Join Date: Jun 2010
Location: Vancouver, Canada
Re: which ohm setting to use?
It's not a stupid question at all...
If the cab is clearly labeled as a 4 ohm cabinet then easy enough - connect it to the 4 ohm plug on your amp...but if the cabinet isn't labeled and you can see that the speakers are labeled as 4 ohm speakers then they are likely wired in series, which would create 8 ohms of resistance and you could use the 8 ohm plug.
Here is my understanding of speaker wiring and matching impedance: two 4 ohm speakers wired in series (likely in this scenario) would create 8 ohms of resistance but two 4 ohm speakers wired in parallel (less likely in this scenario) would create less than 4 ohms of resistance. A saying that helps to remember is "low into high will fly but high into low won't go" meaning that you could run a 4 ohm head into an 8 ohm cab without any problems but running a 8 ohm head into a 4 ohm cab would be inviting problems. So in your case if your aren't sure of the wiring and resistance of your speaker cab then the 4 ohm setting will likely be the safer option but if you can confirm that the two 4 ohm speakers are wired in series then I would use the 8 ohm plug.
The safest thing to do is to check with a voltage meter (most voltage meters have an ohm setting to measure resistance). The voltage meter likely won't read exactly 4 or 8 ohms so check the reading and go with the closest lower ohm setting on your amp.
This is all based on my understanding of speaker wiring & impedance but perhaps someone else can chime in and confirm the info in my post to give you some added piece of mind...