The BSRI Audio Hydrazine Harmonic Percolator Fuzz is a modern interpretation of the Interfax Harmonic Percolator, with updates to give players a wider range of tones1. Pedal of the Day describes it as covering "crunch, dirt, boost, fuzziness, breakup", making it a fuzz pedal that reaches into overdrive and distortion territory as well1. It sits in the wider family of fuzz pedals and distortion pedals.
The Interfax Harmonic Percolator circuit behind it
The original Interfax Harmonic Percolator is, in Aion FX's words, "a unique and extremely rare fuzz/drive from the early 1970s"2. Its circuit is a hybrid design: Guitar Pedal X notes that the HP-1 "originally featured 2N404 Germanium, and 2N3565 Silicon Transistors"3. That hybrid silicon and germanium approach is the thread that runs through the Percolator derivatives other builders have traced and reissued, and Fredric Effects describes its own take as "an improved version of that hybrid Silicon/Germanium circuit"4. The Hydrazine follows the same lineage, and Delicious Audio has described the Harmonic Percolator as the fuzz circuit behind other modern variants as well5.
What it does to your sound
Pedal of the Day found that the Hydrazine keeps the character of the original while adding headroom for detail: "The Hydrazine also has that harmonic beautiful clang with a bit more clarity. It ranges from overdrive to heavily clipped" distortion1. The core amplifier circuit has been modified for improved clarity and punch, according to the same review1.
The Texture control is the pedal's main tonal sweep. Pedal of the Day describes "a unique Texture controls allows for continuous adjustment of the clipping threshold for a wide range of tones between transistor overdrive and compressed full diode clipping distortion"1. In practice, that means one knob moves the pedal from a softer transistor breakup to the harder, compressed clipping of the diodes.
Controls and EQ
- Texture: continuous adjustment of the clipping threshold, from transistor overdrive to compressed full diode clipping distortion1
- Active tone stack: bass and treble shaping for guitar and bass1
The addition of the EQ section, Pedal of the Day writes, "really opens things up, allowing you to fit these wild sounds perfectly into any rig or s[tudio session]"1.
What reviewers said
Pedal of the Day reviewed the Hydrazine in April 2026 and came away with a verdict on its range and placement in a rig:
"It works well with clean and dirty amps, too, fitting into that 'experimenter's dream' category with no problem."
The same review highlights the black and gold enclosure and the pedal's breadth across dirt styles1.
Videos
Pedal of the Day and PedalxChange
Questions
What is the Interfax Harmonic Percolator?
The Interfax Harmonic Percolator is a fuzz/drive pedal from the early 1970s that Aion FX describes as unique and extremely rare. It uses a hybrid circuit combining germanium and silicon transistors, and its distinctive clipped, harmonically rich character has made it a circuit that builders keep revisiting in modern interpretations, including the BSRI Audio Hydrazine.
Does the BSRI Hydrazine use germanium transistors?
Yes. Pedal of the Day notes that the Hydrazine is built with period-correct NOS germanium and silicon transistors, along with germanium diodes for clipping, staying close to the component approach of the original Interfax Harmonic Percolator circuit it is based on.
Can the BSRI Hydrazine Harmonic Percolator Fuzz be used with bass?
Yes. The pedal has an active tone stack providing bass and treble shaping for guitar and bass, and Pedal of the Day found that the EQ section opens things up and lets the sounds fit into any rig or studio session, working well with both clean and dirty amps.
What does the Texture control do?
According to Pedal of the Day, the Texture control allows continuous adjustment of the clipping threshold, covering a wide range of tones between transistor overdrive and compressed full diode clipping distortion. It is what lets the pedal move from overdrive through to heavily clipped fuzz territory.
Sources (5)
- BSRI Audio + PEDALXCHANGE.com Hydrazine Harmonic Percolator Fuzz, Pedal of the Day, 2026-04-25
- Tracing Journal: Catalinbread Karma Suture (Germanium & Silicon), Aion FX, 2021-02-21
- Guitar Pedal X, Guitar Pedal X, 2024-05-13
- Silicon Harmonic Percolator, fredric.co.uk
- Cat Box Customs Bean Machine V2 Fuzz, Delicious Audio, 2022-10-05
