Okay, so I'm rather new to this so prepare for some stupid questions..

For furnishing at school. we're building a speaker cabinet so, I thought id make a 2x12 extension for my Marshall MG102cfx (100w combo) which runs at 4ohms.

In order to make this 2x12 work, what ohm speakers will i need to run? The amp itself already comes with 2, 8ohm speakers.

Cheers, I hope you understand what I mean, sorry for the noob like behavior aha.
Friends don't let friends buy a Marshall MG. Just my two cents.
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Friends don't let friends buy a Marshall MG. Just my two cents.

So? He bought one because he wanted it and liked it. Besides, TS didn't even ask about buying an amp.

OP:I honestly wouldn't make one. You'll only get an extra 3dB of volume. I'd leave my money and buy myself something nice, like a game, or something similar.
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Does the combo have an external speaker output?
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Currently the speakers in your amp are wired in parallel (I assume) which, using Kirchoff's(?) law:

1/R1 +1/R2+....+1/Rn=1/Rtotal

gives you (1/8+1/8)^-1=4ohms in total.

If you want to run the extension cab on it's own without the internal speakers, just wire the cab the same as that with 2 8 ohm speakers.

However if you want to run both at the same time it would get slightly more complicated, it's worth remembering that as your amp has a solid state output section the resistance (technically impedance, but whatever) of the speaker load can exceed what the specified minimum load without causing damage, but should never be run lower. If this is what you want to do I can explain further.
If the amp is stable at 4 ohms, but not at 2 ohms, you're not going to want to add a couple of 8 ohm speakers to the mix.
You're going to have to replace the inner speakers and make the new cab, or disconnect the inner speakers and make the new cab.

You have to have imps that equal 4 ohms. For SS, speakers can have higher (numerically higher, as in 8 ohm speaker to a 4 ohm port) but your amp doesn't run at it's best.

So if you were to use all 4 speaker spots, you would need: 4 16 ohm speakers, wired in parallel. That would give you a 4 imp load (16/4=4).

If you're just using the cab, you need: 2 8 ohm speakers, wired in parallel (8/2=4 imps). Or 2 2 ohm speakers, wired in series (2+2=4).
Last edited by Will Lane at Feb 17, 2015,
Will is right but a little difficult to understand.

Amp impedance is 4 ohms. Any amp, solid state or tube, will handle higher impedance, but it will work a little harder. So you could run an 8 or 16 ohm speaker cabinet on a 4 ohm amp without damage. That goes for tube or solid state. I always prefer to match the speaker impedance to the amp impedance.

Running a 2 ohm speaker cabinet with a 4 ohm amp is a bad idea. Tubes don't like it at all, good chance of destroying a transformer. Last one I bought was $120, around 10 years ago...Solid State will handle it a little better but is not recommended. Still a chance of damage, but not as risky. Still not recommended at all.

Speaker resistance works this way. Parallel divides the speaker impedance by the number of speakers. Two 8 ohm speakers wired parallel will total 4 ohms. Four 16 ohm speakers wired parallel will total 4 ohms. My Super Reverb is 2 ohms, four 8 ohm speakers wired parallel.

Series adds the impedance of them together so two 8 ohm speakers wired series will total 16 ohms. Two 4 ohm speakers wired series will be 8 ohms total.

To run the extension cabinet with your amp, you would need to make the cabinet 4 ohms, unplug the built in speakers and plug in the cabinet but not both. Make the cabinet 4 ohm using two 8 ohm speakers wired parallel. Do not run both cabinet and built in speakers together. If it has an extension speaker jack, most of the time they are wired so that it results in a parallel system, so it would be 2 ohms. Check your amp manual to be sure, it should have info about running an extension cabinet if it has a separate jack for it. If it only has the one speaker jack, unplug the built in speaker there and use that jack for the cabinet.
Hmmm...I wonder what this button does...