Understanding Ohms and Extension Cabs

[Mountain Fever]

I have two cabs that I wonder about use with my Mesa Amps. I have a Fender VirbroKing Extension which is 2x12 V30s @ 4 Ohms and a Vox AC30 Cab with 2x12 AlNico Blues at 16 Ohms.

Can either of these be used with either a Mark IV Combo or LSS Combo?

 

[cb101]

For great diagrams and information look in your Lonestar manual. It gives all sorts of diagrams on how to hook up various types of speaker configuations.

 

[mrd]

Two things to consider:

First, it is OK to overload an amp, that is: present the amp with more resistance in the form of a speaker load than it's "supposed" to have. Example: It's OK to plug a 16ohm cab into an 8ohm speaker output, and it's OK to plug an 8ohm cab into a 4ohm outlet. All it means is the speaker "pushes back" a bit harder on the amp, and as such, the tone will be a bit darker. You might actually like it, and I know some artists, whether live or in the studio, utilize this trick to achieve certain tones or colours. However, MOST amps do NOT like you to under-load them. IE: Plug in LESS resistance than the amp is expecting. From my knowledge, Mesa amps are generally less picky about under-loading. The only downside is that your power tubes will burn hot, very hot. Also, depending on the load, your output transformer (that gigantic square looking chunk of iron in your amp) may or may not receive damage. In most cases, all you're looking at is decreased power tube life and increased power tube output.

Secondly, to calculate the overall load presented to the amp when you have two or more cabs of differing ohm ratings, there is a small and simple equation to use. First, take all the ohms of all the cabs and multiply them together. Then, divide that number by all the ohms added together. Your resulting number is the final ohm load that your amp will see with all these speakers.

A few examples:

Two 8 ohm cabs into 1 amp:
8 * 8 = 64
8 + 8 = 16

64 / 16 = 4

Therefore, your amp sees a load of 4 ohms with two 8 ohm cabs, which is why you always plug two 8 ohm cabs into the 4 ohm speaker outputs.

In your case you have a 4ohm and a 16ohm, so:
4 * 16 = 64
4 + 16 = 20
64 / 20 = 3.2

Therefore, with both of these speakers, your amp will see a load of 3.2 ohms. Likely, you are safe to do this, this is a pretty small under-load. I've run my Road King with an under-load of 2.6 ohms for long periods of time and it's fine.

My guess is it should be safe, but if you're worried about long term effects on your OT, I would call Mesa and ask them.

 

[MJ Slaughter]

Could you not just rewire your Fender cab to 16 ohms then plug both your 16 ohm cabs into 8 ohm jacks on your amp or do you have a need for your 4 ohm cab?

 

[kdorsey]

Fever, the Fender cab must be loaded with 8 ohm speakers in parallel while the the Vox cab's speakers are 8 ohm in series. If you rewire the Vox cab in parallel, you would get a resulting impedance of 8 ohms. Once matched, you can run them together without a concern.

Good luck!