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.
View post (Sweeping (Help with Rolling Technique))
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Start of at a speed that will enable you to make it flow. I'm sure if you just searched "sweeping" on this site you would find loads of stuff. I still find the rolling technique difficult when sweeping back up again.