ProCo™ Rat™

The Rat™ came out of ProCo™ Sound in Kalamazoo, Michigan, in 1978, designed by Scott Burnham and Steve Kiraly. An LM308 op-amp provides the gain, a pair of silicon diodes clip the signal hard to ground, and the Filter control is a low-pass that works backwards from what you'd expect: turn it up and the sound gets darker. The result covers everything from a gritty boost to a saturated fuzz, and the LM308's slow slew rate is part of why it sounds the way it does. The big black box has been on records by Jeff Beck, Radiohead, Sonic Youth and Nirvana, and ProCo™ has kept it in production with changes to the chip over the years.

Our Rat™ replica is called the Rodent, and it came to the site in February 2002 thanks to Scott Swartz, who contributed the project. The kit ships with the OP07 op-amp used in current Rats plus a socket, so you can drop in a vintage LM308 if you find one and hear the difference for yourself. The free schematic and wiring diagram are below, and the Rodent remains one of the most-built distortions on the site.

with a smoother, more tube-like attack, no "plinking". The dismal slew rate may be responsible for this - the opamp simply cannot reproduce the pick attack at high gain. The LM108 or a LM208 should work the same as the LM308, the specs are the same except the 108 and 208 are guaranteed over a wider temperature range. There are also other opamps like the LM10 or the OP-08E with very similar specs. This kit uses the OP07 opamp that is used in the current model Rat™ distortions. We compared the sound of this circuit with the LM308 IC and the OP07 IC and found little or no difference in the sound. Your results may vary, depending on your guitar and rig. For that reason, the kit does include a IC socket, so if you wish to find and purchase a LM308 or other IC on your own and try them out, it will be easy to do. The project also includes a nice little add-on that allows you to set the bias on the JFET. The voltage divider of R12 and R13 sets the voltage on the 1 meg gate resistor, which in turn sets the source voltage. The original had that 1 meg resistor set to ground voltage and if you want your Rodent to be as original, leave R13 blank and use the included 0 ohm resistor (jumper) for R12. This will set the voltage into R9 at ground voltage as original. We recommend the values listed for R12 and R13 as we believe it sounds a little better. Total current consumption of the entire effect is about 3 ma, which means long battery life.

Populating the PCB

Use the project documents provided, starting with the General Build Instructions. There are a couple of options for populating the small ceramic capacitors. Since they are usually available in two different lead spacings and it's sometimes difficult to source the caps with just the right lead spacing, there are optional lead spacing holes for C3 and C4. See the diagram below for placement.