When you are using an inverter, you are wasting energy (= battery life) generating extra weight and carrying around unnecessary weight.
You're basically taking DC voltage, converting it to an AC sine (or other) wave, using a transformer to boost the voltage up to 120V, using another transformer (in the amp) to take it back down to around 12-20v, and then rectifying it back to DC. Each step converts electrical energy into heat, electrical energy you could be using to create sound.
I'm lucky in that my little VOX does the 120VAC to 12V DC conversion externally, so I can just hook up a 12V battery directly. Or since it also runs on 6 AA batteries, I could replace them with a larger 9v source.
IF you are handy with a soldering iron AND the transformer in your amp only has two outputs, you could determine the output voltage of the transformer and hook your appropriate voltage DC source directly into the amp, preferably on the down side of the rectifier, avoiding excess heat generation,and waste of energy.
This would work best for most solid state amps, although some of them have preamps that take +g- (I haven't figured out how to do the + over - thing on my keyboard, such as ±5V ) type power supplies. Most tube amps have multiple voltages, a separate one that may not be rectified for the filament heaters, so they are much more difficult to deal with.
