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ohm question....

Discussion in 'Amps and Cabs [BG]' started by xshawnxearthx, Aug 8, 2005.

  1. xshawnxearthx


    Aug 23, 2004
    new jersey
    ok. 400+ head. i have 2 of the following outs. 2, 4 and 8 ohm.

    this weekend i used a 410hlf(that is 4 ohms) and an ampeg 15(which was 8ohms)

    i just wanna make sure i was using the right inputs.

    for the 15, which was 8 ohms, i plugged into the 8 ohm sections, and same for the 4 ohm.

    now, was this correct?
  2. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    If you mean you plugged each into the matching ohms output, no, that's not correct for any amp.

    When you plug the speaker into matching ohms output, you are setting the amp up to optimally put anything up to full power into that speaker with all the internal signals at the levels where the designer wanted them.

    if you do that TWICE, you essentially did the same as plugging two 8 ohm cabinets into the 8 ohm output (or two 4 ohm into the 4 ohm output, etc).

    For a 4 and an 8, the formula of 1/(1/4 + 1/8) gives you 2.66 ohms, which would mean you are best off to plug both into the 2 ohm output, with an alternate of possibly using the the 4 ohm output.

    There might be a tone difference between the hookups, and the mis-match isn't too bad either way, although 2 ohms is "closer".......I'd choose on tone, then but I don't know the innards of the 400+ very well, one may be significantly worse for it.
  3. xshawnxearthx


    Aug 23, 2004
    new jersey
    i'm just an idiot i guess...

    so if i have a 4 and an 8, next time plug both into the 2 or the 4?
  4. cheezewiz

    cheezewiz Supporting Member

    Mar 27, 2002
    NO. Plug both into the 2ohm plugs. The plugs you use on the Mesa 400, should be near what the TOTAL resistance of BOTH cabs is. Say you are using two 8 ohm cabs. Then you'd plug each cab into a 4ohm jack. If you are using two 4 ohm cabs, you'd plug
    each one into a 2 ohm jack. I'd contact Mesa and see if they suggest using the 2 ohm or 4 ohm jacks for a 2.66 load.
  5. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    What they said...I've heard people both suggest either the 2 ohm taps or the 4 ohm taps...PBG I think suggests the 4 ohm taps for 2.66. Your amp can take a mismatch but you don't want to have too much of one (like I wouldn't want to try 100% mismatch). I'd stay away from running offa differently rated taps at the same time.
  6. IvanMike

    IvanMike Player Characters fear me... Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    i would plug a load like that (2.67 ohms) into the 2 ohm tap of a tube head. (but see below)

    unlike solid state amps (which only have one set of outputs rated for a certain impedance or higher), tube amps often have multiple outputs for differing impedances, (or a switch to select the correct load on one set of outputs)

    unlike ss amps, tube amps do reasonably ok with up to a 100% mismatch (i.e., you can get away with running 8 ohms or 2 ohms from a 4 ohm tap). However, unlike ss amps tube amps don't like running much above the stated impedance. using a load above the impedance of the stated value of the output can increase the plate voltage an cause bad thngs to happen.

    2.67 shouldnt bee too big of a deal off of a 2 ohm tap, but to be sure i'd contact the amp manufacturer. they may even suggest you use the 4 ohm output instead to keep the plate voltage down.

    overall, when using a tube amp it's best to stick to the impedance of the outputs as written. that's the best way to avoid any mishaps. See PBG for more (and better) information.