Cletis,
here is a more lengthy explanation.
The 6V is used in three places in the radio:
1) The vibrator supply
2) the tube heater supply
3) the dial light
So for instance, if you wanted to convert to a 12v operation, you would change the tubes to the 12v version, change the dial light and change the power transformer.
The truck radios did not use the 6v for bias supply. However, in the very early days of radio, they used two and sometimes three different battery voltages and in those early radios, sometimes the bias supply came from the lower voltage batteries.
