#1
I recently bought a used Marshall MC212 to use as an extension cab for my fender pro junior III. The Fender gas has an 8 ohm impedance. The Marshall has two inputs on the back, one is for mono and says it is rated at 4ohms while both are also labled stereo at 8 ohms.

This is my first time moving away from a combo amp so I just wanted to ask here, which input do I use? I'm just confused because I'm not sure if the mono channel is 4 ohms per speaker or what. Sorry if this is a dumb question.
#2
That cab is only 8 ohms if you pair it with a stereo amp (or two amps) and use both inputs. Using it the way you want to is going to run it at 4 ohms. It is my understanding (but I'm not an expert, so maybe wait and see if someone else chimes in, too), that you don't want to have a mismatch where the cab is a lower ohm rating than the amp output, as that can hurt, or at least put undue wear on the power tubes.

It's also my understanding that a cab can be pretty easily rewired to a different ohm rating, so maybe look into that. I don't know how to do that myself, but a tech guy I know did it for my bass cab, as it was not a good match for my amp before. Took him like 5 minutes.
Guitars
Schecter Hellraiser C-1FR, C-1 Classic, Hellraiser Hybrid Solo-II, Special Edition E-1FR-S
Orange Rockerverb 50 212
Basses
Yamaha RBX374 and Washburn MB-6
#3
Quote by the_bi99man
It is my understanding (but I'm not an expert, so maybe wait and see if someone else chimes in, too), that you don't want to have a mismatch where the cab is a lower ohm rating than the amp output, as that can hurt, or at least put undue wear on the power tubes.
That can potentially **** your output transformer (or a fuse if your amp's smart).
Quote by the_bi99man
It's also my understanding that a cab can be pretty easily rewired to a different ohm rating
It depends on the speakers' impedances.

That cab can either be wired to have a 4ohm (in parallel) and a 16ohm (in series) mono input.
If you don't know what that means TS, don't do that.

Connect the 4ohm output from the amp to the 4ohm mono input of the cab.
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#4
The amp itself doesn't have a 4 ohm output, only 8 ohms. So by my understanding the only way to safely use this amp+cab combination would be to switch the 8 ohm speakers for two 16 ohm speakers and then use them in parallel to achieve 8 ohms? Thanks for the help by the way, guys.
#5
If I understand the original post correctly, the amp doesn't have a 4 ohm output jack, just 8 ohm.

If it's like any Fender I've played, if it does have a second jack, it will be wired so it results in a parallel situation, so both sides of the cabinet would still result in 4 ohms.

If this is like the Peavey stereo cabinet I played through a while back, it works as a stereo cabinet, each side being 8 ohms. Each of the 8 ohm jacks only connect to 2 speakers, one left and one right, for stereo usage, 8 ohms each. Running it that way worked fine on my amp, it's a 4 ohm amp. Parallel output jacks on the amp, 8 ohms each was fine, 4 ohm total load.

With your 8 ohm amp, you can unplug the internal speaker and use ONE of the 8 ohm jacks, you'll be using 2 speakers on one side of the cabinet, and it will match the impedance of your amp, that will work fine.

If it's a tube amp, running a 4 ohm cabinet is a bad idea. Very good chance you'll fry a transformer. Last one I bought was $120, over 10 years ago...they've gone up...

If it's solid state, not quite as bad, but still highly not recommended, still a good chance of damaging the amp, probably a transformer but I'm not sure. When I did this in the 70's with a Kustom SS amp, it fried a diode. I got lucky, it was still under warranty and I got it fixed free.

Whether you can wire the entire cabinet for 8 ohm depends on the impedance of the individual speakers. Probably not. If one side is twp speakers at a total impedance of 8 ohms, that means two 16 ohm speakers wired parallel. Four wired parallel would be 4 ohms, so it stands to reason you have four 16 ohm speakers. Series wiring would add those together, 64 ohms if I added right, really bad idea. Wouldn't hurt the amp I don't think, just work it to death trying to produce a little sound. And get it pretty hot...

I'd just use one side at 8 ohms, and make sure you unplug the built in speaker. If it has a second extension speaker jack, still unplug the built in and use the main speaker jack. The extension speaker jack is probably wired parallel, so using both would be 4 ohms. Most also have a cut off on the main jack, so if you unplug it, the amp sees no speaker load.
Hmmm...I wonder what this button does...
#6
Quote by Paleo Pete
If I understand the original post correctly, the amp doesn't have a 4 ohm output jack, just 8 ohm.

If it's like any Fender I've played, if it does have a second jack, it will be wired so it results in a parallel situation, so both sides of the cabinet would still result in 4 ohms.

If this is like the Peavey stereo cabinet I played through a while back, it works as a stereo cabinet, each side being 8 ohms. Each of the 8 ohm jacks only connect to 2 speakers, one left and one right, for stereo usage, 8 ohms each. Running it that way worked fine on my amp, it's a 4 ohm amp. Parallel output jacks on the amp, 8 ohms each was fine, 4 ohm total load.

With your 8 ohm amp, you can unplug the internal speaker and use ONE of the 8 ohm jacks, you'll be using 2 speakers on one side of the cabinet, and it will match the impedance of your amp, that will work fine.

