Includes a switch which will supply the circuits with either 9V or 18V to give you even more versatility in the dynamics and headroom of the effect, and the main effect and boost have been done on separate boards so you can just make the main circuit if you want to, or maybe even add your favourite boost instead of this one. The main board will fit in a 1590B and there's a prize for the first person to do it with all 7 main board pots! :o)
Tip: I have heard a recommendation that the 100R resistor from Q3 source can be replaced with a 1K trimmer to help bias things up, but again I'll leave that choice to you as to whether you add a trimmer or experiment with resistor values (800 ohms worked for the guy who suggested it).
Info about the original from Okko:
Besides the usual Gain, Tone and Level knobs, the Diablo has some more interesting controls:
FEED affects the amount of bass in the input signal. You know the problem: an overdrive sound gets muddy rapidly if the input signal is too bass heavy (that’s why the treble booster was invented). With the FEED control you can make even the fattest neck humbucker sound clear and transparent or beef up weak pickups a little.
The BODY control takes effect on mids and compression. Another important feature is the HEADROOM SWITCH. Set to the "HI" position, an internal electronic voltage doubler is activated – the Diablo runs on 18 Volts from a regular 9 V battery or power supply. The higher voltage gives more dynamic range, more headroom and tighter bass – just try it!
The Diablo comes in two versions: the regular one and the Diablo Gain+ with two footswitchable gain settings.