All Flavours of Fuzz, Overdrive and Distortion
by Phil Taylor
Distortion is the key to obtaining all those classic guitar solo sounds by the players such as David Gilmour, Jimmy Page, Brian May, etc, but is has to be the ‘right’ kind of distortion and applied judiciously. This article takes a look under the hood of the Effectrode ‘Mercury’ fuzz, ‘Tube Drive’ overdrive and ‘Blackbird’ preamp pedals to investigate what they actually do to your guitar signal, advises how to unlock the full potential of these pedals to create legendary guitar sounds and debunks a few tired old tone myths along the way!
Technically (good grief, have I lost my audience already!?) fuzz, overdrive, distortion and ‘blues breakup’ and ‘crunch’ and ‘metal’ pedals all do the same thing – they distort your guitar signal. It’s a simple principle that has been intentionally (and accidentally) exploited by guitarists ever since the first guitar was fitted with pickups and plugged into a tube amplifier way back in the 1940s.
Signal distortion occurs because tube amplifiers are imperfect. Distortion is introduced by the amp’s tube preamp and phase splitter sections, transformer circuitry and the speaker too. It sounds great! It’s no exaggeration to say that distortion can literally work miracles for your sound, adding body, richness, texture and sustain, altering the fundamental character of guitar transforming it into a powerfully expressive instrument, putting it on the same footing for solo work as sax, violin and even layered keyboards.
Even within the preamp section there a many different variations of circuitry, each with it’s own unique flavour of distortion. Not only that, in use, these circuits ‘feel’ and respond differently too affecting the guitarist’s performnance. Overdrive, metal, fuzz pedals and all the other distortion variants are all essentially preamp circuits. Let’s take a much closer look at the distortion they generate and how it affects guitar tone.
The Mercury fuzz is creates distortion by massively amplifying the input signal (roughly +60dB) from the guitar with two cascaded tube sections and then feeding it into a germanium diode clipping circuit. This generates symmetrical clipping distortion with rounded edges as shown in the oscilloscope shot shown in Fig. 1 below (input signal is 1KHz 100mV P-P sinewave).
The sonic signature of diode clipping distortion is very distinctive and sounds distinctly different to tube distortion. To my ear it sounds more artifical, less natural than distortion generated purely with tubes. I accept these are are purely subjective terms and I’m not suggesting clipping created with diodes isn’t as ‘good’ as pure tube distortion, just different. That said, my preference is for pure tube distortion and there will a lot more on this when we discuss the ‘Tube Drive’ overdrive pedal below.
Some additional distortion is also introduced by the second tube stage in the Mercury, which is slightly hot biased to introduce some non-linearity into the amplified signal. This has the effect of limiting the dynamic range of the signal (compressing it) and adding some subtle even order harmonic distortion to make the sound richer and more robust. This tube distortion blends very nicely with the ‘colder’ sounding diode clipping distortion creating a pleasingly robust, bluesy fuzz sound.
Working flat out the germanium diode clipping distortion becomes more asymmetric as shown in Fig. 2 and the solid-state character of the Mercury becomes even more pronounced.
This more extreme fuzz can create some fiery and devastating, almost synth-like sounds.
Tube Drive Overdrive
The Tube Drive is based on an all tube overdrive circuit. There are six tube stage stages in total. Four of these are the tube clipping stages biased to generate symmetrical distortion and the remaining two tube stages constitute an active Baxandall tone circuit. From Fig. 3 shown below it can be seen that the clipping resembles that of the Mercury fuzz pedal to a certain extent, however it has harder more defined edges.
This goes some way to explaining why the pedals sound different – the distortion generated by the Tube Drive is richer in higher order harmonic overtones.
But that’s just part of the explanation – there’s more to why the quality or character of distortion generated by these two pedals does not sound the same. Comparing the output signal waveforms of the Tube Drive and Mercury it can be seen that shapes of the waveforms are also very different shapes. Engineers describe waveforms in terms of ‘crest factor’ and ‘mark to space ratio’. The waveform generated by a germanium (and silicon) diodes has a definite underlying shape, a signature that is unique to solid-state diodes. The softer clipping characteristic of germanium diodes is often described as sounding more tube-like than silicon, however, although germanium diodes transition into clipping more smoothly than silicon diodes they do not sound anything like the signal clipping that occurs when a tube is operating in its non-linear region.
Just out of interest, Fig. 4 shows what the output waveform of the Tube Drive looks like with the tone control maxed out. The crest factor increases and the shape of the waveform changes from a square-wave to a ‘sharkfin’ wave, which is rich in higher order harmonics.
That’s still not the complete picture though. There are many other factors in the circuitry of these two pedals which not only affect their sound, but also how the pedals feel, respond and transition into overdrive. Break frequencies, flatness in the passband, feedback coupling (intentional and intrinsic) within the circuitry, resonances, damping and all kinds of other mind-numbing technical stuff. I’ve said it many times, but the devil really is in the details when it comes into designing guitar effects pedals that sound ‘right’. Some might argue that good tone is subjective and that there’s no such thing as a ‘bad’ or unsable sound in the right context and I have to say that I agree. However, from numerous conversations with other guitarists, there’s also a definite consensus of agreement about what a good guitar tone should be for clean, blues, rock, metal, etc, which is is why all these names for distortion were made up in the first place – guitarists expect to hear these signature sounds from their pedals. A metal pedal will have high gain and more mid scoop, a blues pedal will have relatively low gain and more mids, etc. There are no hard rules but you can generalise. Anyway, to summarise, the Tube Drive can best be described as sounding more like a hot-rodded tube amplifier, that is an amp that has been modified to incorporate an extra tube gain stage to increase it’s sensitivity, distortion and sustain, whereas the Mercury fuzz sounds, well, more like fuzz pedal.
