Original factory radios can be converted to 12 volt and FM can be added while keeping the look of the original case and dial, knobs, push buttons etc. The internal parts will be replaced with solid state components. This is by far the best way to go if you want to keep the original look and still have a modern stereo. The cost may be more expensive compared to a modern aftermarket radio, but you get what you pay for. Check publications like Hemmings Motor News for radio conversion pricing.

Tube Radio Polarity -
When you decide to change the polarity of the electrical system, you need to make sure your radio will accept the change. Most post-war radios are NOT polarity sensitive - here is why. Electrical power is distributed at two places inside of the radio, at the tube filaments which function much like a light bulb, and the vibrator which is a mechanical device that turns DC current into AC current to allow a voltage-boosting power transformer to be used. Modern solid-state vibrators are sometimes used to replace the original mechanical vibrators. All solidstate vibrators ARE polarity sensitive and need to be the same polarity as the rest of the electrical system.

Beware of Synchronous Vibrators -
Many pre-war radios used what are called synchronous vibrators. Synchronous vibrators will NOT work with reversed polarity. The high voltage they produce will be negative instead of positive and the result will be a large BANG” after a minute or so of radio operation. The BANG is caused by the electrolyte capacitor turning inside out literally. . . . . . .not a pretty sight.

If your vibrator has more than four pins on the base or your radio does not have a rectifier tube, your radio has a synchronous vibrator. Some radios will allow you to remove and rotate the vibrator 180 degrees and plug it in from the other direction, which will reverse the polarity. (Remember to pay attention to the pin locations when you first remove the vibrator.) If you cannot reverse the polarity of the original mechanical vibrator, you can replace the original with a modern solidstate vibrator of the correct polarity.

1955 Chrysler -
Signal-seeking radios are a rare example of a postwar radio that will not work on reversed polarity. The problem is with the signal-seeking motor.