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Posted on Tuesday, July 02, 2002 - 06:54 am:   

i have an rd112 with a 4ohm ev speaker in it.what would happen if i used another 4ohm extension speaker cab with it?would it hurt my amp?
Steve Kennedy
Posted on Tuesday, July 02, 2002 - 01:52 pm:   

The 112RD amps were designed to support external speakers that get connected in parallel with the internal speaker. It should say something to this effect in the text labeling the speaker jacks on the rear panel.

The Music Man amps can only support 4 ohms as a minimum, which means that unless you re-wire your external speaker jack for a Series connection you will never be able to plug anything into your external speaker jack!

The amp originally came with an 8 ohm speaker, and the amp could be plugged into an external 8 ohm cabinet for a total of 4 ohms (the minimum).

If you plug in an external 4 ohm cabinet, the amp will see 2 ohms in your particular circumstance and the result would most likely be an output transformer and/or tube failure over time.

This isn't to say that it wouldn't work, just that you would be over-stressing the output tubes and output transformer. This is not recommended! Output transformers are extremely hard to come by and expensive to get rewound.


Stephen S.
Posted on Thursday, April 22, 2004 - 10:46 pm:   

I'm running my HD130R into a 2-12 cabinet wired parallel for 4ohms. Would it be better for the amp if I rewired the 8ohm speakers in series?
Paul Bradley
Posted on Thursday, April 29, 2004 - 08:47 am:   

Yet another post regarding Ohms. I have an early (it seems) MM HD130 (2100-130 no reverb) serial #
000143(or thereabouts - the 143 is accurate, I forgot the exact leading numerals in the serial #). There is no indication @ speaker jacks as to series or parallel. This being a head (not a combo amp), I am assuming it is wired in series. I run both a MM 2-12RH (stock-listed as 4 Ohms on the MM chart) and a MM 1-15RH (this cabinet is not stock, it has been reloaded with a Peavey 15" Black Widow, the MM chart has it listed as 8 Ohms) off the main and extension jacks. From what I have gleaned from these discussions, I should run the Ohm selector switch at 8 Ohms, correct?

Thanks, Paul
lance c.
Posted on Monday, March 28, 2005 - 04:23 pm:   

how do i make a 3 subwoofer system into 2 ohms
michael kaus
Posted on Tuesday, March 29, 2005 - 06:52 am:   

Won't work unless you hjad 6 ohm speakers, which I don't think exist. Formula is 1 over the sum of the recip total. R= 1 over 1/r1+ 1/r2+1/r3. Mike.
Posted on Tuesday, March 29, 2005 - 08:14 am:   

OK Now I have to chime in too. 410-sixty five with stock speakers - switch should be on 8 ohm setting, correct. Now, if I plugged in an 8 ohm extension cab, then switch should be at 4 ohms?

Is there any way I could plug in a 16 ohm extension cab?
michael kaus
Posted on Wednesday, March 30, 2005 - 06:38 am:   

I don't know what thet 4-10's used but Steve has one. Still, it's probably the same. The low side for internal, and switch it to 8 for use with external. You are putting the speakers in series which means that the impedances ADD together. Mike.
Posted on Sunday, August 07, 2005 - 08:51 am:   

I have a MM 130 212 combo no ext.speakers do I run it on 8 or 4 ohm.
Thanks Tim
Posted on Monday, August 08, 2005 - 10:08 am:   

Steve Kennedy (admin)
Username: admin

Registered: 03-2002
Posted on Friday, August 12, 2005 - 07:18 pm:   

The rule of thumb (assuming all speakers are the factory impedances) is:

Single and 4 speaker combos should be set for 8-ohms if used by themselves or to 4-ohms if used with an external 8-ohm cabinet.

Two speaker combos are set to 4-ohm if used by themselves or to 8-ohms if used with an external 4 or 8-ohm cabinet.

The difference is because the first group (singles and 4-speaker) have their two speaker output jacks wired in parallel. The two-speaker combos (and heads) have their jacks wired in series.

Factory Music Man speaker drivers were all 8-ohm.

I have seen exceptions but most of them are wired in this fashion.


Andre Rangel (dreezy2250)
Username: dreezy2250

Registered: 02-2014
Posted on Friday, February 28, 2014 - 11:33 am:   

Hey! I have an RD210 and on the back of mine it says that the outputs are wired in parallel. It also says "Main Speaker 8ohm"; "Extension speaker 8ohm only"; "Switch to 4ohm when extension speaker is used. Jacks are wired in parallel."

So this means that the total load of the stock speakers are 8ohm (16ohm each wired in parallel). If i use another 8ohm cabinet, then the 8ohm cab would go into the extension jack and then i would have to switch to 4ohm.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Friday, February 28, 2014 - 03:46 pm:   

Exactly. Unless of course the two tens are 4 ohm and they are wired in series. Then its STILL the same!
JoJo's picture

Two amps what ohm setting

Hi, I'm playing in a big band, loads of brass, almost too much however there is no keyboard player. So to get more spread with my guitar I was thinking of using my two amps. One a 212hd and the other a 212x75. What ohm setting do I use and any other gems that may be helpful.
I attach two photos of the rear panels of the amps. Probably not helpful. Thanks for the assistance.

mm210's picture


Uh-if you are using both amps, they are self contained so they should be on 4 ohms each, unless I'm misunderstanding you. I ASSUME you are going to use the AMP in each one. If you are just going to use ONE amp but the speakers from both, without the second amp, you will plug the speakers of the second amp into the ext jack of the 130hd and put it on 8 ohms.

JoJo's picture


Thank you Steve for that concise reply. Yes I tried using both amps and they worked fine. I assumed 4 ohm. Good guess!!! However I think I will probably just use the speakers of the 75watt and as you say I will set the 130 to 8ohm. I will need a connector to take the jack from the speakers in the 75watt amp and to receive the jack from the external speaker outlet on the 130.

Is there a preferred set up? I expect that again is a personal choice. Thanks.

mm210's picture

I am NOT Steve but you are

I am NOT Steve but you are welcome. My name is Mike.

JoJo's picture

Not Steve

My apologies. Mike. Your information is a great help.

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