uhhh, what exactly does OHM mean? i was looking at reviews for the Epi Valve Junior Head (see my other thread) and it said somethign about 4,8,16 ohms? so i looked at Wikipedia's definition, and got lost. searched it on UG, lost again.
any help?
It's a unit that measures electrical resistance. All you need to know is that if the number of ohms on the speaker box doesn't match the number of ohms on the speaker jack, bad things could happen.

Very bad things.
Hi, I'm Peter
I know that when head ohms are greater than cabs, you can blow up the cab. What about if head ohms are less?
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i know its electrical resistance but ive always been sorta unsure exactly what it means in terms of sound. i'm thinking less resistance (lower # OHMs) louder possible sound, more resistance (higher # OHMs) not so louder possible sound?

i'm asking a question, not answering one. its something ive always wondered.
yeah, less resistance at the same wattage would mean louder sound.
Hi, I'm Peter
It has nothing to do with loudness.

Think about it: if you connect a head with high output impedance to speakers with low input impedance, a higher electric current would pass through the speakers (since they have less resistance than the source), potentially destroying them.
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OHM's are the unit for electrical resistance. It is important to get the OHM's (or impendence) right when adding an extension cab. If the amp puts out 8 you can match it with a cab with 8 or even less, like 4 ohms. But not any more like 16 cause then your speakers will fry. It doesn't really affect the tone so much. You can tell a little bit of difference by hooking up with a different impendence but not alot. And when it says 4,8,or 16 that basically means the you can pretty much hook up to any cab. So don't worry about it so much, just don't overload.
I think you have that backwards man, at least for tube amps with an matching OT. Running a lower impedance than the amp is expecting, will result in the transfomer putting out more current than it was designed for. If you run it with a higher impedance, it will push less current and run cooler. It's supposed to create a "slightly darker smoother tone, with less output and attack" according to Mesa.
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I'm saying that if you run the cable from the 4 ohm jack into an 8 ohm cab it will be fine. However 16 into 8 is bad
no, the impedance setting you use on the head, should be equal to or less than the impedance of the cab you use. Your best bet is to match them.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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