archangels thunderbird is based on vintage silicon Schaller fuzz as a platform for sound well balanced of depth, dynamic, texture and sustain, without getting mushy or overly trebly. As a classic European two transistor fuzz circuit it does have an oldschool appeal yet I made sure it covers sonic needs of contemporary music.
With both toggles down and bias knob set around noon it is fairly close to the original Schaller circuit with extra output volume.

Switch up the intensity toggle to engage additional transistor boost stage in front - think of MKII Tonebender arrangement, but not exactly the same. This mode delivers lots of gain, wide range of textures, more aggressive character and is actually the focal setup of this design.

Tone toggle at bottom position omits tone circuitry to maintain raw fuzz spectrum. Center position engages muff-type tone stack, voiced for very little midrange loss and wide range from angry-bees-in-metal-can flavor to dark growl with lots of reasonable options in-between. Toggle up position allows to clip signal after tone stack, what reduces knob usability but offers additional, nasty saturation and a couple of extremely interesting sounds at ends of the knob dial.

Bias knob around noon provides full fuzz sounds. It gets more grainy when turned anti-clockwise, with weaker gain but sharp attack and brass-like textures close to minimum. Turned clockwise it drives transistors toward aggressive velcro and forces to start gating when set around 8.

There is also 'debuffer' (pickup simulator circuit) switchable by internal slide switch. Engage it to maintain proper fuzz sound and behavior when pedal is placed after buffer or active pickups. In this setting pedal sounds just a bit different than placed right after guitar without debuffer - with a bit more focused midrange (so it actually can be desired setup). Also it doesn't clean up with guitar vol knob so well, but this is a low price for gaining ability to use classic transistor fuzz with no harm from loop switchers and other buffered devices. When debuffer is turned on, it is automatically engaged together with fuzz effect and does not affect true bypass operation.