Safe Ohm Usage

Joined: 06 Jun 2007 Posts: 12287 Location: Twin Cities

I have a 2 cabs/1 amp question - I have an 8 ohm cab and a 16 ohm cab that I am trying to run safely into a single 100 watt Marshall head. How can I accomplish this? The amp in question is a JCM2000 DSL. Any help would be appreciated - thanks!

Here is a pic of the back panel:

Joined: 03 May 2008 Posts: 6629 Location: Minnesota

Here is my guess:

Set your amp switch to 8 ohms, and use both outputs, one to each cab.

My logic tells me that 8->8 and 8->16 is safe.

However, just wait for someone smart chimes in, to verify my theory or not.

Joined: 06 Jun 2007 Posts: 12287 Location: Twin Cities

the cabs are 8 & 16 - sorry

Joined: 27 Feb 2006 Posts: 13396 Location: Earth

The 8Ω cab goes into the 4Ω output and the 16Ω cab into the 8Ω output. You'll be running a load of about 5.3 but it might be safe. It's not going to be the most efficient way to run it but it shouldn't hurt your amp. That all depends on how the outputs work. It's a mismatch for sure. The question is do you NEED to use both cabs? Better safe than sorry. I wouldn't personally do it but the info above applies. Also running both cabs with that mismatch you'll notice one cab not running as hot as the other. I don't know if there would be any issues with sound having one quieter than the other.

Joined: 03 May 2008 Posts: 6629 Location: Minnesota

But they are both either 4/4 or 8/8 according to the switch.

I dno't believe you can have it 4/8???

Joined: 06 Jun 2007 Posts: 12287 Location: Twin Cities

doesn't the 4/8 ohm switch affect both the jacks not labeled 16 ohm? don't wanna jack anything up and want to be sure before I try them.

I do not "need" both cabs no. chances are I may get rid of the 2x12 but I just want to play around a bit before a decision is made

Joined: 03 May 2008 Posts: 6629 Location: Minnesota

rridner wrote:

doesn't the 4/8 ohm switch affect both the jacks not labeled 16 ohm? don't wanna jack anything up and want to be sure before I try them.

I do not "need" both cabs no. chances are I may get rid of the 2x12 but I just want to play around a bit before a decision is made

you could just buy another Marshall, and run a stereo rig.

Joined: 06 Jun 2007 Posts: 12287 Location: Twin Cities

if I did, it wouldn't be yours Laughing

Joined: 07 Aug 2007 Posts: 2740 Location: Southeast MI

The safest way would be set it to 4 ohms and have one 4 ohm output going to an 8 ohm cab and the other 4 ohm output going to the 16 ohm cabinet. The total load of the cabs in parallel is 5.3 ohms, so since that's a little higher than the 4 ohm output, you should be fine...

Joined: 27 Feb 2006 Posts: 10203 Location: New Jersey USA

Here is the issue. The amp will be sending a s#%t load more power to the 8 ohm cab, and the 16 ohm cab will hardly be working. I would recommend matching cabs properly, especially with a good tube amp. Solid State you can get away with more shenanigans with the speakers. Strictly from a total impedence standpoint, though, if you were to daisy chain the speakers in parellel, you should be fine, as the amp can go down to 4 ohm.

I dont know how the 16 ohm output works in relation to the two switchable 4/8 ohm outputs too, as I dont have a JCM2000 series. But you may be able to work something there. Maybe put the 16 ohm cab into the 16 ohm output, and the 8 ohm into one of the 8 ohm outputs? Not sure if that works or not on that head.

Good luck.

Joined: 06 Jun 2007 Posts: 12287 Location: Twin Cities

if plugged into the 16 ohm jack, the others are canceled out

Joined: 27 Feb 2006 Posts: 13396 Location: Earth

rridner wrote:

if plugged into the 16 ohm jack, the others are canceled out

What does that mean? "Cancelled out". If you run both cabs you're at a 5.3 ohm load total.

Joined: 03 May 2008 Posts: 6629 Location: Minnesota

Lanning is Killdozer wrote:

rridner wrote:

if plugged into the 16 ohm jack, the others are canceled out

What does that mean? "Cancelled out". If you run both cabs you're at a 5.3 ohm load total.

Looks like if you use the 16ohm jack on the back of the amp, it stops the output to the other 4/8 ohm jacks.

Posted: Sat Dec 01, 2012 3:45 pm

Robocop be the man who needs to chime in here...

Joined: 28 Dec 2007 Posts: 1463

bassmantim71 wrote:

Robocop be the man who needs to chime in here...

Hope I can help...................

GuitarGoat wrote:

The safest way would be set it to 4 ohms and have one 4 ohm output going to an 8 ohm cab and the other 4 ohm output going to the 16 ohm cabinet. The total load of the cabs in parallel is 5.3 ohms, so since that's a little higher than the 4 ohm output, you should be fine...

I am not familiar with this head, but if it is all solid state the above would be my guess too

Another few questions though:

What are the two cabs being hooked up (ie: 8 ohm 4X12 and a 16 ohm 2X12).

this info will also help in determining if the impedance mismatch is going to be a problem or not.

*IF* it is a 8 ohm 4X12 being combined with a 16 ohm 2X12, then it is fine as each individual speaker wil be getting the same amount of power.

Joined: 06 Jun 2007 Posts: 12287 Location: Twin Cities

amp is tube not SS. 4x12 is 8 ohm, 2x12 is 16 ohm.

Joined: 28 Dec 2007 Posts: 1463

Hmmmmm....................

Just looked the head up, and it is all valve.

With tube amps it is important to match the impedance to the correct output taps.

*From what I know* you can have a mismatch, but tube amps are the opposite of solid state.

With SS amps, you have have any impedance that is HIGHER then the minimum rating of the amp (you just get less power).

With tube amps, I believe *IF* you are going to mismatch, you want to do it at a lower impedance.

Therefore, my best guess would be to place the ohm selecter switch to 8 ohms, and run one cab in each of the "non 16 ohm" taps.

NOTE - in any case of mismatching with a tube amp you will be working the output transformers out of spec range which could cause problems as well as shortened tube life.

I would not recommend it, and would suggest speaking to either someone at Marshall or to a good tech for their opinions.

Joined: 28 Dec 2007 Posts: 1463

rridner wrote:

amp is tube not SS. 4x12 is 8 ohm, 2x12 is 16 ohm.

See my post above.............

As far as that pairing of cabs, if it is safe to run the amp that way, at least all of the speakers would be getting equal power.

Joined: 06 Jun 2007 Posts: 12287 Location: Twin Cities

thanks for the info. I will contact Marshall.

Joined: 06 Jun 2007 Posts: 12287 Location: Twin Cities

reply from Marshall:

Quote:

Hey Rob,

Thanks for your request. We would recommend only using 2 cabinets of the same impedance. Different impedances like this could hurt your output transformer.