Tube amps distort in a very gradual way when overdriven. The "fuzz" sound is a more severe form of clipping. This is why transistor-based stomp boxes were sough

Tube amps distort in a very gradual way when overdriven. The "fuzz" sound is a more severe form of clipping. This is why transistor-based stomp boxes were sought out; it's actually hard to get a tube amp to clip in this exact way.