It would seem you have a centre tapped transformer. This means you get 12v-0v-12v. If you rectify the two red wires you will get 24v. Your black wire is your ground (or 0v) so in relation to that you will get -12vDC and +12vDC after the rectification. This is used in amplifiers to allow the output to swing both above and below 0v. This means to get you amp to work you would need 2 12 v batteries in series. Neg of battery 1 to -12v, positive of battery 1 to 0v and negative of battery 2, positive of battery two to +12v. I hope that made sence,
Dave