> How do you power an amp with stam?
Who is "stam"?
If you mean "steam": steam is low-energy. Electricity is high energy. You need a big lever.
As Rafe and Franken say: you usually build an engine to turn steam into mechanical power, then a generator to turn mechanical into electricial. If you are in the right place and time with a lot of cash, you could buy a steam-ship like Frankenamp served on. On a more personal scale, you can find old steam engines (but most of them were melted-down for WWII), you can train 30 years as a machinist and build your own (castings are available), or you can modify the valveworks on a small water-cooled infernal combustion engine. The neat way to get a boiler is to use a small home heating boiler: all ready to go except you want to turn-up the pressure. With luck you can get the steam conversion to spin as fast as it did as an IC engine, and then you just couple it to a standard emergency-power alternator.
There was a Chevy V-6 powered backup generator behind my building. This with a 125,000 BTU home boiler is nominal 50 horsepower, which would power the whole band plus a small-arena sound system. Efficiency would be poor. "Looks" would be poor: part of the appeal of steam is the flashing rods and valveworks, all hidden on modern engines. If you can find a circa 1900 steam motor-boat, and get it working again, that would be a good power, but you'd have to belt-up the RPM by at least 1:4.