The problem with using a resistor is that the radio draws a different amount of current (less I think)until the tube filaments warm up. So to set up a resistor to run the radio at 6 volts normaly results in a higher voltage as it is warming up. This shortens tube life. I may have this back asswards but do know it can be a problem. I had my radio converted to 12 volts by replacing the vibrator, transformer and tubes with 12 volt ones. Not as bad as it sounds as the 12 volt tube radios are plentiful for parts.
