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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As i said before ... make sure you keep your wrist as straight as possible. It does help because i've been told this by someone with 17 years of experience and it worked for me.
