Harmonic Percolator

The Harmonic Percolator is a two-transistor fuzz/overdrive circuit that came out of a small electronics repair shop in Milwaukee, Wisconsin in the 1970s. Built as the Interfax HP-1 by Ed Giese, owner of Interfax Electronics, it is an extremely rare and highly sought-after pedal whose basic idea has been rebuilt by dozens of boutique makers and DIY builders¹². Not many production units were made in its short life-span, and the transistors and some components in originals were gooped or obscured, which kept the circuit mysterious for years¹.

What makes it stand out among vintage fuzzes is its hybrid transistor pair: a PNP germanium device working with an NPN silicon device, plus clipping diodes, which most germanium fuzzes do not use¹. The result is a signature even-order harmonics saturation with a dry, husky edge rather than creamy, hot-tube-saturation drive³⁴. Premier Guitar's reviewer described the original's voice plainly:

The circuit has seen a massive surge in popularity over the last few years, helped along by its association with Steve Albini, and Guitar Pedal X counts around 40 or so regularly available variants, mostly from tiny boutique builders⁵⁶.

Where it came from: Ed Giese and the Interfax HP-1

The HP-1 was invented in Milwaukee, Wisconsin in the early 70s by Ed Giese, who ran a small electronics repair business called Interfax Electronics²⁵. Delicious Audio notes that the original "came out of a Milwaukee repair shop in the '70s"¹⁰. Guitar Pedal X dates the Ed Giese designed pedal to the late 70s and describes its enclosure as "absolutely ginormous" compared with modern pedalboard formats⁶.

Production was small. Madbean's documentation for its Pepper Spray projects calls the Harmonic Percolator "a very rare and mysterious fuzzbox" and notes that not many Interfax production units were made in its short life-span⁹¹¹. The transistors and some of the components were gooped or obscured in the original, a common practice for protecting proprietary circuits, which contributed to the guesswork around its history. Delicious Audio calls the Electro-Harmonix Percolator a "re-elaboration of a circuit so legendary that most of its history is still guesswork"¹⁰.

Even with all the clones, Pedal of the Day noted in 2024 that the HP-1 "is still a bit of an under-the-radar effect"².

How it works: a germanium and silicon transistor pair

The core of the circuit is its hybrid topology. Aion FX describes it as a two-transistor fuzz using a PNP germanium transistor paired with an NPN silicon, which allows it to run in a standard negative-ground configuration¹. That is a practical difference from classic all-germanium designs like the Fuzz Face, which typically need a positive-ground supply. Guitar Pedal X confirms the original circuit featured 2N404 germanium and 2N3565 silicon transistors⁶.

The hybrid pairing also has a stability benefit that aNaLoG.MaN explains in its Hybrid PNP FuzzCard documentation: a silicon transistor in the first position prevents the temperature sensitivity germanium is known for, while a germanium transistor in the second position retains the germanium sound¹².

The circuit also uses clipping diodes, which Aion FX points out is unlike most germanium fuzzes¹. Combined with the even-order harmonics saturation Pedal of the Day attributes to the original circuit, this gives the Percolator its own voice rather than a copy of any earlier fuzz³.

Two controls: Harmonics and Balance

The original HP-1 keeps it minimal: just two controls, Balance and Harmonics, with no dedicated gain or tone control⁴. Premier Guitar's review of the Electro-Harmonix version explains how they function:

Electro-Harmonix describes its own Harmonics knob the same way: it controls the gain at the input, and turning it clockwise makes the signal more saturated and distorted¹³. Delicious Audio sums up the range as "from mild overdrive to full destruction"¹⁰.

That two-knob format is part of the circuit's identity, and most clones keep it. Land Devices' HP-2 uses a left knob for harmonics and a right knob for output, with its B knob described as setting "your output level, letting you tame or unleash the chaos"¹⁴¹⁵. Some builders add controls: the EHX Percolator adds Bias and a Gain Range switch¹⁶, and Spruce Effects' Percolator Fuzz adds an Input Depth control billed as a tone control⁶.

No single stock version

One thing every builder tracing the circuit agrees on: there is no single stock Harmonic Percolator. Madbean's documentation states it directly¹¹⁹:

Aion FX identifies two known versions, "stock" and "Albini"¹. The Albini version matters because Steve Albini's actual unit was traced and found to have several different component values than other traced units, and his is the only known unit to use these values¹. Madbean's 2018 Pepper Spray documentation describes the difference in playing terms: the Albini version is smoother and more controlled, with a slightly over-compressed fuzz tone when the Harmonics control is all the way up, and it is more aggressive than a Fuzz Face while keeping unique characteristics¹¹.

This variation is why two original HP-1s can sound noticeably different, and why clone makers have to choose which version to follow¹¹¹¹⁷.

