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Well, I'm not sure how much I can respect any man who takes Lug seriously - King Kashue
Sucking like that is a gift. You couldn't recognize the genius of Suck if it sat on your face and farted. -S400
You don't fix Lug's posts. Lug's posts fix you. - MrJoshua
You can be my HCBF boyfriend forever. - Sugarskull
Well, this about wraps it up. Turned out a little longer than I'd hoped, it takes awhile to explain all the common questions you see with this stuff. But despite its length I hope I've stuck with my goal of keeping it simple, the concepts aren't hard, just stick with it and you'll get it. In summary:
Don't run your solid state amp at a lower impedance than it is rated for.
If you connect two or more speakers to one amp (or one channel of a stereo amp) they are in parallel.
Match your tube amp exactly to the cab impedance it is expecting.
If your speakers are distorting, turn down. 'Underpowering' is a myth, and turning down is NEVER a bad thing.
Your impedance rating doubles if you run a stereo amp in bridged mono, so be careful.
Here's some extra stuff that, while not essential, is probably worth mentioning.
What was that formula for impedances in parallel? 1/Ztotal = 1/Z1 + 1/Z2 + ... , this can be extended for as many impedances as you want. Example 1/8 + 1/4 = .375, so Ztotal = 1/.375 = approx. 2.67. So 8 in parallel with 4 is 2.67.
What about wiring in series? If you know a thing or two about electronics, you know there are two basic wiring schemes, series and parallel. When impedances are in series they simply add, 4ohms + 4ohms = 8 for example. However, you don't really need to worry about this unless you are making your own cab. Individual speakers within a cab might be in series, but we're normally just concerned with the total impedance of the cab, and multiple cabs on the same amp are always in parallel. You could concievably make a special cable to connect two cabs to the same amp in series, but this is rarely done and not something novices should concern themselves with.
I've got a multimeter, why doesn't my cab show the right ohms? Simple multimeters measure DC resistance, which is only part of the story when you're talking about impedance. It is completely normal for a cabinet to have a resistance measurement less than its total rated impedance. So if you measure your 4 ohm cab at 3 ohms, or your 8 ohm cab at 6, or whatever, no need to be concerned. Making an accurate impedance measurement requires more equipment and know how than the average joe can be expected to have, that's why the manufacturer makes those measurements for us.