Curve Tracer Power Supply Puzzle
You may be missing the point here. This section is not a "Power" supply it is a "Sweep" supply. Its purpose is to provide two wave forms (derived from the AC line wave form) to the horizontal and vertical driver circuits.  

The two wave forms shown in the diagram are the important features of the circuit.  

Tying the bridge rectifier group to the center tap gives the signal a 0v reference.   

The loop compensation potentiometers possibly provides some small adjustment to the wave form's phase. 

The low value capacitors in this section are being used to help smooth out the wave forms, not to create a DC supply. 

Applying simple waveforms like this to an oscilloscope XY inputs can create a two dimensional pattern.  In this case the wave form can drive the test transistor which causes the characteristic curve patterns to be displayed on the oscilloscope.  

In the original schematic the power supply to the system is shown on the transformer below this section, (see: Low Voltage Power Supply).