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 have petrucci rock disapline dvd. He does go over chromatics lots. I agree with you here aiwass. They arent really used very much for soloing. They can be good sometimes though. Perhaps for some insane tapping stuff or legato. I usually stick to 3 note per string scales and maybe use some pentatonic stuff when i'm improvising.