Wiring a cab

Discussion in 'Cabinets & Speakers' started by Rock city, Mar 8, 2021.

  1. Rock city

    Rock city Member

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    Hi!
    I have a chance to buy Marshall 1960A from early 90s with g12t-75s in.
    Each speaker is 16 Ohms.
    Cab has that plate with 2 input jacks saying 4 and 16 Ohms.
    Which is funny to me; 4x16 Ohms speakers could be wired 8 Ohms in pair or 16 Ohms all four.
    But ohmmeter shows even funnier: 3.4 Ohms in 4 Ohms input and 6.5 Ohms in 16 Ohms input.
    Beside that, I would like to wire it solid in just one input jack.
    Is there any trick how to wire 4x16 Ohms speakers to get 8 Ohms in input jack?
    If not, I would take out 2 speakers and make 2 cabs, each 2x12 with 150W and 8 Ohms
     
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  2. fitz288

    fitz288 Well, Known Member Silver Supporting Member

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    That's odd. Check the wiring, it may be wired for 4 speakers parallel at 4 ohm & only 2 speakers parallel at 8 ohm, or there's something loose that's not working on the 16 ohm parallel/series side of the switch, or perhaps there's a blown speaker in there somewhere?

    No, but you could rewire for 2 ~ 8 Ohm inputs of 2 speakers parallel to each jack and keep everything in the 4x12 cab.

    Take the back off and post pics. :photos:
     
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  3. scott-colorado2016

    scott-colorado2016 Well-Known Member

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    If you have 4 x 16 ohm speakers, the can be wired in 3 ways, resulting in 3 different impedances.
    1. Series/parallel - each pair of speakers is wired in parallel, then the pairs are wired in series (or each pair is wired in series, then the pairs are wired in parallel, either way is mathematically the same) - resulting in a 16 ohm load.
    2. All 4 speakers are wired in parallel - resulting in a 4 ohm load.
    3. All 4 speakers are wired in series - resulting in a 64 ohm load (not used in common amp configurations).

    There is no maybe that will get you 8 ohms using 4 16 ohm speakers.
    Basically, one jack is series/parallel, the other jack is parallel (as 1 and 2 above).
     
  4. Adieu

    Adieu Well-Known Member

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    3.4 ohms = "nominal 4 ohms" = definitely all 4 speakers engaged

    6.5 ohms is the real reading of "nominal 8 ohms", which means only one side of the stereo cab
     
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  5. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Read all about the difference of Impedance and Resistance.

    What you are reading is DC Resistance, Speakers are given a value of Impedance.

    A little knowledge can be quite confusing my friend.
     
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  6. Rock city

    Rock city Member

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    What does it mean?
     
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  7. Rock city

    Rock city Member

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    I've found out that funny reading could be because of jack plate and/or switch which is known for making a mess.
    Definitely will go for:
    1. Leaving just two speakers in 4x12, wired 8 Ohms to match my other 2x12 cab with g12m-65
    2. 16 Ohms wiring fixed in one jack with no possibility of mess and burning OT of the amp
     
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  8. downstroy

    downstroy New Member

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    dear, i have 4x12 g12t75 speakers in the cabinet ... each is 8 ohms. I wonder which wire to use for their parallel connection? some say 2x1.5mm2, others say 2x0.75mm2 ......? thank you
     
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  9. Rock city

    Rock city Member

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    Use OLD wire, the older you can find. 1.5 mm2 would be just fine.
    A friend reveal me the secret the other day: melting a copper again and again does harm to conductivity so newer wire recycled many times would have pity conductivity comparing to old ones. Find any old cheap device containing old copper wire, cut it out and wire a cab. Significant difference should be noticed.
     
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  10. downstroy

    downstroy New Member

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    thx....:cheers:
     
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  11. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Read about it mate it is all out there on the Net
     
  12. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Your friend doesn't know what he's talking about. Copper wire is never made from recycled copper. For wire, it's always made from virgin copper. All the recycled copper goes to less critical uses such as plumbing pipe.

    Less pure copper is less conductive, that is true. That's why wire isn't made from recycled/contaminated/dirty used scrap copper.

    Impedance is the equivalent of resistance but it applies to alternating current, AC, and not to direct current, DC.

    Impedance involve phase angle (the effects of inductance and capacitance) and is not read correctly with a regular volt-ohm meter.
     
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  13. Rock city

    Rock city Member

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    No, not about your claim.
    Just the way you put it.
    It seemed rude to me.
    And btw you missed the thread point a bit.
     
  14. Rock city

    Rock city Member

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    I hope that's the way in your country.
    My close friend gave 5 different wires to be tested chemicaly.
    Only old 1.5mm2 wire taken outta some military electronics showed copper in good percentage.
    Newer electrical wire used in electronics and home instalation was poor in copper.
     
  15. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Yes ask by all means but don't expect others to do it for you, self leaning is key, as for me being rude well mate i say it as it is and if that upsets you then take a good look at your self, crumbs its 2021

    I had to do it and was only 16, so why shouldn't you.
     
  16. Rock city

    Rock city Member

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    No feeling when to stop being yourself, right?
    Now, read #13 again. Last line twice.
     
  17. neikeel

    neikeel Well-Known Member

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    The problem with your cab is probably within the switch, as was pointed out earlier. Because of the difference between DCR and impedance you will always measure a little low. If you measure 3.4ohms on the 4 ohm selection - that is correct. If you are only getting 6ohm or so that suggests you are only getting two speakers in parallel for 8ohm selection. Your expected observed measurement for the 16ohm option would be 12-14ohms.
    The point Ken is making is that you need to understand the basics behind the calculations.
    To confirm if you have x4 16ohm speakers the only options for all speakers is parallel = 4ohms, series/parallel or parallel/series for 16ohms or all series for 64ohms.
    Otherwise you have to split them.
    Those switches are known to be failure prone over time and some contact cleaner/lubricant May help. The safest way is to hard wire.
     
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  18. BRMarshall

    BRMarshall Member

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    Scott-Colorado2016 and neikeel are correct. If I were in your position, I would be inclined to skip the switch and just wire it for 16ohm. Most Marshall amps I have some familiarity with seem happy with a 16 ohm cabinet. My Vox AC4c1 combo has an external speaker jack - 16 ohm... so there you go.
    Seriously, do what works best for you and good luck.
     

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