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.