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Got an AM radio that works(thanks to a fellow "Bolter), it requires a 10 ohm speaker, replacement speaker is 4 ohms. Do I need 6 ohm resistor or is there a formula I use or what,,the old brain is fuzzy on a lot of that stuff. Brian

You can use a 6 ohm resistor, but put it after the speaker, and wire it IN SERIES, not in parallel. It won't put out as much volume, so be ready for that. The resistor should have at least a 1 watt rating, more would be better if you plan to turn it up more than a quarter of the way. 6 ohm is a nonstandard value, so a 5 ohm would be OK.

If you can't find any resistors that small, you can make a resistor out of Nichrome wire.

Finding an 8 ohm speaker that would fit would be much better overall. You could probably run it without a resistor as long as you didn't turn the volume up past 3/4.

You could also find another 4 ohm speaker and run it IN SERIES with the first speaker. This would give you an effective value of 8 ohms.

There is a distinct reason why you don't want to run a 4 ohm speaker. As the resistance drops in a speaker, it gets harder and harder for the amplifier to drive it. An old radio will not have the power available to drive the 4 ohm speaker, so it will end up destroying the amplifier and most likely the speaker.

If anybody wants to know why this is, I'll explain the math formulas if you request it. Otherwise I won't bother.

i believe the the ohm rating of a speaker is actually an impedence rating as the voltage to a speaker is ac versus dc. in that case, the 'ohm' rating is actually a product of the vector of the frequency and the inductance of the voice coil, which is an inductor.
Z = 2 *pi*f*L
so its a bit more complicated than working with pure dc resistances where you could cascade in series to increase and shunt in parallel to divide.
as vaughn says, finding the correct speaker is the best bet to run the full dynamic range of the amplifier that's driving it.
tony

Wow, its more complicated than I thought, anyone know where to find 10 ohm speakers? I ran one AM before w/a 4 ohm speaker, sometimes it got warm I guess & shut off occasionally. Brian

I'd get two 4Ω speakers and wire them in series.

Small 4Ω speakers are pretty common.. i'm not really sure what year truck you're dealing with, or where you're wanting to put the speaker.. but if it's in the dash like the AD trucks, then two 4Ω speakers bolted together will fit nicely.

old blue is exactly right.

as said before just wire two speakers in series. just don't try to get to much volume out of it or you will overheat the radio.

find some with the highest sensitivity rating you can.