HEAD TO CAB OHMMS ?

Discussion in 'Marshall Amps' started by JCMRobbo, Feb 26, 2020.

Tags:
  1. JCMRobbo

    JCMRobbo New Member

    Joined:
    Jan 17, 2020
    Messages:
    12
    Likes Received:
    15
    Hey chaps! I’ve never actually known this. So I thought you lot would know.

    head to cab:

    if I set my head to (4) 16 what do I need to set my cabinet too?

    What if I set my head to 8 what do I need to set the cabinet too?

    AND WHY DOES THAT MATTER?

    Cheers! All for learning!

    Chazz
     
    SkyMonkey likes this.
  2. SkyMonkey

    SkyMonkey Well-Known Member

    Joined:
    Oct 28, 2018
    Messages:
    1,726
    Likes Received:
    2,035
    Location:
    England
    It would help if you state the head and cab models.
    Not sure what the (4) in brackets is supposed to mean?

    But I will jump in and guess that it is a stereo 412 cab like a 1960, and the head is mono.

    [​IMG]

    If the head is set to 16 ohm output, connect to the right hand cab jack with the switch to the left (MONO). Parallel pairs connected in series.
    If the head is set to 4 ohm output, connect to the left hand cab jack with the switch to the left (MONO). All speakers connected in parallel.
    If the head is set to 8 ohm output, connect to either of cab jacks with the switch to the right (STEREO). Isolated pairs wired in parallel.
    In STEREO mode (8 ohm) each of the cab jacks will only connect to one pair of speakers in the cab, i.e. stereo split L/R.
     
    Last edited: Feb 26, 2020
  3. JCMRobbo

    JCMRobbo New Member

    Joined:
    Jan 17, 2020
    Messages:
    12
    Likes Received:
    15
    thanks very much
     
    Mitchell Pearrow likes this.
  4. Lance Chambers

    Lance Chambers Well-Known Member

    Joined:
    Jun 23, 2019
    Messages:
    1,151
    Likes Received:
    1,696
    Location:
    The Land That Time Forgot
    General rule: The speaker cab can operate at equal/double the amp setting.:D

    Ex: 4 Ohm amp output into an 8 Ohm cab is safe.:agreed:

    NEVER run a higher amp Ohm rating into a lower Ohm rated cab...................BOOM!:run:
     
  5. LPMarshall hack

    LPMarshall hack Well-Known Member

    Joined:
    Nov 18, 2009
    Messages:
    10,381
    Likes Received:
    9,087
    Location:
    San Diego, CA
    The (4) on the Marshall impedance selector is for combos. They used the same chassis for heads and combos. This so what I was told by the forum many years ago.
     
    Mitchell Pearrow and Micky like this.
  6. Ryan Morris

    Ryan Morris New Member

    Joined:
    Jan 23, 2020
    Messages:
    14
    Likes Received:
    5
    This is true of solid state amps, but NOT tube amps. Most tube amps can tolerate a 2:1 mismatch in either direction (ex. 8ohm head->4ohm cab or the other way around), however it will cause the tubes to run out faster. The one thing to NEVER do is to run a tube amp with no cab connected, because that presents the head with infinite impedance, which can definitely damage your amp. On the other hand, solid state amps are perfectly OK with not being connected to a speaker, but if you match a SS amp to a lower impedance speaker, the amp can overheat and get damaged.

    As for the (4)16 switch, LPMarshall hack was right in saying that the (4) is for combos. Basically, for combos that switch will change the amp from 4 ohms to 8 ohms, while on heads the switch goes from 16 to 8 ohms. In fact, heads will say this: (4) 16, while combos will say this: 4 (16). The reason for the second number in parenthesis is because you can rewire the output transformer to work for the opposite setting. For example, you could rewire your head to have a 4ohm option instead of the default 16ohms.

    In short, both heads and combos have the capability of being run at all three impedances (4, 8, 16) but they only put a switch with 2 options which is honestly pretty dumb.

    The reason they picked 4 & 8 for combos is because the internal speakers are 8 ohms, so connecting to an extension 8ohm cab will give you 4 ohms total. For heads, the old greenback-loaded 1960 cabs were rated at 16ohms, so you unless you had a full stack, you needed that 16ohm option. Modern 1960 cabs can run at 4 or 16 ohms mono so its not really a problem anymore.

    If you are interested in rewiring your head to run on 4 ohms, check out this article:
    https://paul1179.wordpress.com/2016/04/22/jcm900-output-transformer-hookup-instructions/
    I dunno if you have a JCM 900 or something else, but I'm guessing t=other amps are similar.
     

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