You will be OK running the 8 ohm load on the 4 ohm head. 

Technically speaking the load should match the head for maximum efficiency. I doubt that you will notice a difference.

If the head is a tube out put (specs tend to indicate it is solid state)  you have nothing to worry about running a higher load on the head.

The problems arise mainly with the solid state (transistor) outputs WHEN you run a load LOWER than the output is rated for.

The load (while reactive in nature is resistance to the current flow) determines how much current will be allowed to flow from the output stage.  An 8 ohm out put into an 8 ohm load is matched and the correct amount of current to develop the rated wattage will flow, allowing the speakers to get the maximum that are designed for while not exceeding the output's ability to handle the power/heat generated.

If the load is higher (as yours is) the power will be slightly less that the maximum 100w that the amp is rated to deliver. On a solid state amp this is OK to do.

If the load is LOWER than the rating of the amp..... your amp is 4 ohms..... you would NOT want to run a 2 ohm load on the amp. The lower load will cause the amp to try to deliver more than it's rated power since the resistance is lower... it looks like a slightly shorted output.... over heating will occur and damage to the amp can be the result. This would normally occur at high outputs, when you're cranking it up.

The good news is..... YES... you can run the 8ohm cab on the amp without any problems..... The better news is, if you want to add a second 8 ohm cabinet to the amp, you can.... that will give you a 4 ohm load with 2 cabs each rated at 8ohms attached.

I would not rewire the speakers..... run the 8 ohm cabinet with out fear.

As far as sound quality, you won't notice a difference. It will be loud, it will have the same tone characteristics, you will be satisfied.

If you think one cab sounds good.... get another just like it when you can afford to do so.

richard.ingraham


I'm not sure if I am asking this on the correct thread or not, but here goes. I have a Behringer GB412s speaker cabinet with 4- 12 inch speakers. The cabinet is rated at 8 ohms. I just ordered a Behringer Virtube VT100FXH amp head online. Ok, so I am a dummy. After placing my order, I read that the speaker output in the amplifier is rated at 4 ohms. There is not a lot of clear information out there about the negative effects of running a 4 ohm amp through an 8 ohm cabinet. Based on what I have read, it appears that I can change the wiring in the cabinet to parallel (resulting in a 4 ohm load), but will only be able to use 2 of the speakers. I hate to do that. I understand that if I run the cabinet and amp together as is, I will be getting a reduction in the amount of power the amp puts out.
Can someone explain to me what I can expect from running the amp and speakers as is, in regard to the sound quality.

If you are getting enough volume don't worry about it. An 8 ohm load is safe. If you need to get more volume out of the system you should be able to wire the cab for 4 ohm.  Do you know what ohm speakers are in there ? You may be able to still use all 4.  

If the speakers are 4 ohm then you can wire them in series as pairs of 8 each pair and then those pairs in parallel to get back to 4.
If the speakers are 16 ohm you can wire the pairs in parallel for 8 each and then the pairs in series to get back to 4.

tlw
I'd imagine the amp in question is a pretty standard solid state (as in no valves in the output stage amp) type so giving it an 8 Ohm load shouldn't be a problem. Going below 4 ohms would probably not be a good idea though. You'll see a bit of a volume drop at 8 ohms which may even be benefical - 100 watts is a lot of power and being able to push the amp's master volume at lower sound pressures may be useful.

If it were a valve amp, or one of the (very rare) solid state amps with an output transformer, then the speaker impedance must match the impedance the amp expects to see. If it doesn't then serious damage to the amp is more than likely.

Behringer Virtube VT100FXH

Agree about amp in question being solid state so it should be no problem to the OP.

For valve amps with an O/P transformer, the really dangerous thing to do is to not connect a load at all - ie no speakers - which is a bit counter-intuitive! This can cause spectacular failures to O/P transformer, valves and valve holders due to very high voltage spikes with often visible arcing. Some amps have safety resistors to protect the amp when no speaker is connected to the output socket, but don't count on it.

Generally it is good practice to match the impedances, but valve amps can happily tolerate mismatches within the usual 4/8/16 ohm ranges.

And to the OP - don't bother trying to re-wire your cab - it is most likely to be 4 x 8 ohm speakers in series/parallel and you won't get 4 ohms with any combination using all of them. Stick with what you have  - and enjoy...