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Dallas Arbiter “Fuzz Face” Vintage Pedal T he Fuzz Face was first released by small British manufacturer Arbiter in 1966, using a very similar circuit to the ‘tone bender’ products on the market at the time. Manufacturers such as Vox and Sola Sound were already offering near-identical products, as can be seen by comparing Figs.1-3. The Fuzz Face therefore owes its popularity not to a novel design but rather to its uptake by prominent musicians of the era. Pete Townshend (The Who), Paul McCartney (The Beatles), and Hendrix are all known users. If you aren’t aware, distortion effects are widely used by guitar players (including bass guitar) to alter and enrich their sounds. They may be looking to create a unique sound for themselves, create different sounds from the same guitar in different sections of a piece, or just ‘beef up’ their sound with some extra harmonics. Soon after the release of the Fuzz Face, Arbiter was purchased by Dallas, who continued production as the “Dallas Arbiter Fuzz Face”. In 1993, American conglomerate Dunlop took over manufacturing, offering versions with either silicon or germanium transistors. The input stage All images have been reproduced with permission from Pre Rocked Pedals (www.prerockedpedals.com). The Fuzz Face has used many different transistors over the years. This example employs NKT275s, but it was not uncommon for early models to use AC128s or SFT363Es, all PNP germanium types. These substitutions were likely made for part availability reasons. In the era, it was more common for germanium transistors to be offered in PNP, in contrast to modern silicon transistors, which are more typically NPN. Both types can be made with both semiconductors, but NPN transistors require higher crystal purity and can be trickier to dope correctly, so in those early days, manufacturers preferred to stick with the easier-to-make PNP types. This circuit therefore has a positive ground, with a negative Vcc from the 9V battery power source. The guitar connects via a ¼-inch (6.35mm) input TRS (tip, ring, sleeve) jack. The signal is AC-coupled by the 2.2μF electrolytic capacitor before being applied to the base of PNP transistor Q1, which operates as a common-­ emitter voltage amplifier Australia's electronics magazine siliconchip.com.au Popularised by Jimi Hendrix, the Fuzz Face (from 1966) is considered by many the gold standard for foot pedal distortion effects. While it is a simple circuit, it is unusual by modern standards. The topology offers an insight into the compromises circuit designers had to make when working with early semiconductors. Vintage Electronics by Brandon Speedie 94 Silicon Chip with a 33kW collector load and no degeneration resistor. Many readers will note this is a poor choice for an input stage; the common-­ emitter configuration has a low input impedance, which will strongly load the relatively high output impedance of guitar pickups. Oddly, the Fuzz Face has gained a reputation for only sounding good when plugged directly into a guitar, not after other signal processing that might present a lower output impedance. This goes against conventional wisdom but is likely due to the interaction between the Fuzz Face and the guitar pickup resistance/reactance. As is typical in audio electronics, the ear is the litmus test. The decision to omit an emitter degeneration resistor is another ‘poor’ choice by modern standards. Adding one would raise the low input impedance mentioned previously but, more importantly, stabilise the stage against gain variations due to manufacturing differences and temperature changes, among other things. So why would the designer opt for such a crude topology? To understand this choice, we need to be aware of the limitations of early transistors. Germanium is directly under silicon in group IV of the periodic table and therefore shares many of the same properties (eg, both are semiconductors). However, as it is a larger atom, its outer shell is further from the nucleus and therefore not as tightly bonded. Thus, its electrons tend to break free more easily, increasing conductivity. Therefore, Germanium devices have lower forward voltages but are more ‘leaky’ than their modern silicon counterparts, meaning they are more prone to conduction without any base drive. This leakiness was exacerbated by manufacturing tolerances, which were not as tight as we might expect with a modern semiconductor fabricator. Lower purity of the feed stock and imperfections introduced in the manufacturing line contribute to additional charge carriers in the germanium, also increasing conductivity. These impurities serve to lower the effective gain of an amplifier built around a germanium transistor. The circuit designer is therefore compensating in this case, trying to maximise the available gain by omitting siliconchip.com.au Fig.1: the Fuzz Face circuit is deceptively simple, using just two PNP transistors (the types varied over the years of production) and a handful of passives to create an effective and popular adjustable distortion pedal. The distortion was created by a high gain combined with asymmetric limiting and clipping. Power is switched on when an input plug is inserted. Fig.2: the Vox Tone Bender circuit configuration is almost identical to the Fuzz Face, although many of the component values are different, as are the transistor types. Pressing S1 feeds the input signal straight to the output. Fig.3: the SolaSound Tone Bender again uses a virtually identical circuit to the Fuzz Face but with OC75 PNP germanium transistors this time. Some later pedals used NPN silicon transistors in a similar circuit, but they are not considered to sound as good. Australia's electronics magazine December 2024  95 the emitter degeneration resistor. This compromise makes the Fuzz Face sensitive to temperature changes and transistor hfe variations, which can differ significantly between devices. The