I've been thinking about maybe building one, but I don't know much about 'em. I'd like it to be pretty small and I've got a great 10" speaker in the closet doing nothing.
Anyone build one before? Any preferred designs? Should I consider any mic other than the SM57? What about a Mic pre so I can plug headphones right into it?
I was going to build one, but found a used Jet City one for like $200, so that killed that dream.
Use a flex arm(s) for the mic and put an XLR jack on the outside, you can use any mic you want. If I were to build one, I'd build it firing forward like a traditional cab as opposed to the JC upwards design, and I'd put more sound dampening on it, possible in removable panels so I could suit it to the room/venue.
I don't really think the Mic Pre is going to do you a pile of good unless you've got it completely damped, as you're still going to here it if you've got anything bigger than 10 watts cranked past half.
I built one out of a 2 x 12 subwoofer box that used to be in a car. It has two compartments, with one speaker firing into the other compartment. The sound is pretty good, yet fairly quiet even when cranking it. It is carpeted on the outside with hinges for the lid, two XLR jacks with cables, a 1/4" speaker jack, and two goosenecks inside. I don't use it nearly enough as it is kind of a heavy beast but it does work well.
A fun fact for ya, in my line of work I get to see a lot of major acts and their equipment. One particular group liked their ISO boxes but found they sounded a little "dead." The solution? They threw some pizza boxes inside the chamber to add more reflections. 8)
The church I play in has a pair of them. One is a big iso box that contains an Orange 1x12 and an SM57. The other is a Randall box with a mic capsule pointed at a 12" Celestion. Both sound superb and although they are not totally silent the volume is totally manageable even with a 100 watt head cranked. I wish I sounded this good when I played bars. The toughest part is getting used to not "feeling" the impact of the amp on your guitar (or your body), and you have to have a good monitor system. We use in-ears with separate monitor mixes (Aviom) which is the cats ass once you get the hang of how in-ears sound. Good ears are pricey little devils.