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JCM900 Impedence question

Discussion in 'Miscellaneous [BG]' started by i_got_a_mohawk, Feb 17, 2008.


  1. Right, thought i'd post here.

    I did a search around some guitar forums, and the amount of BS just made my head hurt too much.

    Anyways, my mate has a marshall JCM900. On the back, at the impedence switch it is as follows :

    (4) 16 [-] 8



    (the [-] is the switch).

    He currently has an 8 ohm 2x12, and is thinking of adding another 2x12 so he can have a 4x12 when he needs it, and a 2x12 when he doesnt. Going for a modular approach (he's quite bright for a guitar player).

    But anyways, he was asking me about impedences (I said he was bright for a guitar player ), and, i was wondering, but looking for some clarification.

    Would it just be a case of sticking the switch to "(4)16" and plugging the two cabs in so they were in parallel giving a 4 ohm load?

    Read some stuff on guitar forums claiming that no amps can handle 4 ohms, that you would want 16 ohms anyway as it sounds better. And the (4) is only there because it is possible to mod the amp to use a 4 ohm load. (which seems a bit wierd that they'd list it on the back when it cant do it).

    So, anyone know the score on these amps?
     
  2. Should also add, that i looked in the marshall manual, and it isnt entirely clear. Unless they print "(4)16" on the backs of all the amps in the series, and the 4 only applies to combos with the 16 applying to heads.
     
  3. I can't answer you specific question above, but I *can* tell you my Laney VC30 handles 4 Ohms quite nicely, thank you. :)
     
  4. Erm, thanks :p

    My TE Speedtwin handles 4 ohms fine aswell :) , playing guitar through a 2x15 or 8x10 is good fun!
     
  5. My guitarist has a JCM900 head, if i get time to tomorrow ill pop round and check his out. I do remember his one has the exsact same switch, and i even explained to him what it means, but that was a while back.

    So unless someone else replied before i do, ill have a look.
     
  6. Cheers it would be much appreciated!
     
  7. Hi.

    On the schematics of some Marshalls there's a note that says quote:

    Output impedance select can be wired for either 8 & 16 Ohm (normal) or 4 & 8 Ohm.

    So in order to use 4 ohm load an internal modification is necessary if not done already.

    Here's the schematic:

    http://www.schematicheaven.com/marshallamps/jcm900_25xx_45xx_50w.pdf

    Regards
    Sam
     
  8. That makes sense. The schematic didnt load, but im sure i could find another one (my internet is being dodgy right now).

    So its just a case of they are wired for 4 and 8 ohms when used as a combo, and 16 and 8 ohms when used as a head. I still think its a bit wierd that they list it as (4) on the head versions when it needs to be modified to be used as such.

    Its all good tho, if get gets another 2x12, it'll be the same as what he already has, so ill just point him in the direction of a series cable.
     

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