Interfax Harmonic Percolator

The Interfax Harmonic Percolator is an extremely rare and highly sought-after fuzz and overdrive from the mid-1970s, made in Milwaukee, Wisconsin by Ed Giese¹. Originals turn up so seldom that Premier Guitar's reviewer, writing about the circuit in 2026, put it plainly: "Like just about everyone, I've never played an original"². What has kept the pedal alive is its unusual circuit, a two-transistor design that mixes germanium and silicon, and its famously minimal two-slider control panel.

The pedal sits between fuzz and overdrive territory. Fredric Effects, whose own versions are traced to the original, describes it as "an oddball overdrive/distortion pedal from the 1970s"³, while Aion FX calls it a fuzz/overdrive¹. Its signature, according to Pedal of the Day, is "the original circuit's signature even-order harmonics saturation"⁴.

A rare hybrid silicon and germanium fuzzbox

The Harmonic Percolator's circuit is its main claim to fame. It uses a PNP germanium transistor paired with an NPN silicon, allowing it to run in a standard negative-ground configuration¹. The transistors identified in traced units are a 2N404 germanium and a 2N3565 silicon⁷³. That pairing matters to players: most classic germanium fuzzes need a positive-ground supply and can be fussy about power, while the Percolator's hybrid arrangement works like a normal modern pedal¹.

The circuit also uses clipping diodes, which Aion FX notes is unlike most germanium fuzzes¹. Jack Orman's technical analysis at AMZ identifies it as another commercial example of PNP transistors operated off a positive supply, an unusual arrangement for its era⁸.

Part of the pedal's long-running mystery was deliberate. In the originals, "the transistors and some of the components were gooped or obscured," according to Aion FX, which kept the exact circuit hidden for years¹. The builder behind it, Ed Giese, "built a unique circuit using a rare mix of silicon and germanium transistors," as Gearnews put it when covering a modern reissue⁹.

There is no single definitive version to copy. The Percolator "does not seem to have one 'stock' design," according to the madbean pedal documentation, which notes that slight changes were made in component values over its life-cycle⁵. Aion FX's research found several circuit changes throughout the run, with two known versions: "stock" and "Albini"¹. The Albini version, named for the unit owned by recording engineer Steve Albini, "was found to have several different values than others that had been traced. His is the only known unit to use these values"¹. The madbean documentation describes the Albini version as "smoother and more controlled," with "a slightly over-compressed fuzz tone when the Harm is all the way up," and more aggressive than a FuzzFace with unique characteristics¹⁰.

The controls: Harmonics and Balance

The original Interfax HP-1 kept its interface to two sliders: Harmonics and Balance⁶. There is no dedicated gain or tone control, as Premier Guitar's reviewer noted². That two-knob format is carried through almost every modern version.

How those two controls behave is described consistently across builders' versions. On the Vaderin Pedals HP-X, "Balance (volume)" and "Harmonics (distortion)" are the two controls⁴. On the Land Devices HP-2, "the right one controls the output" and "the left knob controls harmonics"¹². Pedal of the Day, reviewing the Bowman Audio Endeavors version, observed that "Balance and Harmonics are the only controls, but there's a wide sonic spectrum just waiting to be explored"¹³.

Why originals are hard to find

Not many Interfax production units were made in the pedal's short life-span⁵. Combined with the gooped, obscured components that kept the circuit from being easily traced¹, scarcity has made the HP-1 a collector's item. Guitar Pedal X counted "around 40 or so regularly available variants of this circuit, mostly from tiny boutique builders"⁷, which tells you both how sought-after the design is and how few people can actually buy an original.

Modern versions and clones

The Harmonic Percolator has one of the larger clone ecosystems of any vintage fuzz. Named versions from boutique builders include the Nine of Swords Headache, the Bowman Audio Endeavors Harmonic Percolator, the Malaise Forever Clubber Lang, and the Vaderin Pedals HP-X¹¹. Moreland Magnetics' 202a is "Moreland Magnetics' take on the inimitable Interfax Harmonic Percolator"¹⁴, and Paperboy Pedals builds a clone in a white enclosure¹¹. Aion FX offers the circuit as a DIY project, the Particle Vintage Fuzz¹⁵.

The most established modern builder is Fredric Effects, which has been making versions of the circuit for over a decade³¹⁶¹⁷:

• Harmonic Percolator (Interfax Spec): an exact clone using NOS 2N404 germanium and 2N3565 silicon transistors, with normal rotary pots instead of sliders, plus a switch offering three different clipping options³.
• Utility Perkolator: Fredric Effects' own improved version of the hybrid silicon/germanium circuit, first announced in 2014 as "our latest version of the fabled Interfax Harmonic Percolator from the 1970s"¹⁷¹⁶.
• Silicon Harmonic Percolator: a part-for-part clone of the original circuit but built with entirely silicon transistors manufactured 40 years ago in the former East Germany¹⁸. Guitar Pedal X's Stefan Karlsson described its control set as "Gain, Volume, Diode On/Off"¹⁹, while Delicious Audio lists Volume, Harmonics (tone shaping), and Drive knobs²⁰.

The Percolator does not seem to have one "stock" design: slight changes were made in component values over its life-cycle⁵¹⁰. The Harmonic Percolator is also available from Aion FX as the Particle Vintage Fuzz¹⁵¹⁰, and the Albini version is smoother and more controlled, with a slightly over-compressed fuzz tone when the Harm control is all the way up¹⁵¹⁰.

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