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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I supose its good that kirk doesnt solo anymore. They werent any good anyway. i think almost every solo he played was pentatonic above the 12 fret and sometime the odd note below.