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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also you should be able to tell the difference from a tube amp and solid state amp from listning to the sound. The tubes will be seen if you look at the back of the amp if there are any !