Jcm900 4100 Ohm Selector Question(s)

Discussion in 'Marshall Amps' started by Gutch220, Oct 27, 2018.

  1. Gutch220

    Gutch220 Well-Known Member

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    In the rear ohm selector there is an ohm switch to choose either (4)16 or 8.

    -Why/how can one choice be both 4 and 16 ohms?
     
    Last edited: Oct 28, 2018
  2. Gutch220

    Gutch220 Well-Known Member

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    Nevermind. Found the answer to this but I'll explain it so people with the same question can get an answer as well.

    For heads, the switch shows (4)16 or 8, which means it can run at 16 ohms or 8 ohms, with the option for 4 ohms with internal wiring changes.
    For combos, the switch shows 4(16) or 8, which means it can run at 4 ohms or 8 ohms, with the option for 16 ohms with internal wiring changes.

    *basically, ignore the number in parenthesis if no 'work' has been done on the amp.

    The JCM800 switch for 4, 8, or 16 ohms is so much better in my opinion. This JCM900 set-up basically makes the stereo option for the 1960 cabinet useless unless you mod it for 4-ohms for two separate 8-ohm cabinet halves, same for the mono 4-ohm option. I never use a 1960 in stereo anyway, but still.

    *Note: I removed some of the questions from the original post so as not to further confuse people.
     
    Last edited: Nov 8, 2018
  3. iron broadsword

    iron broadsword Well-Known Member

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    A 1960a set to stereo is 8 ohms on each side, and you've got a setting for that so not seeing the problem. But everything else was right and having the 4ohm option should have been implemented
     
  4. Marshallhead

    Marshallhead Well-Known Member

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    I believe the reason for that very poorly understood and therefore downright risky 4(16) nonsense was so a two position switch mounted to the rear pcb could be used. The usual 3 position rotary switch would need mounted off the board, and that comes at a cost in chassis space/£££.

    Also, with a single head and cab, you should not use both speaker inputs - just the 16ohm input with the cab set to mono and the amp set to 16.
     
  5. Marshallhead

    Marshallhead Well-Known Member

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    No he doesn’t, not if it’s an amp head - it has no 4 ohm option unless the amp’ is altered inside.
     
  6. Gutch220

    Gutch220 Well-Known Member

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    it's weird that 1960 cabinets still have a stereo option (8 ohms on each side thus requiring 4 ohm amp setting) when there's no 'stock' 4 ohm option on the head.

    Plus, to me there's really no point in having something in stereo when the speakers are 20 inches apart. just sayin'
     
    Mitchell Pearrow likes this.
  7. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member VIP Member

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    I am one of those people who would turn the cab into a non stereo cab, don’t really need that type of set up! I would run two cabs one on each side, with a pair of heads:coffee:
     
  8. Marshallhead

    Marshallhead Well-Known Member

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    The stereo cab setting was intended for use with the stereo valve power amps, and the mono/stereo plate remains a feature even though those power amps were discontinued.

    Using it in stereo mode with a single head set to 4 ohms through two speaker cables is a pointless exercise, and doesn’t produce stereo - that’s not what the stereo option was intended for.
     
  9. Gutch220

    Gutch220 Well-Known Member

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    yea, for a normal amp, it's pointless. I ran a delay into it once to ping-pong back & forth between sides, and it's pointless. but it works as described. it's probably more useful to run two amps into the cabinet and experiment with contrasting tones
     

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