AC128 data sheet lists an acceptable gain range of 55 to 175 for a newly manufactured device, an enormous variation of more than three to one. For this reason, Jimi Hendrix was known to purchase 10 Fuzz Faces at once and play each to determine the best one or two from the batch. He was experimentally determining which products had good transistors, with adequate gain and reasonable matching between the pairs. Their sound will also fluctuate due to ambient temperature changes – one of the many difficulties sound engineers and musicians faced back then. The output stage 96 Silicon Chip The transistors, capacitors and resistors were mounted on a small phenolic PCB. Much of the assembly work would have been in wiring up the stomp switch, sockets and potentiometers. The output signal of the first stage feeds directly into the base of Q2, another common-emitter amplifier, except this time, a 1kW potentiometer acts as the degeneration resistor. The wiper of this pot connects to ground via a 20μF bypass capacitor, which provides a low-impedance path for AC signals. This potentiometer is therefore the ‘fuzz’ control. With the control set at minimum, AC signals must pass through the degeneration resistor, providing a lower gain and less distortion. With the pot at the maximum setting, AC signals are fully bypassed, so gain and distortion are maximised. Negative feedback is applied to the input stage via the 100kW resistor, which also biases the DC operating point. Australia's electronics magazine siliconchip.com.au The reader may note that this voltage will be quite low and won’t bias Q1 fully into conduction. That is by design. Negative-going peaks from the guitar will be cut off earlier than their positive-going counterparts, providing an asymmetry to the distortion. This gives a more progressive effect, a characteristic musicians enjoy. The series combination of the 470W and 8.2kW resistors forms the collector load. Two resistors are used here to divide the output signal to obtain a more appropriate signal level. The output signal is AC-coupled through the 10nF capacitor and applied to a 500kW log taper volume potentiometer, another voltage divider to provide final control of the output signal level. Considered in its entirety, with the fuzz control at maximum, the circuit offers the highest gain configuration possible from a two-transistor solution, aside from the modest effect of the 100kW feedback resistor. Why early germanium transistors were mostly PNP types In the early days of semiconductor electronics, germanium was the material of choice for manufacturing transistors, predating silicon. PNP types were far more common among these early transistors than NPN types due to germanium’s inherent material properties and the era’s technological limitations. As a semiconductor, germanium has a higher hole mobility than electron mobility. That makes it easier to manufacture PNP transistors, where the current is primarily carried by holes moving from the p-type (positive) areas to the n-type (negative) area. In contrast, NPN transistors rely on electron mobility, which is less efficient in germanium. The doping process involves adding impurities to a semiconductor material to change its electrical properties, with the type of impurity determining whether the semiconductor becomes n-type or p-type. The dopants used to create the p-type material in germanium transistors were more readily available and easier to work with than those needed for n-type material. Elements such as indium and gallium, used for p-type doping, could be more easily incorporated into germanium during manufacturing. This was partly because the processes developed early on were optimised for the materials and dopants that were most accessible and well-understood at the time. Thus, in the early days, when manufacturing processes were less refined, PNP transistors offered better performance and were easier to produce with the available technology. Germanium transistors are more temperature-sensitive than their silicon counterparts, influencing operational stability. By virtue of their construction and the nature of germanium, PNP transistors had better temperature stability than NPN types in the early transistor designs. That made PNP germanium transistors better for applications where thermal stability mattered. Silicon, with its superior thermal stability, higher electron mobility, better resistance to environmental degradation and much greater abundance, became the preferred material for transistors. This shift was facilitated by improvements in manufacturing technologies that allowed for the efficient production of high-performance NPN transistors in silicon. Silicon transistor variants More recently, Dunlop offered the Fuzz Face with silicon transistors such as the BC108 or BC109. These are NPN devices, so the battery is swapped to a more conventional negative ground arrangement. While these more modern transistors have much more stable gain, their differing characteristics from germanium (mainly the higher Vbe of 0.7V compared to 0.3V for germanium) make for a fundamentally altered tone. These variants are known for their harsher and less progressive distortion and are not held in very high regard. A modern silicon version can be purchased for $200, much cheaper than the $10,000 (yes, ten thousand) early germanium versions fetch. Of course, many readers of this magazine will be more than capable of building one version for a fraction of that. If you can find the right vintage germanium transistors, you could easily make one with the ‘vintage sound’ for a small fraction of what a genuine SC early pedal costs! siliconchip.com.au ◀ The Fuzz Face case has an attractive shape that betrays its origins in the mid-1960s. Modern pedals generally come in ‘wedge’ shaped cases; this disc shape appears to be quite ergonomic but would probably be more expensive to manufacture. Australia's electronics magazine December 2024  97