Sola Sound Power Boost... but with MOSFETs?!?!

[yetionbass]

Like the title may suggest, I've been somewhat keen on trying out a Power Boost type circuit and took to the breadboard. I started out with BC183s (it's what I had handy and I don't think they're terribly dissimilar to bc108's, 109's, or 184's like would have been in the old ones) and went through a couple days of researching the variations in schematics that I'm sure many of you are aware of. Once I got that working, thought to mysell, "well, that sounds ok," and stepped away for a bit, it dawned on me I've got some BS170's that it might be fun to try to drop in the circuit and see what happens. First thing I notice: doesn't work at all at 9v. At 18v, BAM!

It definitely needs some tweaking, but I'm feeling a lot of potential from initial results. Below is a sound sample at roughly 4 different settings: High-ish gain, Maxed out fuzz, completely clean at noon, and slightly over the edge of breakup. Guitar used is a strat in position 4 and the amp is a Supro Black Magic with the gain at 3 0'clock. Mic is an SM57 roughly halfway between the dust cap and the edge of the cone.

https://www.dropbox.com/s/5896i7h4w2ehat2/bs170PowerBoostdemo_session.wav?dl=0

In the first and last clips you should definitely notice the crackle/fizzing noise that accompanies the clipping. I'm not sure if that's a biasing thing or something I can filter out with strategic capacitor placement. I'm hoping some of you with more experience with either MOSFETs or the Power Boost circuit might have some suggestions on where to go.

Drain voltage for each transistor is as follows:
Q1: 9.86v
Q2: 9.06v
Q3: 13.91

From what I've researched, the same circuit with BJT's properly biassed would have collector voltages along the lines of 6v, 10v, 10v's for Q's 1, 2, 3 respectively, so perhaps that will be my next step, but given there's some feedback biasing going on in this circuit, I'm not sure exactly the best way to bias each transistor. Helpful suggestions would be much appreciated and I'll be sure to share my findings along this particular journey as it's got me pretty excited.

[PRR]

Welcome!

> variations in schematics that I'm sure many of you are aware of

So many, that I/we have to ask: What schematic are you using??

[yetionbass]

the middle schematic here. only variation is it's a 22 uF cap on the emitter of Q3. I had definitely planned trying the three different values. I'm assuming mostly that will function as an hpf.

[Rob Strand]

There's many ways to spin it.   Some may sound better than others.

These will push it closer, although you might need to tweak a bit more,

- For Q1 move the gate (was base) from the lower side of the 470R to the top side.
  [I mean move the 150k to the other side of the 470R.]

- For Q3 change the 33k to about 47k.

Q1's drain/collector voltage will sit a little higher than the BJT version.  If you want the emitter of Q2 to sit at a certain voltage then that forces the drain/collector of Q1 to sit at a higher voltage - since a BJT has a VBE drop of 0.7V and a MOSFET has a VGS drop of 2V.