simple series pass regulator on PA supply line.
i would probably go with 4 zeners on a 4 pole rotary. ie: 5.6, 9.1, 12.0 and 15 volts so then the output is "adjustable," with fixed known intervals. 100uh rf choke on output should choke the rf backfeed from the supply. a hidden benefit is this is also a capacitor multiplier that will use the transistors gain to multiply its effective capacitance. ie, a 100 ufd cap on the base of the pass transistor will be like a 1000 ufd cap based on hfe gain of 10. using a npn darlington with a gain of 1000 makes the effect crazy like 100 ufd x hfe 1000 = effective capacitance like 10,000 ufd. smooth baby smooth output.
i miight draw this up if interested
if using a 12v supply maybe 2 or 3 position switch might do it for you. the darling will lose .7 volts so if you have a 12.00 volt supply you will get ~ 11.3 out