We don't have any amps at any of our nights.
We do have a mixing desk wired up to two PA speakers.
We will run any cables directly from your instrument (e.g. your guitar) into the mixing desk.
Where there isn't a jack socket (e.g. it's an acoustic guitar, without an input) we'll attempt to mic up the instrument.
However, due to other elements (e.g. how loud or softly the performer plays, whether they keep the instrument in the same place pointing at the mic, how loud the venue is that night, etc) there is quite a large chance of feedback (so the instrument will often need to be turned down quite quiet!).