Wiring Schematic diagram

In this schema, a full-wave bridge is switched to a full-wave center tap to reduce regulator dissipation. SCR D6 switches between configurations. When D6 is off, the schema is an FWCT rectifier using Dl, D2, and D5. It applies 17 V plus ripple to the regulator input. 

The drop across the regulator supplies base drive to Q2. If Q2 is on, Ql is off, and D6 is off. If the regulator voltage drops below about 3 V, Q2 turns off, and turns Ql on, which turns on D6. This changes the schema to an FW bridge using Dl through D4.