The Tycobrahe Parapedal was unique in that it used two pots to create it's own incredible wah type sound. The Parapedal was made in the mid seventies by Tycobrahe Engineering and has become quite a collectors item, that is - if you can find one. They were made in such small quantities that their value has sky-rocketed. Ask any vintage effect dealer for one(if they can find one).
What I do know is that Geezer Butler and Tony Iommi both used one, and that's enough of a reason for me to build one. Just listen to N.I.B. and you'll hear it on Geezer's bass solo intro.
Original was Positive Ground: