8ohm cab measures 10ohm

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Norma Schock, Oct 8, 2019.

  1. Norma Schock

    Norma Schock Member

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    Hi, I have a marshall 1965b 4x10 that is rated at 8ohms (four 10" 8ohm speakers). I used a multimeter to make sure the cab ohm is 8, but it measures out as 10ohms. Measured by putting the probes to the speaker cable jack. From my understanding it should measure in the 6-7 ohm range if cab is 8ohm. Does anyone know why it's higher? Thanks
     
  2. BluntForceTrauma

    BluntForceTrauma Member

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    No idea unless one speaker is wired wrong. Or the wrong impedence.
     
  3. gldtp99

    gldtp99 Member

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    Some possibilities:

    1) weak battery in meter

    2) dirty jack in speaker cab not making good contact

    3) dirty speaker cable not making good contact

    4) dirty leads on meter not making good contact

    5) wrong speakers in cab

    6) speaker cab wired incorrectly

    Any or some of these conditions could very well throw off your DC Resistance measurement.

    Set up the conditions so that you are able to make sure, reliable measurements.

    Then you will know if there is a problem with the speaker cab itself or not.
     
  4. skytrench

    skytrench Member

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    Or one of the speakers might be shot.
     
  5. pdf64

    pdf64 Member

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    So what does the meter read if the probes are shorted together?
     
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  6. Johndh

    Johndh Member

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    It is possible that one speaker is not connected or bust. 6.5 connected in series to 2x6.5 in parallel is 9.75.

    Could check that by listening closely to each speaker to check it works.
     
  7. Tron Pesto

    Tron Pesto Member

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    And if you have a known good resistance source (like a new high tolerance resistor), do you get a correct reading when you measure that?
     
  8. Norma Schock

    Norma Schock Member

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    Thanks for the responses, I'll check the speakers today. I have a couple other cabs and those were in line with what you would expect.
    Also, when you say what the meter reads when probes are shorted together, is that just touching the red probe with the black one? Sorry just got the multimeter yesterday, first time using one. Also, I took the measurement by putting the red probe to the sleeve and the black one to the tip of the input jack. I don't have that mixed up, or does that matter? I seen examples online with red to sleeve, black to tip, and vice versa.
    Thanks!
     
  9. pdf64

    pdf64 Member

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    Just press the tips of the red probe and black probe together, with the meter set to measure low ohms.

    When measuring resistance, it doesn’t matter which probe goes to which end of things
     
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  10. Richard Guy

    Richard Guy Member

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    it should display something way close to 000.0 ohms
     
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  11. easyed

    easyed Supporting Member

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    If one of the speakers is blown, a 10 Ohm resistance is expected - assuming that the cab is wired series-parallel. It could be anything from a loose connection to a blown speaker. It only takes one break in series circuit to make it non-functional. Two 8 Ohm speakers wired in series will produce impedance of 16 Ohms.
     
  12. Norma Schock

    Norma Schock Member

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    I tested the speakers and they sound ok. No characteristics of a blown speaker, even on the low e. I guess I'll bring it in somewhere to have the jack/wires/connections checked. The wires are soldered as well, no clips. Thanks for the responses!
     
  13. pdf64

    pdf64 Member

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    I think your measurement will include any resistance in the speaker lead, and contact resistance between the cable plug and cab socket. Try it with a different speaker cable.
    If it’s one of those cabs with switchable mono-stereo / impedance, there’s also resistance in the switch.
    Low ohm measurement is pretty challenging for regular DMMs; my Flukes read about 0.2ohm with the probes shorted (the cal button can be used to tare that out), so I wouldn't expect much less than 0.5ohm from a cheap meter / probe set.
     
    Last edited: Oct 10, 2019

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