I'd have to guess there are high voltage winding with center tap for the power amps, and probaly a low voltage winding with cente tap for the preamp.
Are the primary wires still connected?
I'd power up the transformer and chart what wires make what voltage. I guess I'd use an ohm meter first to group the windings.
Once you know what is what there, look at the circuit board. it should be easy to spot the main rectifiers. The high voltage wires would go to the heftier rectifiers or bridge, and ultimately to the larger filter caps. Then the low voltage wires to the smaller rectifiers. The center taps would be grounded.
If you have something other than two center tapped windings, tell us what voltages there are.