So here's my question: If I have a Bass-Head that has an Output of 300W to 4 Ohms, can I use a box with an impedance of 8 Ohms and a capacity of 300W?

I know, maybe it's a really babyisch question but is there anyone who can help me please?
The way I understand it goes something like this:

1 4-ohm head @ 300w puts out something like 180w to an 8-ohm cab (don't quote me on the exact number). to get the full 300w out of the head, you'll need another 8-ohm cab.

basically, 8 + 8 = 4 in impedance land.
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OK. thank you very much
I just wanted to know that I won't destroy anything, but if it's like you said, It's OK.
He's got it right. However, you will damage something if you plug a pair of 4-ohm speakers into the head. This will bring the resistance down to 2 ohms, and smoke your amp.
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Here goes...
As far as impedance goes, if you're running two cabs in parallel, you can use this formula:
(AxB) Divided by (A+B) = Total impedance

Where A=Impedance of cab 1
and B =Impedance of cab 2

Running cabs with a higher impedance than your amp's minimum capability is ok. However, if you run one with a lower impedance, you run the risk of overworking your amp and burning it out.
As for wattage, if you are running with say, 8 ohms impedance from cabs on a 300w 4ohm head, you will only be getting around half (as a rule of thumb) of the power from the amp, so you would be perfectly safe with a 150w cab. If you then plug in another 150w cab, the total impedance would be (8x8)/(8+8)=4ohms, so you would be getting full power from your head, and the 150+150w would be 300, so you would be fine with that too.
You can sometimes get away with having a lower wattage on your speakers than your head as long as you're careful with how you push them. Having a lower wattage on your head than your speakers is usually fine.

Thats a pretty basic explanation, but its got the key details =]
Hope it helped
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Oh thank you James. Now I figured it out how to get the right cab and which cab is usefull.
So I guess my question is answered completely
With the developement in materials especialy vioce coils the old adage of you need twice the handling power in speaker wattage than the amps max output is now history, it is quite safe to sensibly use an amp rated at a higher output/wattage than the speakers rating, using a higher wattage amp means that your speakers will receive a better quality signal (not clipping at the output stage).
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Everyone pretty much has it summed up nicely, I just want to note that as long as it is a solid state head without a impedance selector switch you are fine. Staying at or above the amps rated impedance load when all of the cabs are wired to it, being in series/parallel/various combination's of that, you will be fine with the ohm load.

Of course you have to watch power and listen for any clipping, or voice coils hitting the backplate etc, but that's a different story you aren't asking about and I can't explain lol.

Anyways, most tube heads have a impedance switch and you must match the heads impedance up to the cab(s) total impedance to avoid damaging anything.
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Last edited by AfroMan Skeeter at Oct 30, 2008,
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I just want to note that as long as it is a solid state head without a impedance selector switch you are fine

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