Steve Albini and the circuit's popularity

Steve Albini, the Nirvana and Pixies engineer and Big Black and Shellac frontman, is closely tied to the Harmonic Percolator's modern reputation: Delicious Audio credits him with the circuit's massive surge in popularity over the last few years⁵. Guitar Pedal X's tribute roundup of 23 fuzzes he helped popularize leads with the Interfax HP-1⁶¹⁸.

Albini's influence shows up in the clones too. Seward Park Electric's Arabica Harmonic Percolation takes the classic circuit and enhances it with additional controls while maintaining the character, influenced by the Albini modifications¹⁹.

Pedals based on the Harmonic Percolator

The clone field is large. Delicious Audio's buying guide tracks nine HP-1 clones and evolutions²⁰⁶, and Guitar Pedal X counts around 40 regularly available variants, mostly from tiny boutique builders⁶. A sample of the approaches:

The Vaderin HP-X shows how builders handle the germanium question: Pedal of the Day notes it uses silicon transistors in the PNP position instead of the classic germanium, describing it as "a vintage circuit, modernized for today's differing climates"³. Land Devices' HP-2 adds a switch that lets you choose between the stock tone and a louder but less compressed one, with switch left for the original circuit and switch right for doom¹⁵.

The Electro-Harmonix Percolator

In 2026 Electro-Harmonix released the EHX Percolator, which it calls a Harmonic Saturator and "a fuzz pedal based on the 1970s Harmonic Percolator"¹³. Guitar Pedal X describes it as "a budget take on the Interfax HP-1 Harmonic Percolator, with two extra controls"¹⁶. Those extra controls are Bias, for character and texture, and a Gain Range switch choosing between Drive and Fuzz, alongside the Balance and Harmonics knobs¹⁶.

Premier Guitar reviewed it as an attempt to "make the HP-1 a little more accessible"⁴. It is a re-elaboration rather than a part-for-part copy, unlike Fredric's Interfax Spec build¹⁰²⁰. For context on how Electro-Harmonix handles circuit lineage, its Small Clone chorus has similarly been reissued in economy versions like the Nano Clone and Neo Clone, and adapted for bass as the Bass Clone²⁹.

DIY layouts and the Harmonic Jerculator

The circuit is also a DIY staple. Tim Escobedo's Harmonic Jerculator is an adaptation of the Interfax Harmonic Percolator, published as vero (stripboard) and tagboard layouts on the Guitar FX Layouts blog³⁰. The layout has practical notes for builders: Q2 must be a PNP transistor such as the 2N3906 shown in the layout, and a 2N3904 will not work³⁰.

In sound, the Jerculator behaves like the parent circuit: at lower gain it does a half-overdrive, half-fuzz thing, and there is quite a bit of oscillation on super high gain settings³⁰. Madbean's Pepper Spray and Pepper Spray 2018 are printed circuit board projects based on the Interfax Harmonic Percolator for builders who prefer PCBs over stripboard⁹¹¹.

Fuzz, overdrive, or something in between

The Harmonic Percolator resists a tidy category. Aion FX calls it a fuzz/overdrive¹, Fredric Effects calls the original an oddball overdrive/distortion pedal from the 1970s²⁰, and the Jerculator's half-overdrive, half-fuzz behavior at low gain³⁰ sits alongside Delicious Audio's description of the EHX version's range from mild overdrive to full destruction¹²⁰³⁰. Zander Circuitry's Custom Shop Cafetiere, in its Low Gain transistor pairing, aims for a more open and overdriven fuzz sound closer to the gain level of the original circuit¹⁸.

Whatever you call it, the combination of even-order harmonics, clipping diodes, and a hybrid germanium/silicon pair gives it a voice distinct from the classic fuzz circuits it shares era and rarity with³¹³¹.

Pedals on this page

Interfax Harmonic Percolator

Interfax · Overdrive

Electro-Harmonix Percolator

Electro-Harmonix · Fuzz

$99 list price

Spruce Effects Percolator Fuzz

Spruce Effects · Fuzz

$149 list price

Dunlop Fuzz Face

Dunlop · Distortion

$149 list price

Vaderin Pedals HP-X

Vaderin Pedals · Distortion

Land Devices HP-2

Land Devices · Distortion

$229 list price

Interfax HP-1 Harmonic Percolator

Interfax · Fuzz

Electro-Harmonix Small Clone Chorus

Electro-Harmonix · Chorus

$107 list price

Electro-Harmonix Nano Clone

Electro-Harmonix · Chorus

$56.3 list price

Electro-Harmonix Bass Clone

Electro-Harmonix · Bass

$83.5 list price

Electro-Harmonix Neo Clone

Electro-Harmonix · Chorus

$86.2 list price

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