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 (I'm not sure if I am going to break my amp.)
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Yes. You should be able to replace the old tubes with new tubes if the new ones are the same type of tubes without re-biasing the amp. I dont know about parts inside the amp carrying an electric charge though.. someone else will have to answer that question !