Headlight voltage

[Yarder]

Is there a quick way to tell by looking at a headlight to distinguish between a 12v and 6v bulb?

I got a complete headlight assy in a box of parts that include older M38 stuff as well as some M38a1 parts.

I don't want to plug in the light if the first thing that it does is go out.

[wesk]

The filament appearance is not much help. You should be able to see the 4 digit part number on the rear of the bulb. They were usually ink stamped and if you look very closely you should be able to still make it out.

If you have a few marked bulbs you can make a comparison in output of the unknown bulb against a known bulb. IE start with 6 volts applied to the unknown bulb's high beam. Then compare that to 6 volts applied to a known 6 volt bulb high beam. If they are the same output then the unknown unit is probably 6 volt. If the unknown bulb is quite dimmer then try the same comparison with a known 12 volt bulb.

Or if you own a digital VOM then compare resistances across the filaments with a known voltage bulb.

The second and third options can have very varied results since bulbs in the same voltage rating also come in several wattage (candle power) outputs