How to regulate battery power

Hello guys,
I need to get +/- 17v dc from batteries for experimentation purposes to power my ss preamp. Not too tech savvy.
How do make this regulator to get +/-17v from two banks of batteries that have about 18 to 20v per bank (I will use total of 6 x 6v lead acid batteries).
arrange 3 x 6v like this positive voltage +- +- +- earth
and 3 x 6v negative voltage -+ -+ -+ earth

to get very close to 17v without any complication, use a forward biased diode for each
supply, rated at required current or 2x diode to drop typically
.6v for one or 1.2v for two observing anode from battery cathode to load for the positive supply
and cathode from battery and anode to load for the negative supply.

You should end up depending on how close batteries are charged to 6v with
your required 17v positive and negative

Cheers / Chris