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Exploring tube tones sans tubes

15 Jul 2015  | Paul Pickering

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along with a dose of mojo and voodoo, of course.

In my previous article, I tackled guitar tone and the almost magical qualities attributed to vacuum tube amplifiers. As we saw, mixed in with the science there's a healthy sprinkling of musician opinion, lore and superstition. And some stratospheric prices for legendary amplifiers.

Given some of the issues surrounding vacuum tubes – reliability, lethal voltages, size, weight—for decades designers have been trying to replace them by more "modern" components as semiconductor technology has improved; since the distortion characteristics of classic amplifiers from the 50's still represent the "gold standard" of guitar tone for many, subsequent designs have concentrated on duplicating the characteristics of tube distortion.

In the late 1960s, the widespread availability of germanium diodes and transistors led to the first "fuzz" boxes. Later designs made use of discrete FETs and then op amps. As more powerful microcontrollers came on the market, DSP-based digital modelling came to the forefront, although most of the earlier approaches are still viable.

In this article, we'll take a quick look at some of the classic designs, tracing their evolution from germanium technology to modern digital techniques. Along with a dose of mojo and voodoo, of course.

Early designs: The Dallas Arbiter Fuzz Face
This design was one of the first "legendary" pedals, introduced in 1966 and made famous by Jimi Hendrix. The earliest versions used NKT275 germanium transistors, but their parameters varied widely, both part-to-part and over temperature, and they were later replaced by BC183, BC130, BC108 and similar silicon devices. These were more stable, but had a harsher sound to many as well as an unfortunate tendency to pick up AM radio broadcasts.

The Fuzz Face circuit is extremely simple, and based on the shunt-series-feedback amplifier topology with R4 providing feedback. The two transistors are direct-coupled; C1 blocks the DC component of the input signal.

The original designs were discontinued around 1976, but later reissued and are still being manufactured in both silicon and germanium versions and boasting a long list of famous users. As can be seen, both the design and fabrication are extremely simple compared to later designs, yet an original Fuzz Face can fetch over $1000.

Figure 1: The Dallas Arbiter Fuzz Face. (source: SJ Effects)

Why do many guitar players prefer germanium for that vintage sound?

There are certainly differences in the technical specifications. Compared to silicon BJTs, germanium transistors have: lower base-emitter voltage (Vbe) (150mV vs. 600mV); lower frequency response, due mainly to their large junction capacitances; lower current gain; higher leakage current; and lower temperature stability.

This combination of factors, which has doomed germanium in the broader marketplace, produces a more pleasing distorted sound which has ensured their continued usage in distortion pedals. What do you think? Decide for yourself – here's an audio comparison.

If you want the best of both worlds, though, pedal manufacturer Robert Keeley has produced a pedal that allows you to switch between silicon and germanium channels.
Opamp-based design: The Ibanez Tube Screamer
Once opamps became popular in the 1970s, manufacturers started using them in distortion pedal designs.

The Ibanez Tube Screamer, first introduced in 1979 and now available in multiple versions, is arguably the most well-known distortion pedal and has spawned numerous derivative designs.

Figure 2: The Ibanez TS808 Tube Screamer. (source: Electrosmash)

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