From Marc’s Site:
The Skreddy Pedals™ LZ-129 is based on the circa 1969 Univox Uni-Drive*, which took the form of a volume pedal with a couple of controls on its front face. The two controls a) set the drive level (by means of 6-way rotary switch that acted as a volume control for the incoming signal) and b) the bottom range for the volume that was controlled by the treadle.
The original Uni-Drive was evidently intended to push a cranked, dirty amp into overdrive, seeing how it offered no clipping of its own, negligible added sustain (on the contrary; a very dynamic and un-compressed response), and a very loud output signal that was fuzzy when the gain was up high but with still very prominent signal peaks. So I added a unique clipping section that will help emulate some “cranked amp” softness, distortion, and compression into a clean amp that has plenty of headroom without the need to shatter windows or eardrums.
The tone tends to be clear on the high strings and fuzzier on the low strings. You will hear this effect on many songs from the first two Led Zeppelin albums, like the breakout solo in “Heartbreaker” and many more, such as “Good Times, Bad Times” and “Communication Breakdown.”
You can control how much of the signal is clipped by the proprietary, amp-like clipping section by means of the “HEADROOM” control, and, if you like the full, loud signal peaks of the original Uni-Drive, you can take the clipping section completely out of the circuit via the mini toggle.