Impedence for stereo cab?

[RobDavies]

Morning all.

An impedence query for you....

I'm intending to re-wire my Marshall MF400 cab for stereo use. It's currently rated at 8ohm and each speaker is 8ohm (Celestion G12k-100's).

How will this affect the impedence? If at all?

[ICBM]

You will have to wire it for either 2x 4-ohm (speaker pairs in parallel) or 2x 16-ohm (speaker pairs in series) depending on what your amp can be set to. If it can do both, use 2x 16 since then the cab can be run in mono at 8 ohms - you can connect to both inputs at once from a single amp even if you don't put switching in the cab itself. If you make it 2x 4 the combined impedance is 2 ohms, which is less useful.

[RobDavies]

The amp (Marshall 8008 power amp) runs at 4ohms on each channel. So wiring the speakers in a parallel pair is the way forward, I guess?

[ICBM]

How loud do you need it to be? That amp will be fine with a 16-ohm load - and under less stress, which is an advantage since their one failing is occasionally blowing power transistors or the rectifier at full crank - but it will drastically reduce the available power. I don't have the figures to hand but I think something like 25W/channel into 16 ohms (it's not quite a quarter, but close). Although 50W is still quite a lot, into four G12Ks! It also might reduce the bass response a bit due to the 16-ohm load producing more feedback into the amp (which is how the Valvestate circuitry works, to give a more 'valvey' response.) On the other hand if you're not cranking it at all, it will be fine at 2x 4 as well...

If it's easy enough to do - I think it should be, since the cab will be wired with push-connectors - I'd be inclined to try it both ways and see which sounds better... if there's no real difference go with 2x 16. You can wire one side each way temporarily if you want a direct A/B. (Bear in mind that the 4-ohm side will be louder, so make sure you equalise the volumes to get a true tone comparison.)

[RobDavies]

Thank you Mr ICBM - very much appreciate your knowledge!

[Wonks]

The 8008 has a linear/valvestate switch, so if you feel some bass response is lacking at 16 ohms, you could always try the linear mode.