There are three main decisions to make about mic'ing your guitar amp; what kind of mic to use, whether you use more than one of them and how far away you place your mic or mics.
The most common approach is to use a dynamic mic as they are rugged enough to place close to speakers as they can handle the high sound pressure levels. They also help isolate because the cardioid pattern of a dynamic mic focusses on the sound ahead and not to the back and sides. Again the Shure SM57 is an industry standard candidate for the job, but there are plenty of other options on the market from the likes of Yamaha, Rode or AKG and more.
Single mic setup
The simplest method of mic'ing an amp is to place one dynamic mic close to the speaker. The best start point for positioning is to face the mic directly at the centre of the speaker, almost touching the grille cloth. Keep the mic at that distance and now experiment through the monitor headphones in moving it from the centre of the speaker's cone to the edge of it. The changes you make now greatly affect the tone; dead centre will be a direct, punchy tone but positioning to the edge of the cone will produce a darker tonality. Angling the mic up to 45 degrees will also affect the tone. Experimentation is key to fine tuning here. And once you've found your ideal spot, it's worth marking on the speaker clothe with tape for future reference.
Multiple close mic's
You could also try two or three close mic's that can be blended together for your tone for even more potential variations, especially if these mic's are different models too. If you have a combo amp with a 2 x 12 speaker for example it might be worth trying a mic positioned on each to spread the sound for a wider effect.
Close mic with distant mic combination
Using the previous one mic setup, you could also add an additional mic that's further away, to add core ambience and air to your guitar tone. The second mic doesn't actually have to be placed too far away though when you start experimenting with the effects. Start with several inches, moving to a few feet for the second mic.
But remember, your options for multiple mic's will be dependent on the interface you're plugging into. Our Steinberg interface has eight XLR ports, and we're using four mics for drums so we have four mic options left, and if you have less options with your interface, think carefully how to get the best sound with a one mic setup and take the time to get one you're happy with.
Bass amp mic'ing
In a traditional recording studio environment bass guitars are often recorded with a DI box or from an amp's preamp out jack. But as we're recording live we'll be close mic'ing the bass amp speaker, again experimenting with mic positioning on the speaker cone to find the desired tone.
We're dealing with low end frequencies now of course and we need that bass to be prominent in the sound, so distance mic'ing alone is not going to cut it. If you're eager to start playing it's easy to rush through with the mic'ing stage but you need to make sure you're happy with the sounds of each instrument's amp being mic'd, so keep checking the signal through monitor headphones and fine tuning; a small movement with your mic at a speaker grille can make a surprisingly big difference.