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post #1 of 8 (permalink) Old 08-24-2010, 10:47 PM Thread Starter
 
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Choosing Ohms for 2 cabs

Ok, heres the deal. I have a Peavey JSX head which has a 4-8-16 ohm switch on the back and a marshall cab which is set to 8 ohms mono. I set the head to 8 ohms accordingly.

I'm looking to get another cabinet (Peavey) and this cab is 16 ohms mono, 8 ohms stereo. I want to run both cabs but I am unsure how I should set the head and cabs. If I set the peavey cab to 8 ohms it will only run one side as its a stereo cab. Any help is greatly appreaciated.
Rick
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post #2 of 8 (permalink) Old 08-25-2010, 01:06 AM
NSR
 
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Re: Choosing Ohms for 2 cabs

You'll need to find out how ohms each speaker has in each cabinet and how they're wired.

Series: R = R1 + R2 + ... + Rn ohms
Parallel: R = (R1 x R2) / (R1 + R2) for 2 speakers; or for more than 2 speakers 1/R = 1/R1 + 1/R2 + ... + 1/Rn

Eg.
(i) Two 4 ohm speakers wired in series gives you 8 ohms.
(ii) Two 8 ohm speakers wired in parallel gives you 4 ohms.
(iii) If you were to wire in parallel 2 sets of two 8 ohm speakers (which are wired in series), that'll give you 8 ohms altogether:
Ra = 8 + 8 = 16, Rb = 8 + 8 = 16
R = (Ra x Rb) / (Ra + Rb) = (16 x 16) / (16 + 16) = 256 / 32 = 8 ohms

Sounds to me like the speakers in each cab may have a different rating. You would need to look inside your marshall cab and determine how many ohms each speaker has and how they're wired up to see if you can rewire it to give the cab a 16 ohm rating. You'll not get a balanced sound otherwise when using all the speakers of both cabs, no matter what you set your amp to.

Best that you speak to / have the work carried out by a professional to make sure that you don't break anything.

Better yet, you could try and and find a different cab rated at 8 ohms or trade your marshall cab for a another peavey cab.
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post #3 of 8 (permalink) Old 08-25-2010, 11:19 AM
 
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Re: Choosing Ohms for 2 cabs

Sorry, just realised I misread the original post, you could use half of the new peavey cab as you suggest by setting it to stereo and plugging one side in together with the Marshall. Make sure you set the impedance switch to 4 ohms.
Basically your peavey has 16 ohm speakers while the Marshall has 8 ohm, that's why you can't have all the speakers on at the same time.

Jim

Last edited by Vim Fuego; 08-25-2010 at 11:27 AM. Reason: Correction
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post #4 of 8 (permalink) Old 08-25-2010, 12:47 PM
 
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Re: Choosing Ohms for 2 cabs

Not true--run both your 8-ohm Marshall cab out one side and your 16-ohm mono Peavey out the other side. They will be in parallel and this will produce a resultant load of 5.33333 ohms. Set your head to 4 ohms. You will be fine and not blow your output transformer.

The taps on your head are MINIMUM ratings, meaning if you set your selector switch to 4 ohms, you need AT LEAST a 4-ohm load. A higher load is safe, but may not be to your liking. Many famous artists run their 16-ohm cabs out of an 8-ohm tap because they like the way it sounds.

Learn your math, kids... key to the universe.
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post #5 of 8 (permalink) Old 08-25-2010, 08:09 PM
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Re: Choosing Ohms for 2 cabs

Quote:
Originally Posted by racerevlon View Post
Not true--run both your 8-ohm Marshall cab out one side and your 16-ohm mono Peavey out the other side. They will be in parallel and this will produce a resultant load of 5.33333 ohms. Set your head to 4 ohms. You will be fine and not blow your output transformer.
Agree with the load on the amp, however, with the cabs being imbalanced then running the peavey in mono mode at 16 ohms will mean (speaker characteristics aside) that the marshall may be significantly louder than the peavey. This is because you will need twice the power to drive the peavey cab. Another solution of course would be to place a power attenuator in line with the marshall cab and then you can dial in whatever sounds good to you.

Quote:
Originally Posted by racerevlon View Post
Learn your math, kids... key to the universe.
No need to get flippant... really.
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post #6 of 8 (permalink) Old 08-26-2010, 04:01 PM
 
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Re: Choosing Ohms for 2 cabs

Quote:
Originally Posted by NSR View Post

No need to get flippant... really.
Not trying to be flippant... it's a quote from a movie... "The Prophecy" -- Christopher Walken plays the angel Gabriel and is talking to a bunch of elementary-school kids.

No disrespect meant...
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post #7 of 8 (permalink) Old 08-26-2010, 04:08 PM
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Re: Choosing Ohms for 2 cabs

Quote:
Originally Posted by racerevlon View Post
Not trying to be flippant... it's a quote from a movie... "The Prophecy" -- Christopher Walken plays the angel Gabriel and is talking to a bunch of elementary-school kids.

No disrespect meant...
Thank you for clarifying that. With not seeing the movie your quote did get misinterpreted.
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post #8 of 8 (permalink) Old 08-26-2010, 10:36 PM Thread Starter
 
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Re: Choosing Ohms for 2 cabs

Thanks for the info people. Looks like I'm gonna pass on the peavey cab. Doesnt suit my needs.
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