amp ohms?

[noctilux1]

I was reading on the Marshall web site (pro tips) when running two unmatched cabs--8ohm and 16ohm, set your amp head to 4 ohms. That's ok.
Anyone doing this???
Does it sound right?

[VaughnC]

Yes, 4 ohms would be best as the total load impedance would be 5.3 ohms with 8 & 16 ohms in parallel.

The math behind this is: the total impedance of two parallel impedences is equal to the product of the two impedances divided by the sum of the two impedances....or Z(total)=Z1xZ2/Z1+Z2.

So, in your case: Z(total)=8x16/8+16 or Z(total)=128/24 or Z(total)=5.33 ohms

However, because the 8 ohm cab is pulling more current than the 16 ohm cab, you may noticed that the 8 ohm cab being louder than the 16 ohm cab. Consequently, 2/3 of the amps power will go to the 8 ohm speaker and 1/3 will go to the 16 ohm speaker....so you have to make sure the individual cabs will handle the corresponding power levels being applied to them. So, for instance, if your amp is rated 100 watts, 66 watts will be applied to the 8 ohm cab and 33 watts will be applied to the 16 ohm cab (if you are playing the amp loud enough to produce 100 watts).

[noctilux1]

VaughnC, thank you very much for your detailed answer. My two cabs in question will work perfect together with 2/3 power going to my 8 ohm cab.
This is great!