The simple answer is preamp distortion results when you distort/overload a guitar amp's preamp, and power amp distortion results when you distort/overload the power amp section. In general, power-amp distortion is considered to be richer, more dynamic, punchier, and less compressed.
In this manner, do all guitar amps have distortion?
The answer to the question do all guitar amps have distortion is no; not all guitar amps have distortion. In fact, there are different categories you could place guitar amps in depending on how they generate their distortion.
Subsequently, question is, how do you set an amp distortion?
If you turn up the level of preamp, then there will be some distortion in the preamp itself, and also you'll be putting a powerful signal into the main amp causing further distortion there. Turn knobs marked "gain" or "overdrive" all the way up. Turn other volume knobs down to get the loudness you want.
Is gain the same as distortion?
Your gain setting determines how hard you're driving the preamp section of your amp. Setting the gain control sets the level of distortion in your tone, regardless of how loud the final volume is set.
What causes amp distortion?
Common Emitter Amplifier Distortion of the output signal waveform may occur because: Amplification may not be taking place over the whole signal cycle due to incorrect biasing levels. The input signal may be too large, causing the amplifiers transistors to be limited by the supply voltage.