Yes it will, however a computer psu won't supply the amps for the amplifier that I will add to the system. This is a good idea though.Will your radio work ok on a 12Volt supply, if so why dont use an old ATX psu from a computer?
This ^^^The circuit will work as drawn, not with great regulation but you can use NPN transistors as an emitter follower after a regulator like this and get better performance than you have seen. But, as wayneh says you need more base current - lower your 100 Ohm resistor or eliminate it altogether. You should allow for an amp from your regulator - it will need to be on a good heatsink.
Be careful with the current sharing between the transistors - perhaps add a small resistor in series with the collectors of each - 0.3 Ohm would drop perhaps 1V at full current and swamp out the differences between the transistors ensuring they share properly.
How much current do you need ?, an ATX will give out 20Amps easy!Yes it will, however a computer psu won't supply the amps for the amplifier that I will add to the system. This is a good idea though.
This is good circuit with LM317/337 it is very stable,emitter resistor will also allow transistors to share better.Here is a circuit for a 12 V 15 A power supply. To make it 15 V, change the voltage regulator.