
The sound of the tube amp was always synonymous with a rock guitar tone. It is said that the sound of the tube amp is "warmer" compared to solid state amps like transistor or mosfet. The saturation of the tube increases gradually with the increase of the volume, this is something that doesn't happen with solid state amps, which by the way weren't even designed to produce distortion. In solid state amps the distortion is simulated with electronic circuits.