AKG C12 Power Supply Question

I am building a point to point N12K AKG power supply(no regulators) for a point to point C12 http://www.groupdiy.com/index.php?action=dlattach;topic=44737.0;attach=8315 and I am wondering if an alternative circuit is available for the 50 ohm tapped rheostat(R4).  I am also not 100% clear on what the 2 diodes (G3) are used for, I would assume for balancing/ ground reference.  Could a resistor be used at R4 and a 470R  linear pot for balancing with the wiper to ground instead of the 2 diodes and C3?  Thanks, ChrisP