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Impedance question : is it ok to plug an 8 ohm cab into a 4 ohm amp ?

Discussion in 'Amps and Cabs [BG]' started by Inconnu, May 10, 2010.

  1. Even if it's an all tube amp ? I was just wondering if I could hook on a smaller cab, 8 ohms, to my SVT-CL...
  2. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Covered, in pretty good detail, in the FAQ.
  3. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Shorter answer is no.
  4. 1kinal


    Jan 18, 2006
    Montreal, Qc. Canada
    Endorsing Artist: SIT strings
    In fact short answer would be YES.

    The amp is 4ohms MINIMUM which means the lowest ohms you can go is 4 ohms. But there's absolutely no problem to hook up a 8ohm cab, you'll just get less power (volume) than if you used a 4ohm cab.
  5. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Aaargh, no, not with valve amps, doesn't work like that. Go read the FAQ.
  6. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    The details:

    So - if you have a tube amp with a tap for 8 ohms, you will get the nominal power of the amp only with a "matched" 8 ohm load. If you hook 16 ohms there, the power tubes "see" a proportionately higher ohmage on their plates, and can only put out about half the nominal power. If you hook up a 4 ohm load to the 8 ohm tap, the power tubes "see" a load about half of the matched one, and again will put out only about half of the nominal power. This "half the nominal" power is not fixed because of the 2:1 change in load, but varies from amp to amp and tube to tube, and may not be exactly 2:1. In addition, speakers are NOT single impedance loads. It is convenient to think of "8-ohm" speakers, but the plain fact is that the speaker's impedance varies with frequency and also with the acoustic loading (cabinet and other things) that the speaker sees. That impedance meter is not going to be a huge help, because you have to specify the frequency being tested as well as the impedance to have something meaningful.

    The potential damage:

    If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

    There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

    This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.


    It's not guaranteed, and in all likelihood, an 8 ohm impedance is not going to cause this damage on low volume, moderate use. But, unless you need to, why put your self in that situation?
  7. breakHARD77


    May 6, 2010
    Correct me if I'm wrong, but what I'm getting from that is that it's okay to (for example) match a 4 ohm amp with a single 8 ohm cab (with the assumption that you're going to lose some juice) but not with a 2 ohm cab, since there is no risk in overloading it. This is all assuming that you're not using an all-tube head.

    Am I correct in saying all this?

    By the way, where can I find this FAQ? :oops:
  8. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    No. If you're not using a tube head (ie a solid state head without an output transformer) the rules are different. In that case, you want to be above the minimum stated impedance, and anything above that minimum should be ok.

    If you were using a tube amp, going below the impedance setting will cause the tubes to run less efficiently, shortening their life, but is less likely to cause big damage to the amp (ie voltage spikes across the tubes as mentioned above). In general, the advice I hear is that a quality tube amp will tolerate a 2:1 mismatch in either direction (in this case 2 or 8 ohms with a 4 ohm tap) with ease, given optimum conditions and light use.

    I'd start with the section labeled in large bolded letters "Ohms & Impedance - how many cabs can I use with my amp and what does all this mean???" and then scroll down the page from there to find a lot more.
  9. breakHARD77


    May 6, 2010
    Ah, okay. I didn't know that you were referring specifically to all-tubes. Thanks for the clarification.
  10. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    The OPs question was about an SVT-CL, so I was keeping that as the frame of reference. ;)
  11. Sorry I haven't properly checked FAQ. Thanks !

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