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Using mismatched cabs. Am I hurting my amp?

Discussion in 'Amps and Cabs [BG]' started by johnbegone, Sep 16, 2004.

  1. johnbegone

    johnbegone Supporting Member

    Sep 16, 2004
    Chicago, IL
    Hey all, I am playing a Fender Active Jazz Bass through an Ampeg SVT-3 Pro and two 4x10 cabinets.

    The first cabinet I got is an SVT-410CA4 which is rated 600w @ 4ohms. The other cabinet is a newer SVT-410HE rated at 500w @ 8ohms. After I bought the 410HE Idid a lot of research but couldn't figure out whether it would be harmful to run both of these cabinets with my amp. So, as any other would've done, I just plugged it in and went for it. It seems to be working and I can't really tell a difference in volume between the cabs. I've never A/B'd it to an 8x10 but my setup sounds loud enough. I'm worried that this is pushing my ohm load before 4ohms on my amp, which could be harmful to it?

    Do I need to sell the 4ohm cabinet and buy a matching 410HE? Thanks for any input you can provide me.
  2. It looks like you are running a total load of 2.67 ohms. I don't think that head is stable at loads under 4 ohms. I would recommend finding another 8 ohm cab to give you a total load of 4 ohms. Hope that helps.
  3. Tames


    Dec 31, 2002
    Decatur, IL, USA
    Should have just gotten an SWR. :)
  4. cetera


    Apr 29, 2004
    Surrey, England
    Endorsing Artist: Spector Basses & Cort Basses
    Yes, your amp is unstable. Get another 8 ohm cab....
  5. I agree the SVT3 PRO is only capable of running into 4ohms minimum, use one cab or get a second 8ohm cab and run together. Stop using it how you are as you will fry it !!

    Good amp, served me well till i gout the Classic

  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Open up the 4 ohm box and rewire the drivers to a 16 ohm load. You probably have four 16 ohm drivers parallel wired. Wired as two sets of series/parallel pairs you'll have 16 ohms, which when run along with the 8 ohm box will give you a 5.3 ohm load.

    The 16 ohm box will be down 3dB in output from the 8 ohm box, so to compensate keep the 16 ohm box on the bottom to get boundary loading that will pretty much even out the levels of the two.
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The amp is not rated for that load.......so you are surely "skating" on that. "Sure" to blow it? Not really. "Sure" to be OK? Not that either.

    The re-wire to 16 is a practical, low-cost solution. The mis-match of 8 and 16 is no worse than an 8 and 4, so that isn't an issue.

    And if you don't like it, you can put it all back the old way and get another 8 ohm cab.
  8. johnbegone

    johnbegone Supporting Member

    Sep 16, 2004
    Chicago, IL
    Wow, lot of information here. I appreciate all your input and I have ceased playing with both cabinets for now. Using one for practice, and I'm borrowing a friends' 8x10 for some live gigs coming up. How complicated would rewiring the cabinet be? I've never ever done anything like that, so are there tutorials out there or anything?

    Also, could someone give me the question you use to figure out the ohm load on an amp? Like how an 8 + 16 equals 5.3 or whatever? Thanks all!
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Get a book on loudspeaker design and theory; the benchmark is by Vance Dickason, available at Parts Express. While it is hi-fi oriented it does give you the basic information that as a professional musician you should always have on hand.
  10. Selta


    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Parallel (2 x 4ohm load = 2 ohm, 2 x 8 ohm load = 4 ohm etc.):

    Series (2 x 4ohm load = 8 ohm, 2 x 8 ohm load = 16ohm etc.):

    Series & Parallel (4 x 8 ohm load = 8 ohm etc.):

    That's what I use. Just put total impedence of you first cab next to "row 1" and you total impedence of the 2nd next to row 2 and so on. If you daisy-chain, go across instead of down.

  11. JOME77

    JOME77 Supporting Member

    Aug 18, 2002
    Parallel Resistance:

    1/r1 + 1/r2 = 1/r3 (r3 equals the sum of 1/r1 + 1/r2)

    In your case:

    1/16 (.0625) + 1/8 (.125) = 1/.1875 = 5.33
    (16 ohm cab) + (8 ohm cab) = 5.33