Pete briefly used a mkIII USA-made Electro-Harmonix Big Muff Pi fuzz pedal in his rig occasionally in 1976, in the studio in 1977, and in the Gaumont/Shepperton gigs for The Kids Are Alright film. Rather than using this as a traditional floor pedal, the pedal was taped down on top of the Hiwatt amp rack, and was engaged by hand, as can be seen during Pete’s solo in Won’t Get Fooled Again in the The Kids Are Alright film.

Two heavily modified V3s were part of Zappa's rack gear. This was mentioned by Zappa in this May-June 1979 Guitar Player interview (as transcribed on Zappa Wiki Jawapa) and identified and detailed by Kit Rae on his Big Muff Pi Page from Mick Ecker's Zappa's Gear: The Unique Guitars, Amplifiers, Effects Units, Keyboards and Studio Equipment. The latter includes the above photograph of one of the two pedals' circuit boards.

Guitar Player May-June 1979

What's in the large console you use on stage? It looks very complex.

Well, actually it's not all that complex. It's a little rough to maintain. Things can get broken where it's shipped around. It's got a pair of Dynaflangers, a pair of MXR Flangers. It's got one input and four outputs – two dirty outputs and two clean outputs, all stereo. There's also two Big Muffs, Systec Harmonic Energizer – all these things are in pairs – Oberheim ECF, Eventide Harmonizer, MXR DBL, Mutron, DBX 162 compressors, Gain Brains, Kepexes, a Theremin and a Biphase. That's about it, and there's about 24 switches on the floor. I add to it every year. The Dynaflangers are on the newest thing. Oh yes, there's a Mutron Octivider and a DBX Boom Box.

Kit Rae's Big Muff Pi Page

Shown above - This is the pcb from Frank Zappa's V3 Big Muff. Mick Eckers, author of the upcoming book Zappa's Gear took these photos in Frank's storage space in Los Angeles in 2012. Frank had the pcb's from his pedals mounted on cards which slotted vertically into a custom 19" rack unit with a connecting bus for input/output, footswitch connections, and power. Pots and switches were mounted on the front panel. Frank had two Big Muffs in his 1988 rack. Mick found some of the boards in a dusty old flight case labelled 'Junk', including this V3 pcb. Components and values match the ,ost [sic] common V3 circuit variant, but this one has large 47k limiting resistors, a value I have never seen in any stock Big Muff. (Photos © Mick Eckers)

At 1:09 in the video, Ian can be seen auditioning a Big Muff Pi, which has a distinct red 'Pi' symbol.