Be sure to ask them exactly "where" in the radio circuit the resistor is located. The WRONG way to do this is to put a high resistor in between the battery and the radio power lead... the correct way is to put a resistor in between the switched B+ supply in the radio.
There are two circuits inside the radio which are both switched by the on/off switch. One is the tube heater circuit and the other is the B+ supply.
The
proper way to do a 12 volt conversion like this is to change all the tubes to 12v equivalents: 6V6=12V6, 6BE6=12BE6, etc. That way the radio operated exactly the same, but the tube filaments will light on 12v instead of 6. No problem.
The resistor is a must unless one is to rewind the B+ primary. Most GM radios can be adapted with a 2.5 ohm 100W resistor between the power switch and vibrator. This resistor is much smaller than one, say for instance, required to run the WHOLE radio, filaments and all! If I have a customer who is REALLY stickler, I'll track down a stock radio from a '57 or '58 chevy (which are a dime a dozen) and use the B+ transformer out of it.
I wouldn't jump down their throats just because they added a resistor to your radio... but it certainly should not be so big as to necessitate mounting to the firewall. I do many 12v conversions on stock radios and find that I can hide the resistor in the box somewhere.
Probably more information that you wanted to know, but from someone who does also does radios I can see both sides of this dilemma.
