[sdiy] Power Supply question

[DCMagnuson at aol.com]

I've recently gotten a dead organ from a friend that I'll be turning into a 
monosynth.  In the base of the organ there is a transformer / fullwave 
rectifier and filter caps that outputs bipolar 30VDC.  I wish to keep this 
portion of the powersupply, but recreate the regulation portion.  I will be 
using 79xx and 78xx regulators and creating bipolar 15V and bipolar 5V rails.

What is the best method to cut the input voltage to the regulators (I don't 
want to fry them by applying 30V).  Can I simply create a voltage divider 
with a resistor and appropriate sized zener diode before the regulators?  I 
was planing on creating ~6V to feed the 5V regulators and ~17V to feed the 
15V regulators.  Is this a good method to use?

I assume the resistor in the voltage divider will need to be 1/2W?  What is 
the method for deriving the value of this resistor?  I know how to do it for 
resistor-based voltage dividers, but unsure of what to do with a 
zener/resistor combination.  I want to avoid drawing excess current from the 
unregulated 30V supply lines.

Thanks in advance,
Dave Magnuson