What voltage are you trying to get out of the regulator?
Here are the things you need to think about:
The minimum voltage at C1, (reg input) Vc cannot drop below VoutMax + VdropOut with heavy loading, where VoutMax is the maximum you want out of the reg, and VdropOut is on the regulator's data sheet.
Minimum Vc occurs while C1 discharges between peaks from the full-wave rectifier.
The amount C1 discharges depends on the tranny sec. voltage, the value of C1, and the regulator load current.
The max Vc occurs when the load current is minimal.
I'm guessing that you would have done better to choose a tranny with a secondary voltage of ~ 16V.
Peak Vc