Zonk Machine

The Zonk Machine is a three-transistor germanium fuzz box made by the Leeds firm John Hornby Skewes between 1965 and 1966, and discontinued in 1968¹². It is one of the more famous fuzzes influenced by the Sola Sound Tone Bender MKI, and multiple sources describe the two circuits as nearly identical²³. D*A*M calls it "the freakier, weirder looking cousin to the Tone Bender MKI"⁴, and its sound is famously bright and searing⁴¹.

A separate Italian Zonk Machine also exists: created in the late 1960s by Jen Elettronica as part of its Super Sound series, with a colorful psychedelic enclosure common among fuzzes of the era⁵. This page deals mainly with the British John Hornby Skewes pedal, the circuit most builders mean when they say "Zonk Machine."

How it works: germanium and leakage bias

The Zonk Machine is a three-transistor fuzz circuit in the MKI family. An original unit traced for a Guitar FX Layouts project measured Q1 as a TI AO2 with 180 Hfe and 0.12 mA leakage, Q2 as a Mullard OC75 with 78 Hfe and 0.29 mA leakage, and Q3 as an OC44 with 58 Hfe⁸. Those leakage figures matter more than they would in a silicon pedal: in this kind of circuit the transistors are biased through their own leakage current, which is why the pedal behaves the way it does.

Germanium leakage is temperature-sensitive, and that is the root of the Zonk Machine's reputation for shifting behavior. D*A*M, whose FZ-673 is built on the same circuit, states it plainly⁹¹⁰:

D*A*M, on the Zonk Machine/Tone Bender MKI circuit type¹⁰

D*A*M also notes that this circuit type is prone to shifts in output due to heat, and that the fuzz control can be "all on nothing at times" in stock form¹⁰. ZVEX describes the same class of problem from the builder's side, saying the germanium transistors normally used in this circuit "will misbehave when you're playing hot outdoor gigs above 80 degrees Fahrenheit"¹¹. Modern makers have answered this in different ways: Benson Amps' Störkn B0kš uses an automatic thermal bias circuit that gently warms its germanium transistors to a preset operating point, and the Scott McKeon SM Fuzz uses temperature-tolerant circuitry to keep its tone stable in hot climates¹²¹³.

The sound, in the words of those who make and study it:

The PCB, stripboard and tag board versions

Original Zonk Machines were built in three construction phases, and collectors distinguish them by board type¹.

• PCB version, c. 1966-1967: built on printed circuit boards. Particularly early examples used a triplet of Mullard transistors; more typically a Texas Instruments 2G309 or TI AO2 in the first position with Mullard "black glass" OC44, OC71 or OC75 in the second and third. Earlier examples had brown carbon composite resistors, later ones yellow types, likely Iskra. The very earliest known examples had Bulgin open jack sockets, later ones closed-type Cliff sockets¹.
• Stripboard version, c. 1967-1968: built on Veroboard. Earlier units used the AO2/OC75/OC71 transistor set; towards the end of production the second-position transistor was replaced with a silicon 2N4061, along with a trimpot to adjust its bias. Component date codes reveal these stripboard units were actually among the youngest built¹.
• Tag board version: one unusual tag board Zonk Machine surfaced in 2015, using 2G309 transistors, suggesting it may predate the shift to stripboard. At the time of writing it was the only one of its kind known to have survived¹.

Early examples had a battery clasp fastened to the main chassis, while later units had the clasp attached to the removable cover behind the controls¹. Some units also came with a 25k log fuzz pot and a 220k resistor instead of 470k⁸.

The builder community has traced and redrawn the circuit for DIY use. Guitar FX Layouts published a tag-board layout for the Zonk Machine I, describing it as "almost identical in circuitry" to the Tone Bender MKI, "topographically identical sans the 33K resistor"⁸. Builders reporting on that layout found the original 1nf input cap sounded tinny, and that a 10nf cap gave a beefier sound with stronger midrange, while a 1nf cap to ground removed sizzling "bacon noise" artifacts without killing the highs⁸. PedalPCB sells a DIY PCB, the Zonkbox, described as "Compare to D*A*M Zonk Machine"¹⁶.

Power and battery

Like the Tone Bender, the Zonk Machines were powered by 9 volt batteries¹. There is no maker-published current draw or polarity figure for the original pedal; it predates the 9V DC center-negative power supply convention that later pedals follow.

Modern Zonk-based pedals are more board-friendly. The Tru-Fi Zonked Machine takes a negative tip 9V power supply plug or a battery¹⁷, and the BLAMMO! P2P Buzz Tone offers a 9 volt battery option that disconnects at the input jack¹⁸.

Reissues and pedals based on it

The Zonk Machine was made only in small numbers before being updated with a more modern, easier to produce unit², and various companies have produced reissues and clones⁵. Vintage units have become collectible among enthusiasts and collectors of vintage guitar gear⁵.

MusicRadar reported in 2024 that JHS itself has two of its own versions out, a Vintage Series in the larger wedge-style enclosure and one in its Compact Series²¹. The Reeves Electro Z1 is described by its maker as a deliberate homage to the rare John Hornby Skewes MK1 Zonk Machine and its MK1 Tone Bender cousin²².

Tru-Fi's Zonked Machine gives the circuit three voicings on a switch: the Zonk setting has a bright bite, the MK1 setting has more bottom end, and the maker reports cleanup is available with the guitar volume knob¹⁷.

The Zonk II and the circuit's later life

The Zonk Machine was followed by the Zonk Machine II, the result of a re-engineered Zonk Machine I¹⁵. Early Zonk II pedals, from around 1967, were built with various two- and three-transistor germanium circuits²³. By the 1970s the Zonk II featured a two-transistor silicon circuit, and a c. 1969 revision used two 2N4061 silicon transistors on stripboard before construction changed to a dedicated printed circuit board; that silicon circuit was used until the model was discontinued²³. One traced Zonk II used two PNP M2N 4403 silicon transistors instead of the earlier three-germanium Zonk Machine I arrangement¹⁵.

Where it came from

The circuit's lineage runs from the Maestro Fuzz-Tone through the Tone Bender. BLAMMO! traces it as the Maestro FZ-1 inspiring Gary Hurst's Tone Bender, "which was later taken by John Hornby Skewes and further tweaked into the Zonk Machine"¹⁴. Fuzzboxes.org agrees the Zonk Machine's circuit resembled Gibson's Maestro Fuzz-Tone but was much more similar to the Tone Bender MKI, likely its true inspiration¹. British Pedal Company's history adds that Skewes approached Charlie Ramskirr of Wilsic Electronics in Doncaster to build his new fuzz box⁶, and that the pedal was housed in a simple steel box with two controls⁶.

The enclosure was not as heavy duty or robust as the MKI's². Melody Maker reported in August 1966 that the company had been exporting fuzz boxes¹, and original Zonk Machines overwhelmingly resurface in Canada¹.

Pedals on this page

Sola Sound Tone Bender MKI

Sola Sound · Fuzz

D*A*M FZ-673

D*A*M · Fuzz

Benson Amps Benson Stonk Box

Benson Amps · Fuzz

$279 list price

Hornby Skewes Zonk Machine I

Hornby Skewes · Fuzz

Tru-Fi Zonked Machine

Tru-Fi · Fuzz

$225 list price

Hornby Skewes Zonk Machine

Hornby Skewes · Fuzz

Reeves Electro Z1

Reeves Electro · Fuzz

Gibson Maestro Fuzz-Tone

Gibson · Fuzz

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