Correct, doing a fixed voltage linear regulator is certainly a better way to go for running a 6V radio off 12V instead of using a dropping resistor. There are a number of issues when using a dropping resistor with a vibrator radio although it will work to a degree.
The problem using a 3 terminal regulator is that they typically can't handle the current or power that is required for running a radio (appproximately 6.9V at 6 amps), which is going to drop about 40 watts across the regulator. The regulator pointed to from Chevs of the Forties is actually pretty reasonably priced if it is well built.
If you're comfortable around electronics and a soldering iron and you want to roll your own, you can use a standard 3 terminal regulator and add a pass transistor and a big heat sink to handle current and power needs. Here is an example that shows the general idea:
http://www.mitedu.freeserve.co.uk/Circuits/Power/boosti.htm Also, Linear Technologies makes a 3 terminal regulator LT1083 that can handle the current by itself. It could do the power too if you can adequately heat sink it.
To eliminate this adapter need altogether, I've decided to convert my radio to run on 12V by replacing the tubes, vibrator, and transformer with 12V equivalents which Delco used on the '53 - '55 Buick radios. Not a simple task but doable in a couple of hours and the results are cleaner IMO.
-Ed
