help with ohms

Discussion in 'Amps and Cabs [BG]' started by Harrod182, Aug 29, 2005.

  1. Harrod182


    May 2, 2004
    im really really confused about this and know one i speak 2 ever seems to explain this in a way that i can understand, so here is my problem i have an AMPEG SVT CL and an ORANGE OBC410 the AMPEG head is 300watts at 4 OHMS and the cab is 400watts at 8OHMS so if i wanted to buy another cab can i and what amount of OHMS would it need to be?
  2. im not sure if the CL can use an 8ohm load, i think it might just be 2 or 4 ohms

    If you were to get another cab, it would be best to get another 8ohm cab, because two 8 ohm cabs would give you 4 ohms

    but, as its a tube head, it is the same wattage no matter what the ohm rating
  3. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    Its "what IMPEDANCE should the new speaker cabinet be", not "how many ohms should it be". Speakers don't "have" ohms, they have an impedance rating, measured in ohms.

    Knowledge is power.
  4. Reminds me of when your 5 year old comes and asks you where babies come from. You simplify a bit for the intended audience.... :D Few here are electrical engineers.

    So anyway, you have an Amp rated 300watts at 4 OHMS and the cab is 400watts at 8OHMS, and they love each other very very much. But the amp doesn't love the 8 ohm cab all 300 watts, probably more like 200 watts worth. But they still love each other a lot, so much that they finally have another cabinet, also rated at 8 ohms. And they make a family. A family of 2 8 ohm cabs has a combined impedance of 4 ohms. And the amp loves both cabs EVEN MORE than it loves just the one cab, the amp loves the 4 ohm family of cabinets all 300 watts worth. And they all gigged happily ever after.


  5. Steve Clark

    Steve Clark Supporting Member

    Jan 9, 2004
    London ON
    And if they had twins? :)
  6. Then you have a family with less than 4 ohms, and social services has to remove one of the cabs for the sake of the family...

  7. before the other one burns the house down or kills its parent :smug:
  8. If you want to

    1. Keep the Ampeg head and
    2. Keep the Ampeg head from burning out

    you should sell the 8 ohm cab and get a 4 ohm cab. Tube amps are very different from solid state amps when it comes to the output load. A solid state amp will tolerate any impedance cab as long as it is above the minimum impedance of the design. You can even run the amp with no speaker connected. A tube power amp MUST have the correct impedance load attached to its output transformer or you run the risk of damaging the power section of the amp. Damage here is usually very expensive to repair.

    Rick B.
  9. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    That's quite possibly the best thing I've ever read . . . .
  10. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    hi there. you'll notice that the second sticky down says "check here before asking". in that thread you'll find this link on ohms aqnd impedance that should answer your question in a straightforward (and a bit longwinded - :D ) manner.


    your head puts out 300 watts into a 4 ohm load. it puts out less (how much? ask the manufacturer) into 8 ohms. your amp can't power anything below 4 ohms or you will fry it.

    if you want to use a second cab it has to be 8 ohms. sounds crazy, but 8 ohms plus 8 ohm = 4 ohms. that's because the amps speaker outputs (and the jacks on cabinets) are wired in parallel.
  11. Oh crap, I assumed solid state.... I think it is VERY bad to run a tube amp with a HIGHER impedence load than it expects, a 8 ohm cab with the amp setup for 4 ohms is asking for trouble. Either get a 4 ohm cab to run by itself, or get another 8 ohm cab and always run them together for 4 ohm load if its a tube output stage.

    Tube is opposite of SS in that regard, tube hates higher impedance, SS hates lower impedance.

    Too busy being a wiseacre to think things through all the way... :oops:

  12. cheezewiz


    Mar 27, 2002
    I think you'd be best off just sending me the SVT CL. I have a nice 4 ohm cab that would treat it right, support it, comfort it, and provide a comfortable place to stay. I wouldn't even expect any child support.
  13. He using a tube head, the output is the same at either impedance, you need to check the impedance switch tho

    And using a higher impedance is just as likely (probably moreso) to burn out a tube amp that a lower one
  14. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    yup i missed the "cl" part of the post.

    i think the cl is switchable form 8 ohms, 4 ohms, and 2 ohms (maybe not two ohms)

    yes runnign a higher impedance on a tube head is bad. but, you can get a second 8 ohm cab (which will bring your net impedance to 4 ohms), set the switch on the back of the amp to 4 ohms (or use the 4 ohms outputs on the back if that's the way the amp is set up - some tube amps have seperate outputs for a NET 8 ohm load or a NET 4 ohm load). Then you'll be fine. the CL puts out 300 watts into 8 or 4, but you'll be a bit louder with a second cab.
  15. I just checked the ampeg site, its like my SVT, only 2 and 4 ohms, i'd switch to the 4 ohms on the selector on the back and highly recommend getting another cab or a different cab
  16. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    wow, only 2 or 4???

    well, running a tube amp at a higher impedance than selected can cause increased plate voltages and eventual failure along with a host of other bad things. I'd ask psychobassguy about the particulars.

    that said, there is a general (not total) consensus that reunning withing 100% either way (i.e., an amps selected to 4 ohms but running a 2 ohm load or a an 8 ohm load) is withing tolerable limits. But, i'd get ampeg to give you a definite answer on that.
  17. Keeaumoku


    Dec 29, 2004

    You mind if I use this to explain the birds & 'da bees to my twin daughters? Nah... come to think of it, my wife probably covered that when they were 5 or 6... if not earlier! :D
  18. bmc


    Nov 15, 2003
    Aw cut him some slack Randy. He's just doing his ohm work. (ba da bing)
  19. yeah, goin by the 'peg site anyway, i think its the mesas thats are 2/4/8 switchable