Sentinel

The Sentinel is a PT2399-based pitch modulator, delay, reverb and drone verb designed by Arnoud Philip (~arph) as part of the DIYSB 10th Anniversary design contest. It's based on the Sewer Pipes ring-verb and the Noise Ensemble. Original thread here, schematic here. Should be able to squeeze it into a 1590B. 

Note: LED2 in the layout is a bi-color LED that could be used externally as a rate indicator or even the on/off indicator. For the later, use the footswitch to break/connect the connection between the common cathode (middle lead) and the 1k resistor going to ground at the top of the board. Also, the 25k trimmer is a fine tune control for the Drone pot.

Ashbory Bass Preamp

The Ashbory bass was first introduced in 1986 and is/was a half scale (18") fretless travel bass with a piezo pickup and built in preamp. That preamp is essentially a Baxendall tone stack feeding into the volume control, into an op amp based gain stage. I stumbled across the schematic and thought it would be a good addition to the library. Should fit nicely in a 1590B, or you could wire it into a bass.

DeadAstronaut Black Triangle

Got another take on the bufferless Tube Screamer. DeadAstronaut has added some extra tone shaping options for tone, gain, and clipping. Pretty versatile tones on tap from light drive into heavier territory. He's also got a PCB with board mounted pots and switches for sale on his site here if you don't feel like wiring all those toggle switches.

Parasit Studio 8-Bitar

For Fuzz Friday, here's a really cool design from Parasit Studio:

The 8-Bitar is a minimalistic octave down pedal with a 8-bit sound (glitchy and gated square wave fuzz). It's small enough to fit inside a 1590A without any special parts.

The design goal with this pedal was to achieve the core octave tone from the Arcadiator, but with as few components as possible. I'm happy with the result since it only uses one chip (a CD4069).

Usual caveats apply for getting the best tracking - humbuckers, neck pickup and tone rolled off.

Pigtronix Fat Drive

Here's an interesting one from Pigtronix. It uses a TL072 to push a CD4049, which is followed by another TL072 pushing a Rat-esque tone control. All the gain stages make for natural compression and tube-like tone without using clipping diodes. Schematic for reference on FSB here (note, I've linked the inputs of the unused stages of the CD4049 to +9v to avoid any unwanted noise). Runs great on 9 or 18 volts and will fit in a 1590B with board mounted pots.

Diamanté Compressor

Stumbled across this one over on the old La Revolution Deux blog. It's a simplified version of the Diamond Compressor, removing the EQ and the blinking LED circuitry. Makes for a much smaller footprint and will fit nicely in either a 1590B or 125B. Here's the schematic for reference.

Demeter Fat Control

Looking for a mid-range frequency booster? This one's apparently really good at it. Schematic and discussion on FSB here.