The Keeley Katana has been a popular boost pedal since it was introduced. It's circuit topology is simple yet it manages to produce everything from a small boost to add a bit of life to leads right through being able to smash the front of a tube amp into submission and natural break up. Here's the description from the Keeley website:
"As far as guitar clean boosts go, the Katana is KING TONE. It's the pedal to have for the finishing touch on your tone.
Want a mirror image of your tone, only louder? This is it. The Katana is also a fat, harmonically rich boost, just pull out on the volume knob and stretch your imagination with overdriven tone! You can keep it on all the time to push your amp harder, or as an occasional boost to cut through the mix.
Obsessed or Compelled with the Fattest, Fullest Drive? Well, Before other people started using Blue LED's and non-diode clipping stages, there was the TMB and Katana.... Cascading FET stages do the tube-amp-thing like nothing else!"
And here's a video of the Katana in action:
And here's the schematic redrawn for you:
And a nice PCB Layout by RYANUK: http://www.box.com/s/zqpak9putdef8up82m51
And the transfer: http://www.box.com/s/z6fry11n4g10dldqraug
The dual cascaded Jfet stages do sound good, the charge pump ups the voltage to 18v and gives the circuit double the gain of a similar circuit running at 9v. It does introduce it's own clipping when the 10uF cap is switched into bypass the source resistor on the first stage, I prefer the tone without the cap switched in, a possible mod would be to use a smaller cap here to introduce some treble/mid boost to the tone. I also suggest changing the 4k7 resistors for 10k trim pots to allow you to tune each jfet to it's optimal bia's point around about 1/2 V (9v in this case). Build one up, they sound pretty decent for such an easy build!
Here's the Freestompboxes.org forum topic for reference: http://freestompboxes.org/viewtopic.php?f=7&t=2946