs

JFETs are uniformly depletion mode devices, which means that if you do nothing to the gate to turn them off, they conduct as much current as possible. MOSFETs (the commonly available ones at least) are enhancement mode devices, which means if you do nothing to the gate, they don't conduct at all, and you have to do something to the gate to get them to conduct anything. MOSFETs have much higher transconductance than JFETs, so the gains will usually be very dissimilar.

There's another page or two I could type on this, but before I do that, go to Geofex and read http://geofex.com/Article_Folders/mosboost/mosboost.htm

[screamersusa]

Thanks R.G. will do that.  
Built a lot of J201 stuff but want to try to get more consistent builds for my own needs and I like higher voltages
for Clearer cleaner tones despite my love for hell bent distortion. Built 2 Identical OLC Slow Centurys and a few
Fetzers variants... Amazing how different each build sounds against another same build.
I kinda want to get away from having to tweak em with a scope constantly while experimenting.

I figure I could use some of the interstage filtering on the 201 circuits (And amps)as a start between the BS170 blocks.