I want one foot pedal for crunch and sustain.

I have very basic small amp that has its own distortion button, but I would like to get good at getting nasty crunch sound.
What should I get?
I am currently working on Carol (Stones Ya-Ya), Bitch, Sympathy…, etc., Back in the USA (love both MC5 and Linda Ronstad’s versions), Red-neck Friend, and beginning to do do ZZTops Beer Drinkers…, Legs, and Sharp Dressed Man–you know the basic rockers.

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