Tube Bass Amp into 2.67 ohms ?

Discussion in 'Amps and Cabs [BG]' started by BassDemon, May 26, 2005.

  1. BassDemon


    May 18, 2004
    I have a fair understanding of Ohms with Solid State amps but I have a question on Tube amps.

    I know that most have three outputs, 8, 4 and 2 ohm connections.

    My questions is, what happens if I want to drive an 8 ohm and a 4 ohm cab at the same time.

    I'm assuming you can't (unless the Amp has stereo outputs and thus two output transformers) as the resulting load is 2.67 ohms.

    does anyone know if this is correct

    would it in fact work if you hooked up to the 2 ohm output or would you risk blowing up the amp, cabs or both.
  2. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    Plug it into the 2 Ohm tap. Tube amps can take generally a 100% mismatch, but tube life will suffer a tad.
  3. throbgod13


    Mar 26, 2005

    what will happen is that your output transformer will die..
  4. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    I have a friend who gigged with a SVT-II for years running it at 8 Ohms even though it wasn't rated to run at 8, and it worked just fine with no big issues. Just a retube every few years.
  5. popinfresh


    Dec 23, 2004
    Melbourne, Aus
    One thing, the 4 ohm cab will get 2/3 of the power. (correct me if i'm wrong).
  6. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    I thought it was 3/4...
  7. popinfresh


    Dec 23, 2004
    Melbourne, Aus
    ^^Probably is. I wasn't 100% sure :)
  8. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    Thats OK, I'm not sure on the number either ;)
  9. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY

    What happens if you've got a speaker whose impedance varies from 4 ohm to nearly 40 ohm?
  10. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
  11. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
    The above graphs were from a speaker that has a nominal impedance of 8 ohms. I think we can get a bit too caught up in impedance matching nit-picking. 0.67 ohm is really not a big deal, unless everything is already stretched to their limits, design-wise.
  12. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    When I called Aguilar regarding this very question they told me to use the 2 ohm setting, but said that really either the 2 or the 4 ohm setting would probably be fine.

  13. Iritan


    Jun 3, 2005
    Wilmington, N.C.
    This is kinda off subject, but I'm a bit new to the world of amps electronics and I'd like to know, how'd you get 2.67 from 4 and 8, is there some eqaution. And where the 2/3 and 3/4?
  14. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Nothing unusual, as all dynamic drivers have widely varying impedance with respect to frequency. Impedance values are nominal averages and the actual operating impedance, at least for woofers, is as much determined by the cabinet as it is the driver.

    The reason an output transformer has mutiple taps is so you can better match the source to the load for the most efficient power transfer. With a well made amp it's hard to hurt one. Back in high school when we didn't know any better we'd routinely run our Fenders into 1 ohm loads at full tilt with no ill effect. I doubt that situation has changed much, except you can't do so with SS outputs as the protection circuitry will shut the amp down.
  15. And it shouldn't have had any ill effect. Can't say the same if it were a 16 or 32 ohm load, though. Bassmans short their outputs to ground, ZERO ohms, when no speaker cable is plugged into them to protect themselves.
  16. Kelly Lee

    Kelly Lee Yeah, I'm a guy! Supporting Member

    Feb 17, 2004
    Marana, AZ, USA
    IIRC, isn't it safer to run lower ohms than higher from a given tap on a tube amp? In other words, 2.67 would be safer with the 4 ohm tap than the 2 ohm tap. :confused:
  17. Yes, but the difference isn't as bad. I would recommend the 4 ohm tap just to be safe, but you SHOULD be OK with the 2 ohm tap, too, especially with dissimilar cabs.
  18. ezstep


    Nov 25, 2004
    north Louisiana
    Way back when, I had a Bassman 100 and eventually got rid of the cabinet :meh: . I ended up with one 4-ohm and one 8-ohm cabinet. The repairman at the local music store told me to plug it into the 4-ohm jack and let her rip! I used the same rig for several years with absolutely no problems.
  19. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    As far as the issue with the impedance changing anyway......per the graphs...... Yes that is true.

    But, a speaker starting at a different nominal impedance will be even FARTHER from the tap nominal impedance at its extremes.

    And, it makes a difference what amp it is. An amp run at the limits of the tubes, like the SVT that was the subject last time on this biz, would be more fussy. A smaller amp, run well within the tube ratings, is going to tolerate a lot more mismatch etc than the SVT.

    Just watch out...... many years ago I discovered that if you beat up on a Fender twin, it may sound pretty cool, and be loud, but it also lets the tar out of the power transformer..... which gets pretty hot. Might not always be the best idea.....
  20. kilgoja


    May 26, 2005
    i think i remember someone saying that you need to plug the 8 ohm cab into the 4 ohm jack and the 4 ohm cab into the 2 ohm jack...if you have a mesa 400+ that is