How do I read ohms on a 4x12 that I can't open?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by LowWatt, Jul 2, 2008.

  1. LowWatt

    LowWatt Member

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    The rehersal space my band uses has a beat up Marshall 4x12 cabinet that I need to use with my Bumbox head. The head has taps for 4, 8, and 16 ohms.

    The cabinet has no markings telling me its resistance. I figure it's either 8ohms or 16ohms, but I have no idea which one. Last week I just used the 8ohm out figuring I was still safe if it was 16ohms.

    My sound was a little muffled last week, so I'm figuring it's a 16ohm cab, but I don't want to take chances with my transformer.

    Is there a way I can use a DMM or something to test the reistance of the cab without opening it up?
     
  2. Dave C

    Dave C Gold Supporting Member

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    Yes you can check speaker cabs with a DMM. The DC resistance reading will be approx. 80% of the impedance. A 4 ohm load will read about 3ohms, 8 ohm about 6 and 16 will read about 13 ohms.
     
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  3. LowWatt

    LowWatt Member

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    So I just pop in a speaker cable and test it against the hot and ground contact points on the plug?
     
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  4. Lyle Caldwell

    Lyle Caldwell Member

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  5. LowWatt

    LowWatt Member

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    Thanks! I've got my DMM in my gig bag and I'll solve my problem at practice tonight.
     
  6. 5E3

    5E3 Member

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    You could also plug a speaker cable into the cab and measure the resistance at the tip and sleeve of the other end. There will be more resistance, but shouldn't be that much. Don't have to open the cab that way.
     
  7. doctorx

    doctorx Member

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    This is the way to do it. The resistance of the cable will be minimal. Pulling the back off the cabinet is silly.
     
  8. LowWatt

    LowWatt Member

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    Wait. How is this different from what was already being discussed (see below). One answer says the reading will be lower. One says higher. Both say to test with a DMM against the speaker cable that is plugged into the closed cabinet. If I do this will I get a number under or over the resistance?

     
  9. Cotton

    Cotton Member

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    LowWatt - You are absolutely fine to plug in a speaker cable and read the ohms across the tip and sleeve of the cable that would be used to plug into your amp. You will not read exactly 16 ohms, but you will be close.

    I think everyone here is saying the same thing. But sometimes to be brief, we use subjective terms and kind of confuse earlier postings.... but, you fine to check the cab ohms as noted. I do it ALL the time...

    PM me if you see any further concern and I'll give you a call...

    good luck!

    Cotton
     
  10. 5E3

    5E3 Member

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    oops. my bad. :jo my apologies.

    I need to go back to the Evelyn Woodhead Sped Redding Skool :rolleyes:
     
  11. LowWatt

    LowWatt Member

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    No worries and thanks for the offer. It's either 8ohms and 16ohms, so whether its over or under it should be obvious because of whichever it's closer to.

    James aka LowWatt
     
  12. DGDGBD

    DGDGBD Member

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    The measured resistance will be slightly less than the cab's impedance. Dave C. has it right (or the clearest, I should say).
     
  13. 5E3

    5E3 Member

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    :agree

    I had a new Emi C-Rex arrive today and had ordered a 8 ohm, but couldn't find an 8 anywhere on it. So used the same method and it registered just over 6 ohms.
     
  14. LowWatt

    LowWatt Member

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    Yep. The cab came in at 13ohms at practice last night. I plugged into the 16ohm jack and was treated to a much louder and brighter amp than I had in the week before when I plugged into the 8ohm load trying to be safe.
     
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  15. hamfist

    hamfist Member

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    Peel back the rubber ring around the magnet cover, and the impedance will be revealed !
     
  16. 5E3

    5E3 Member

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    Ah ha!

    Thanks for the tip! :)
     
  17. Belmont

    Belmont Member

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    check all cabs you play through no matter what it says on the plate,because I played through a cab that was labeled as a 16ohm for years,only to find out later that it was actually 8ohms.:jo
     
  18. LowWatt

    LowWatt Member

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    Good advice always. Most rehersal spaces I've been too probably wouldn't think twice about relabelling the impedence after replacing speakers in a cab.
     
  19. vibroverbus

    vibroverbus Member

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    which is another quick and dirty way to guesstimate... trying it in the various taps - most of the time you'll be able to tell which is the loudest brightest combination and is impedance matched.

    between all the possibilities, the only combo that could potentially be way bad for the amp would be 4ohm tap to a 16ohm cab. even then as long as you didn't crank it hard for long (just long enough to note that it's clearly not as good as the others) it's doubtful you'd arc a tube socket or fry a tube in a short test.
     
  20. Guitardan7

    Guitardan7 Member

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    Thanks from 10 years later. Needed to check without taking the back off and my 8 ohm cab was putting out just over 7 ohm by checking with a cable
     
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