By virtue of their electrical properties, tubes generate a special waveform when they're saturated, which is why tube engineering has tremendous tonal advantages over solid state or DSP solutions, particularly for crunch and lead sounds. Tubes enter the saturation zone gradually or softly, which lends tube-driven tone its trademark yet totally unique character.
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You may think his riffs are good but they are simple. I dont think much of his soloing either. I agree that you dont have to shred to be good but he could at least make the solo's a bit more technical !