I'm currently playing a Fender Pro Junior which is pretty loud, but there may come a gig where I need just a bit more power. I've thought about micing the amp running into the PA. Is a Shure SM 57 a good choice? And what do you think about mic+PA at all?
An SM 57 is a great choice. It's a very versatile mic, and if you mess around with the placement, you should be able to find a good sound. Miking your amp for a PA is a great idea, and usually a better idea if there's a sound guy working the event to mix it properly. It allows for better balance of the sound in the space, especially if normally you have a small amp sitting in the back.
A lot of people mic their amps through PA's. Usually the house sound tech will help you get it set up and the proper placement. If that's not available just play around with it when you can and if you have the equipment to do so.
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Thanks for your replies! I think I'm gonna get one of those. Not only for gigs but also for recording. I only own a condenser mic yet.
yeh the 57 is and has been an industry standard since the 70's
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
My last gig had its own amplifier mics - they were SM57's.
The sound was quite good, even though the owner of the bar said the PA was ancient and dying.
Ceci n'est pas une signature.
SM57's are the standard for a reason: they work well in almost any situation.

the sm57 is a great place to start. also, look into your mic placement. depending on what part of your speaker you aim the mic at, and what angle and distance you do so at, you can get very different sounds
yeah the sennhieser 609 is a pretty good one. most people run the chord through their amp handle and it lays down on the grill of the amp face. aparently it works well that way, opposed to a 57 where you need an amp stand.

if you like the sound, it will save you 20ish bucks on the stand for the 57.