"I gots ta know." (Amp Wattage)

You can underpower a cab, but should never overpower one. So if you run a 600W cab off of a 300W head, you'll be fine, but if you run a 300W cab on a 600W head, it will be very easy to blow said cabinet out


As for ohms? Most amps will run between 4 and 8 ohms, with some going down as low as 2 (Check the manual to see if yours can or can't). If you don't know for your particular head, your best bet is to run no less than 4 ohms on it. And as far as running a cabinet at too many ohms? Unless you're building it, you would never have to worry about changing the resistance of your cabinet. Its the cabinet itself thats going to decide the resistance of your rig. Cabinets come from the factory rated at a certain resistance. You can't really change that, unless you rewire it.

if you don't know how to figure out ohms, btw

if you're running them series, add the resistance together. So two 8ohm cabinets run in series will give you 16 ohms of resistance

if you're running them parallel, you figure it out by adding the inverses of the ohms, then taking the inverse of your answer

as in.. for two 8 ohm cabinets..

1/8ohm + 1/8ohm = 1/4. Take the inverse of that and you see that two 8 ohms run parallel is 4 ohms.

 

OP, you'll want to do the following things:
1) Check out the amps forum. Yep, we got a whole forum dedicated to amps.
2) Read the FAQs and Stickies in that forum. Then re-read them.
3) Use the search function and search on the words "underpowering" and "overpowering". There is a lot to read and learn on those subjects, taking particular note of the fact that people disagree about them pretty strongly.

 

Umm, we do have an Amps forum.

Moved.

 

"You can underpower a cab, but should never overpower one. So if you run a 600W cab off of a 300W head, you'll be fine, but if you run a 300W cab on a 600W head, it will be very easy to blow said cabinet out."

This is not quite the way I have understood it to be - while it makes sense I was always told that if you underpower a cab and crank the amp you are MORE likely to blow it as when driven hard a transistor amp will deliver AC/sharp sine waves(?? something like this) into the cabinet. I always thought that overpowering is safer so long as you use your ears sensibly. This probably applies less to tube amps. Happy to be corrected if I am wrong here.....

 

Briefly, not quite. No offense, but this has been discussed a lot here, and if you take the advice given above in post #3 and search, looking particularly for posts by folks such as Bob Lee, billfitzmaurice, and Mark Reccord, you will find good and full explanations of this stuff.

 

+1 with Richard.

 

FWIW I've been under the assumption that if you are under powering a speaker cabinet and clipping the amp you are likely to damage the speakers as well. I believe I read this in the "Sound Reinforcement Handbook" that Yamaha puts out. I also have in my little hands "The Basics of Sound and Sound Systems (Hal Leonard) and right on page 58, and this confirms the effect that clipping has on underpowered speakers. So my guess it would be alright to use an underpowered amp, but don't run it full out to the point of distortion/clipping. Best to match as close as possible power output of the amp to power handling of the speaker with the venues and volume your band will be playing at.

 

Get an amp that puts out twice as much power as you think you might need, then double that, and get one. In the end you will be about right. I run a Carvin DCM2500. 850 watts per channel into two 4 ohm cabs, when I need the big power. If I need less, it has knobs for that.