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They, have what is known as a "vibrator power supply". think of it as a Switching power supply, but with a vibrating relay in place of the switching transistors and oscillator. This is EXACTLY how they work. The 6 volts( or in a few cases--2 volts, I have a couple of old GE "lunch box" radios, model #250 IIRC, that use a single large 2 volt battery. That powers the filaments of the tubes and the vibrator supply--it then produces over 100 volts that is then used for the B+ supply. In a car radio the 6 or 12 volts does the same thing--the car voltage powers the filaments...the vibrator supply raises this to over 200 volts usually--for the tubes B+. The output of the vibrator transformer is then rectified by a tube OR another set of contacts on the vibrator itself, known as a "synchronous" vibrator.
To power by 120 volts, One needs to either simply supply the 6 or 12 volts to the radio from a power supply..( the easiest way...) OR replace the vibrator tranny with a 120 volt unit. I have seen this done before.
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