4ohm amp with 8 ohm cab?

Discussion in 'Amps and Cabs [BG]' started by Bloc Riff Nut, Mar 23, 2011.

  1. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    I don't think we are. I think we're getting a feel for the real problem.
  2. Smooth_bass88

    Smooth_bass88 vaxx! Supporting Member

    Oct 31, 2006
    North AMERICA, USA
    I've been running my all tube Ampeg SVT 2PRO (rated at 300 watts into 4 ohms) into an 8 ohm cabinet for maybe 5 years playing 130 to 150 gigs a year and guess what...NO PROBLEMS!!
  3. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    Well, you've also got electronic phase reversals on either side of the cab resonances. The reactance component of the impedance is going to flip from capacitive to inductive at a resonance, the quicker the higher the Q. I'm looking at the resonance Qs as f0/delta-f where f0 is the resonant frequency and delta-f is the width in Hz of the -6 dB points relative to the resonance peak of impedance, since it is analogous to a parallel resonant electronic circuit. So, without phase information, you can't completely describe the load the amp sees.
  4. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    True, dat.
  5. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    Man, what's worse is that a ported cab has two resonances, so you go from inductive (below 1st resonance) to capacitive (above 1st resonance) to inductive (below 2nd resonance) to capacitive (above 2nd resonance). And, you're pure resistive at both resonances. It's a fine mess for an amp designer. :D
  6. Just to add to the experiences posted here, I had an SVT-CL that I often ran into an 8 ohm cab from the 4 ohm tap. No issues, ever, though I suspected (but could not prove) that there was some tonal difference.

    The sticky (and my amp tech) advises that, on a tube amp, you should be safe to go 100% either way, so putting an 8 ohm load on the 4 ohm tap should be safe. So far, it seems nobody is able to offer evidence to the contrary.
  7. Selta


    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Did you not read Bill's post?
    I could also relate the experience I had with an amp that was set on 2 ohm accidentially, when a 4 ohm cab was plugged in, and died within an hour of use.
  8. GrowlerBox


    Feb 10, 2010
    Nude Zealand
    In response to a PM from greenboy (couldn't reply privately)

    No offense taken, man. If one can't take the cut and thrust of a TB conversation, then one shouldn't be swordfighting in the kitchen :)eyebrow:). I guess I was waxing philosophical on a point that really has a fairly clear answer in "real" terms. Acoustics is not my field at all beyond being an interested amateur, and I enjoy the threads that wander into the esoteric -- it's where I learn the the most. So keep dumping! Same goes for Bill and the other engineers/designers who weigh in.
  9. 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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