The Carl Martin Plexitone has been around for yonks (good word!) and has remained popular with it's high gain tones and wide versatility allowing the Plexitone to cover most bases required when playing anything other than Folk (Yuk) music.
Here's the description from the Carl Martin website:
"The Carl Martin PlexiTone, is as the name might indicate an unusual high gain three step rocket of an overdrive.The PlexiTone offers two step overdrive, the crunch channel and the high gain channel, on top of that it gives you an up to 20dB clean boost channel.
The crunch channel goes from subtle overdrive to hard rock gain stage, the high gain channel goes from rock gain stages to absolutely meltdown, with loads of gain and low end, and all in a tone that brings the memory back to the early 80's high gain guitar hero's.
The PlexiTone has as most Carl Martin pedals a built in regulated (+/-12V) power supply, to secure the necessary headroom that gives you the great tone."
Check out this comprehensive demo video of the Plexitone in action:
As you can see it sounds pretty nice. I bet you wanna know what's going on in there. Well here's the schematic provided by GnognoFasciani:
As you can see we've got the standard opamp type clipping circuit with a couple of LEDs in there to provide the distortion BUT with an interesting change - that dual +12v to -12v supply provided by the onboard transformer. This gives the pedal it's great headroom and dynamic appeal by ensuring that the opamp is never pushed into it's clipping region, this means all the clipping is done via the LEDs and not harshly in the opamp itself.
Here's a PCB layout by Bucksears:
And here's a link to the freestompboxes.org forum topic for reference: http://freestompboxes.org/viewtopic.php?f=7&t=1113