Some of the earlier equipment with 24 volt power and tube radios did use a big ballast resistor. It did put out some heat. There was not that much difference in current draw between send and receive, so you might have 14 volts on receive and drop to 11 or so on send - with solid state the currents are much different - get a dc to dc converter.
Some fire apparatus also had a separate battery and charging system for a water pump. We have connected radio equipment to the pump electrical system but the conductor runs were kinda long and lots of opportunities to pick up noise in the power system. Steve NU5D >> I know I could use a >> droping resistor but I can see it making a lot of heat >>
