Sounds like a machine. Pretty easy build using a layout from freestompboxes.org. This version has a two drive knobs. The original only has a single drive knob. The circuit is built around two identical sections. Each of these sections is made of a Super Hard On, MosFET booster into the Lockhart circuit comprised of an NPN, and PNP transistor pair. In the original Only the first HSO stage uses a gain control. The second stage uses a fixed 470 ohm resistor from Source to ground. This gives the second stage a gain of roughly 10.
The gain here is roughly determined as the Drain resistor divided by the Source resistor. The first section uses a 5K pot. Graphing a few values will show that a reverse log pot is required to spread the change over the pot travel. I had originally used some linear pots which didn’t off much control. I wanted to swap them for C5K posts but didn’t have on hand. As a quick fix I used B10K pots with a 10K resistor across pins 1 and 3. This approximates 5K reverse log response.
Back to that second gain control. This pot doesn’t add a lot to the effect. At the high end there are a few more sounds, in a narrow range. At the full it makes the effect unusable. The value here might be tailored to get more use out of this control. I’m assuming that Zvex saw this chose to leave this control out. On the other there might be some use in replacing this control with a 1K or 500 ohm pot and a fixed 100 ohm resistor. Though, for the sounds this effect produces it might not be worth the trouble.