Weird speaker cable connection JCM900-1960A

Discussion in 'Introduce Yourself' started by StNicolaas, Dec 2, 2019.

  1. StNicolaas

    StNicolaas New Member

    Joined:
    Dec 2, 2019
    Messages:
    2
    Likes Received:
    4
    Hi all thx for having me join this forum.:agreed:

    Last year, after many years of playing, a boyhood dream finally came true, I got myself a Marshall half stack, a JCM900 4100 with a 1960A. Both my 1979 Strat and 2013 LPR8 vos sound awesome and allow me to mimic the brown sound, somewhat.

    It took me about a year to realize there was something odd with how I was told to connect the amp to the cab. I bought the set of someone whom told me to connect it in this fashion and it has been working fine. The speaker cable is connected to the 4 Ohm input. But the amp selector switch is set to 8 Ohm, as if it was a stereo connection? See included :photos:. As I want to connect the amp to another cab, I need to know for sure what is going on.

    It is possible the tech of they guy I got it from (whom I cannot reach anymore), made some mods causing this weird way of connecting it? Could there be any harm in connecting it the proper way, selector at 16 ohm as well as speaker in 16 Ohm input? And then trying it on very low volume? Or could this potentially damage the amp?
    amp.jpg cab.jpg
     
    Mitchell Pearrow likes this.
  2. Micky

    Micky Well-Known Member VIP Member

    Joined:
    Apr 29, 2010
    Messages:
    20,447
    Likes Received:
    16,707
    Location:
    Vermont
    That is wrong.
    Connect it to 16/16
     
    Mitchell Pearrow likes this.
  3. anitoli

    anitoli Well-Known Member

    Joined:
    Jul 9, 2011
    Messages:
    7,003
    Likes Received:
    8,009
    Location:
    Lewiston, Maine U.S.A.
    Take an ohmmeter and measure the resistance on both the 4 and 16 ohm settings, what do you get?
     
    Mitchell Pearrow and Sustainium like this.
  4. DaDoc

    DaDoc Well-Known Member

    Joined:
    Oct 13, 2014
    Messages:
    2,485
    Likes Received:
    2,428
    Location:
    Rancho Deluxe, 3rd stone from the sun
    Your head should be set on 16 Ohms, (should be the one in the middle) along with the speaker cab (In mono) If you add another cab (at 16 Ohms) you need to change the amp impedance selector to 8 Ohms. Your cab don't look like it's been modded, at least not from the outside. My 80's era 1960 Vintage series slant cab has the same setup on the back. I DO wonder what those magic marker thingies on the sides of the selector switch and hi/low power switch are..:scratch:

    Now, my two 70's era cabs are different..They don't have anything but a single 1/4 jack input on the back..Cabs from that era are usually 16 Ohms unless they have been tinkered with. Or had speakers changed.

    You can also wire your speakers up in different stereo/ohm setups, but that can get complicated and can also damage your amp if you do it wrong.

    Hope this helps!
     
    Last edited: Dec 2, 2019
    Mitchell Pearrow likes this.
  5. Michael Roe

    Michael Roe Well-Known Member

    Joined:
    Mar 25, 2017
    Messages:
    1,453
    Likes Received:
    3,499
    Location:
    Springfield, Ohio
    On the cab there is a switch that reads Mono/Stereo. In your pic you have it on "Mono" and are running in the 4 ohm input. If your amp output is 4 ohms then this correct.
    The weird thing about it is that it appears that a mod has been done to the speaker outputs to make it 4 ohm.
    If you notice to the left of the speaker outputs you read "(4) 16 8". The reason is that the output transformer actually has a "4 ohm" tap that can be modded internally to work. From the factory it is set to only be " 16 and 8". That is why there is a " ( ) " around the number 4. This explains why there appears to be some sharpie markings on it.
    I personally would take it to a tech to find out for sure what is going on.

    Oh, and just in case: No Marshall tube amp heads are stereo amps. Those cabs were made that way to accommodate other stereo amps or Marshall's own rack mount stereo power amps of that time.
     
    Last edited: Dec 2, 2019
    Mitchell Pearrow likes this.
  6. Michael Roe

    Michael Roe Well-Known Member

    Joined:
    Mar 25, 2017
    Messages:
    1,453
    Likes Received:
    3,499
    Location:
    Springfield, Ohio
    When you read the cab inputs think of it like this:
    When the Mono/Stereo switch is in the "Mono" position, only read the upper info 4 ohm & 16 ohm.
    When the switch is set to "Stereo", only read the bottom info, Right 8 ohm / Left 8 Ohm.
     
    Mitchell Pearrow likes this.
  7. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,925
    Likes Received:
    8,765
    Location:
    Start the reactor... Free Mars!
    Are you using a real speaker cable? This should be 2 conductor, non-screened (un-shielded) cable.
    All speaker connections must be made without guitar leads.
    Please do not connect the speaker using guitar cables. This can damage the amplifier.
     
  8. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

    Joined:
    Jul 2, 2018
    Messages:
    8,813
    Likes Received:
    13,453
    Location:
    Moreno Valley CA
    What all has been said above is on the money.
    :welcome: To the forum
    Cheers Mitch
     
    DaDoc likes this.
  9. StNicolaas

    StNicolaas New Member

    Joined:
    Dec 2, 2019
    Messages:
    2
    Likes Received:
    4
    Wow, thanks all for your quick and useful input. Yes the speaker cable is a true speaker cable, not a lead and I understand the Mono/Stereo part.

    As suggested, I set it to 16/16, however this seems to result in it producing far more volume than normally, when plugging a guitar in, in the front of the amp.
    But when I connect my guitar in using FX return (the way I normally do, as I am using a Helix, with a dummy plug in the FX send otherwise it does not work), thus bypassing the pre-amp, the volume is far lower than normal.

    As suggested I think I need to get it to a tech, because I can not make heads or tails of this and do not want to risk blowing up anything.

    NB I do realize that using the FX loop this way, I am missing out on what this amp is all about;) and that is exactly the reason I am fiddling with this as I do want to start using its full potential.
     
  10. Michael Roe

    Michael Roe Well-Known Member

    Joined:
    Mar 25, 2017
    Messages:
    1,453
    Likes Received:
    3,499
    Location:
    Springfield, Ohio
    yes, take it to a tech, good idea!
    If it were all stock it would be simple: set amp output to 16 ohm. Set cabinet to Mono and plug into 16 ohm jack. If your amp has been changed I would have tech convert it back to normal.
     
    Mitchell Pearrow and DaDoc like this.
  11. DaDoc

    DaDoc Well-Known Member

    Joined:
    Oct 13, 2014
    Messages:
    2,485
    Likes Received:
    2,428
    Location:
    Rancho Deluxe, 3rd stone from the sun
    When in doubt, call a tech!
     
    Mitchell Pearrow likes this.
  12. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,925
    Likes Received:
    8,765
    Location:
    Start the reactor... Free Mars!
    There's nothing wrong.
    The "Helix" does not have enough output level to drive the power amp.
     
    Dogs of Doom likes this.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice