"The Formula № 5 was inspired by old tweed amps, most notably the 5E3 circuit. The character of the 5E3 is very unique. It has very minimal power filtering, low plate voltages, elemental tone stack, and does next to nothing to control the low end between gain stages, not to mention the under-rated output transformer and speaker. The entire system runs inefficiently resulting in a loose bass, greasy mids, cutting treble, and incredible grit – none of these qualities were ideal or intentional back in the day. In spite of this, over the past five decades the amp inspired some of the coolest, most sought after guitar tones ever recorded."
Here's a demo video of the Catalinbread Formula No 5:
Here's the schematic provided by Coldcraft after a unit was traced over on freestompboxes:
As you can see it's a combination of two cascaded "mu" booster stages with a tone/gain section in the middle - much like the Catalinbread Dirty Little Secret. It sounds great and that mis-biased first booster stage creates some great asymetrical clipping harmonics. I'd build/buy this pedal - it rocks.
Here's a vero layout by Roseblood11:
Here's the freestompboxes.org topic for reference: http://www.freestompboxes.org/viewtopic.php?f=7&t=9373