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I ran my amp at 4ohms into an 8phm cab whoops

Discussion in 'Amps and Cabs [BG]' started by panama, Sep 13, 2010.


  1. panama

    panama

    May 26, 2009
    Detroit
    So this weekend I jammed with a new band and rather than haul my cabs one of the dudes said I could use his Emperor 4x12, which is rated 8ohm. I forgot to switch my VB-2 from 4 to 8 ohms. The cabinet didn't fart or blow or anything, which was a relief, but I did notice my head was pretty hot after about an hour of jamming. No harm no foul? Which piece of equipment would have got the short end of the stick if something went wrong? Did I shorten the life of my tubes?
     
  2. Spinal Tapper

    Spinal Tapper

    Nov 15, 2007
    Chicago
    if it didn't crap out, you'll probably be ok. just remember for the future
     
  3. I am a newbie on tube grounds, but I think you just shortened the life of your power tubes. Maybe some potentional harm to your trannies?
     
  4. panama

    panama

    May 26, 2009
    Detroit
    Yeah I will. I don't have the cash to be replacing someone;s blown speakers!

    That said - the Emperor sounded pretty incredible, but I like the tightness of my 10s.
     
  5. panama

    panama

    May 26, 2009
    Detroit
    Well, after we jammed and I had turned off my amp and realized what I had done I promptly switched it to 8 and turned it back on. Everything seemed to be in fine working order. If I have to replace my power tubes sooner as a result then oh well.
     
  6. Then no harm done!
     
  7. panama

    panama

    May 26, 2009
    Detroit
    I'm new to the tubes too
     
  8. Grooveman1961

    Grooveman1961

    May 8, 2006
    No harm done

    You didn't hurt anything. You ran at lower power and slightly reduced freq response, thats all.
     
  9. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    The thing about tube amps that causes problems with mismatched impedances is that tubes are current output devices. The signal voltage into a tube (on the grid) controls the current through the tube (plate current); within limits, the plate current on output tubes is going to be what the grid says it should be, no matter what voltage that causes on the plate.

    What this means is that if the output impedance is too high, the output voltage will also be too high. (Ohm's Law: voltage equals current times resistance, if the resistance/impedance goes up and the current is constant, the voltage goes up). This can exceed the maximum plate voltage rating of the output tube. If you don't exceed it by too much, or for too long, you won't cause serious damage (tubes are a lot more forgiving this way than transistors).

    I would expect that if the amp has some negative feedback from the secondary of the output transformer (which is pretty commonly the case), you wouldn't even have this problem - you'd just lose some output power because the feedback reduces the current to try to hold the voltage constant.

    In extreme cases, the voltage can get high enough to break down the insulation in the output transformer windings, causing permanent damage. Or it can arc over from the plate terminal on the tube socket to the chassis. I once repaired a tube amp that had been run with no speaker (open circuit) that had burned up the sockets this way.
     
  10. rbbrchkn

    rbbrchkn

    Feb 25, 2009
    Denver, CO
    A properly built tube amp should be able to handle a load 100% above or below the proper selected impedance. (2 or 8 ohms if set to 4 ohms, etc.) Not the best thing to do, especially if you're running it hard, but you can run it like that for an extended period of time and be totally ok. :cool:
     
  11. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    ya, for that short a time, i doubt you did anything to your amp at all, including your power tubes. if you did shorten their life, you probably didn't shorten them by much at all.
     
  12. QFT. Look it up here in 'Amps, Ohms,' etc.
     
  13. Destructobot

    Destructobot

    Jul 22, 2008
    You won't hurt your amp running with too much load on it. It'll just sound bad. The upside is that once you realize what's wrong, it'll sound HUGE when you switch the impedance to the right ohmage.
     
  14. panama

    panama

    May 26, 2009
    Detroit
    I didn't think it sounded bad though! haha
     
  15. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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