If the radio is very old and uses vacuum tubes, it'll need about 5 or 6 amps. If it's solid-state, it'll likely be less than 1 amp.
As stated above, the best way to determine current is to connect it, and measure current. If the capacity of the source is huge, (like a car battery), make sure there's a fuse of some sort in series with the radio wires.
Remember, polarity is important with DC power. Reverse polarity will destroy semiconductors (transistors and the like).