Q1, Q2, and R2 may be regarded as a power darlington transistor. D5 and R1 provide a reference low ripple dc power supply at the base of Q1. 220v Ac to 12v Ac then rectifying circuit and then all the above elements , but the output voltage gets around 1.
It is not able to take load. It’s very good to supply an power amplifier, because the capacitor it have high voltage. Yes 12V 12A transformer can be used. D1-D4 should be able to handle that load current. C1 should be 470uF to 1000uF per Amp of load current.
5 times load current after C1 before R1. Transistor Q1 should be be mounted on a thick heat-sink tightly. Actually I have builded in this circuit. I could not get one matter. How much is this circuit have to output voltage. If i used zener diode voltage 6. 2V then will i get output voltage 6.