Like the Tube Drive the Blackbird gain channel is all tube, however its signal path is based on more orthodox amp circuitry as utilised by the likes of Alexander Dumble, Mike Soldano and Randall Smith (Mesa Boogie), that is, cascaded gain stages with a passive Bass-Mid-Treble tonestack. So the gain channel inside the Blackbird has four cascaded tube clipping stages followed by a tube buffer. There are two modes of operation ‘Classic’ or ‘Creamy’. In ‘Classic’ mode the tubes are biased to clip the signal asymmetrically, introducing some even order harmonic distortion. From Fig. 5 below it can be seen that waveform distortion is relatively low generating subtle or mild overdrive.
The character of the guitar is retained whilst also enhancing and fattening up the instrument’s tone.
This makes the Blackbird an invaluable tool when recording guitar direct to a digital medium such as computer or DAT. It can be challenging, frustrating and unbelievably time consuming capturing good guitar tone when interfacing a guitar directly to computer. There’s seldom enough warmth and body when recording digitally and no amount of cabinet emulation, equalisation, compression or reverb quite manages to capture or recreate that magical sound of a guitar played through a real tube amp. A susbstantial part of a guitar amp’s tone is generated in the preamp section and using a good tube preamp can really help when recording direct, making the session much more productive (and enjoyable!).
The Blackbird’s ‘Creamy’ mode is biased so that the tubes clip the signal more symmetrically introducing a higher proportion of odd order harmonic distortion and a good dose of even order harmonic distortion too so that sound still retains that signature tube richness and warmth. This mode also has higher gain than the ‘Classic’ mode. Again, although the clipping distortion is symmetrical the waveform crest factor is much peakier (see Fig. 6 below) than the Tube Drive and Mercury pedals, and you guessed it, sounds quite different too.
Overdrive and distortion generated in this mode can be subjectively described as lively and ‘fine-grained’. It’s a reactive and energetic saturated drive sound, great for modern rock and even metal too. The pedal is so responsive that hammer-ons and pinch harmonics drop-out effortlessly – it’s real fun to play through. The Tube Drive has similar characteristics, however the the Baxandall tonestack focuses the mid-range for a tighter, punchier response. The Blackbird on the other hand, with it’s passive Bass-Mid-Treble tonestack has the same kind of mid-scoop in frequency response that Fender, Marshall, Soldano and Boogie amps exhibit.
Because many of the big amp manufacturers have been making amps with this type of tonestack for well over 50 years it has so some extent defined the sound of rock and roll as it’s the signature sound that many guitarists (including myself) expect to hear from an amp. Guitar players may not consciously be aware of this but I’m convinced it’s become ingrained in our psyches over the decades – we expect an amp to sound scooped and then usually compensate for this by boosting the mids with a graphic eq! There’s no need to do this with the Tube Drive though as the Baxandall stack has zero ‘insertion loss’, which means no loss of mids.
Sometimes the question gets asked, “Does the Blackbird sound like a British or American tube amp?”. The short answer is “Yes”. This isn’t meant to be funny, it’s just that how do you define a British or American amp ‘sound’, let alone design a pedal to replicate it? For a start there’s large variation in the manufacture of guitar amps within Britain (or within America). Differences such as circuit topologies, that is, is the amp class A or class AB. Differences in tube types, triodes, tetrodes and pentodes. Different size and type of loudspeakers and output transformers. There are literally dozens of variables, even within amps from just one given manufacturer, let alone another manufacturer in another country, and no single attribute in the circuitry of British (or American amps) that uniquely defines their sound. To my knowledge there is no such thing as a British or American amp sound.
What can say about the Blackbird though is that it’s gain architecture is super flexible. It will generate drive, classic distortion, crunch and even saturated clipping distortion [check out the technical demonstration video below for a closer look] making it useable for a wide range of playing styles and different musical genres, plus it has an incredible clean channel too. You don’t need spearate pedals to get these different distortion sounds, and the Blackbird has an excellent low-end frequency response. Couple this with it’s passive Bass-Mid-Treble tonestack, the same kind of tonestack as used on the Fender Bassman, means the Blackbird makes a potent bass preamp too. One pedal really can cover all these bases and do very good job of them.
A final word
So that just about covers everything there is to say about the different flavours of clipping distortion in Effectrode pedals. Sorry if any of this article has blinded anyone with science, but we engineers do love to impress people with our knowledge. Seriously though, I hope you find this article useful and informative for getting the best out of your effects pedals in the quest for a better guitar sound. Thanks for reading.
For a more in-depth look at clipping distortion and how it affects guitar tone check out the article, ‘Guitar Preamp Tone Explained’