ohm question

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by Neumatics, Nov 16, 2005.

  1. Neumatics

    Neumatics Guest

    I have a fender combo that has an 8ohm out to extension cab, and a marshall 1960a with a 4 and 16ohm inputs. Which input should I plug the fender into
     
  2. Turbozag

    Turbozag Senior Member

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    Using the 16 will have a somewhat lower volume level but will not hurt anything. Using the 4 could melt your amp...
     
  3. EADGBE

    EADGBE Member

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    I wouldn't use connect them together if the impedances don't match.
     
  4. axpro

    axpro Member

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    MIsmatched impedances aren't the best for your amp or your soud but....

    as Turbozag stated, a higher impedancespeaker, would be less efficient, but shouldn't harm anything, lower impedance speaker (load) and your amp is trying to push harder than it should, and the speaker isn't resisting... leads to fried output transformers and many unhappy visits to your techician!


    Dave Mc
     
  5. donnyjaguar

    donnyjaguar Member

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    I can't speak for every Fender amplifier but...

    On my 1984 Concert the impedance is doubled when running 2 cabinets, or halved when running on from the primary output only. So, if it were set up for 8 ohm speakers, that means that when running 2 x 8 Ohm cabinets they would be connected in series and the output would be 16 Ohms.

    If you put a shorting plug in the primary or secondary output socket the remaining one will be twice the normal impedance. In other words its very easy to configure your amplifier to run 16 Ohms.

    The issue with tube amps is you don't want to run them into no-load as this will (possibly, but not likely) cause the output transformer/tubes to arc. Many engineers will argue that this is the sign of a bad design and most better amplifiers are okay running into no-load.

    DJ
     
  6. mccreadyisgod

    mccreadyisgod Member

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    When mismatching impedance, you're okay if the speaker ohmage is higher than the amp's rated output. It's inefficient, but fairly safe (just don't dime your amp to compensate).

    Putting an amp into a speaker cab rated lower will be like giving your amp an asthma attack... no matter how hard it breathes, it will never keep up, and eventually will crap out.

    Whenever possible, match the impedance...

    Is your Marshall cab wired for stereo operation? That's usually how you get variable impedance on a 4x cab... which means that one of the two is only half the cabinet (2 speakers).
     
  7. mikecat

    mikecat Senior Member

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    you can always go with a higher number speaker ohm than your amp's ohm setting.

    just don't go with a lesser speaker number.
     
  8. walterw

    walterw Gold Supporting Member

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    that's true with solid state, maybe not so much with tube amps. mis-matching those in either direction causes losses and stresses.

    if that fender combo has an 8Ω internal speaker and an 8Ω extension jack, then it can run 4Ω. you might be able to just unplug the internal speaker, set the 4x12 to 4Ω, and run it that way.
     
  9. Ronsonic

    Ronsonic Member

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    WHAT AMP IS IT??????

    Yes it makes a difference.

    Some newer Fender HR series and the like have switching impedances on the output jacks, classic Fenders don't.
     

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