If it's a tube amp, running a 4 ohm cabinet is a bad idea. Very good chance you'll fry a transformer. Last one I bought was $120, over 10 years ago...they've gone up...

If it's solid state, not quite as bad, but still highly not recommended, still a good chance of damaging the amp, probably a transformer but I'm not sure. When I did this in the 70's with a Kustom SS amp, it fried a diode. I got lucky, it was still under warranty and I got it fixed free.

Whether you can wire the entire cabinet for 8 ohm depends on the impedance of the individual speakers. Probably not. If one side is twp speakers at a total impedance of 8 ohms, that means two 16 ohm speakers wired parallel. Four wired parallel would be 4 ohms, so it stands to reason you have four 16 ohm speakers. Series wiring would add those together, 64 ohms if I added right, really bad idea. Wouldn't hurt the amp I don't think, just work it to death trying to produce a little sound. And get it pretty hot...

I'd just use one side at 8 ohms, and make sure you unplug the built in speaker. If it has a second extension speaker jack, still unplug the built in and use the main speaker jack. The extension speaker jack is probably wired parallel, so using both would be 4 ohms. Most also have a cut off on the main jack, so if you unplug it, the amp sees no speaker load.

The speaker that came in the fender was connected to the only speaker jack, so that's already been unplugged. The cab is only a 2x12 so there are only two speakers to work about. Also, I was under the impression that two 16 ohm speakers working in parallel would be 8 ohms. Seeing as
the two 8 ohm Speakers I have now working in parallel are 4 ohms. Is that incorrect?
#7
I was incorrect, I thought you were referring to a 4x12 stereo cabinet.

You are also right, two 16 ohm speakers wired parallel would be 8 ohms, that's what I was trying to explain above, might not have described it well enough. The two 8 ohm you have would be 4 ohm in parallel, that's what the 4 ohm jack is, so run through either 8 ohm jack and you'll be in good shape. I thought your amp would only have one speaker jack but not positive since I've never played that model. My Super Reverb only has one. Others have 2.

I should have realized from the model number that was a 2x12 cabinet...Marshall MC212...it went right over my head.

But it works the same, any two speakers wired parallel will be half the impedance. The way it actually works is

Impedance divided by number of speakers = total impedance.

So 4 speakers at 8 ohms each is a 2 ohm total load, that's how my Super Reverb is wired.

In series it adds them, so if you wire two 8 ohm speakers in series, it's 16 ohms total.

Your last paragraph is all correct, two at 16 ohm is 8 ohm total, 2 at 8 ohm is 4 ohm total load. Same idea, different values. So your stereo wiring is one 8 ohm speaker each side, mono wiring is two 8 ohm speakers parallel. Use just one 8 ohm jack, that'll work perfect but you'll be running one speaker, I thought it was a 4x12 so it would be 2...should have looked closer...
Hmmm...I wonder what this button does...
#8
^that makes sense. So, the 8ohm output on the amp can be run safely to one of the 8 ohm stereo jacks on the cab. It's just that it will only use one of the speakers in the cab, in that case.

OP, if you want to do that, just use the cab input jack that's NOT labeled for mono. That way, the cab will think it's being run stereo, and give you one side at 8 ohms. Plugging into the mono input will run both sides off the one input, at 4 ohms.

Of course, this makes me wonder if it would even be worth it, at that point. Using only one of the two speakers in the Marshall cab, compared to the one speaker built into the combo amp, probably isn't going to give you the bigger sound that you were likely looking for by going with an extension cab in the first place. Still might be cool, if you like the tone of that speaker more than the built in one.
Guitars
Schecter Hellraiser C-1FR, C-1 Classic, Hellraiser Hybrid Solo-II, Special Edition E-1FR-S
Orange Rockerverb 50 212
Basses
Yamaha RBX374 and Washburn MB-6
#9
OK, cut to the chase, huh?

You're in a bit of a bind because they are 4 ohm drivers. But before getting into details:
When you plug in the extension cab do the speakers built into the amp still work? Answer that and we can go further.

That's not a very good cab btw. They're "custom" Celestions which is code for garbage. And it's an MDF cab. They're just an entry level cab - and Marshall enter very low in the game. Don't expect much from it.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Mar 8, 2015,
#10
Quote by Cathbard
OK, cut to the chase, huh?

You're in a bit of a bind because they are 4 ohm drivers. But before getting into details:
When you plug in the extension cab do the speakers built into the amp still work? Answer that and we can go further.

That's not a very good cab btw. They're "custom" Celestions which is code for garbage. And it's an MDF cab. They're just an entry level cab - and Marshall enter very low in the game. Don't expect much from it.

No, the speaker in the amp doesn't work once I plug in the extension cab because the amp only has the one output jack. At this point I think I may just return it since it's been less than thirty days. It was a bit of an impulse buy, as it was only $200.
Thanks for the help everyone!
#11
You would us the 8 ohm mono input.

Look for a totally different cab, yeah. The 1936 isn't bad. Go used.
If in the US, look at Avatar speakers.
And start a thread. Give budget, location, amp and genres. Lots of options out there.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band