4ohm amp with 8 ohm cab?

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


  1. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Please listen to the OP, and the rest of us. The op is trying to plug in an 8 ohm cab into a head that's only meant for 2 and 4 ohms. Tube amps are not like SS where going to 8, 16 etc. loads doesn't effect them.
     
  2. Loads don't affect them? Of cours they do. Ohm is a measurement of resistance if and amp is not rated for a certain ohm rating then it can't run that ohm rating. It works the same for ss amps as it does for tube amps. Only use what the head is rated for, that's it
     
  3. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    You can plug a 32 ohm cab into a solid state head. You'll only lose power output.
    Plug a 32 ohm cab into a tube head rated for 4 and 2 ohms, and you'll murder it. Plain and simple.
     
  4. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    We're talking about a 2:1 mismatch here, not 8:1 or 16:1.
     
  5. GrowlerBox

    GrowlerBox

    Feb 10, 2010
    Nude Zealand
    He's right, guys -- Ohm's law still applies, it's just the different effects on the different components in SS and tube power amps that count for everything here.
     
  6. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Right, but when I gave that example above, people didn't get it, so I had to resort to an extreme example to open eyes. Even 2:1 can be fatal to a tube amp. I would advise NOT doing that.
     
  7. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    Ohm's Law is not sufficient to describe the behavior of a tube amp running into a mismatched load. Lem8r had it right when he said you need to do a load line analysis. Most TBer's can't do this. Tube amps can and do run safely into mismatches of 2:1 or less provided they are sufficiently well constructed.
     
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    +1. SS will take higher impedance loads than their minimum rating without complaint, tube are exactly the opposite.
    Harry, Aprod and Nightlife, i don't like being forced to say that someone doesn't have the slightest notion of what they're talking about, but you don't, and you're doing no one any good by making pronouncements that have no basis in fact. Please stop.

    As for running an 8 ohm load on a 4 ohm tap with a tube amp, sometimes it won't bother a thing, sometimes it will toast the amp. Why? Because there's no such thing as an 8 ohm speaker. Load ratings are imprecise to say the least. For instance, here's an impedance chart of two cabs. Care to say what their impedances are?
    sealedventedimp.jpg

    They're both 8 ohms. The green trace is an Alpha 10 in a sealed cab, and it would not bother a 4 ohm tap on a tube amp at all. The red trace is a Kappa Pro 10 in a ported cab, and not only would it cause problems with a 4 ohm tap, it would cause problems with an 8 ohm tap as well.

    If you have a sealed cab you don't have to be nearly as concerned with impedance matching the cab and tap as you do with a vented cab. Amp manufacturers post tap ratings based on the fact that they can't assume that the user will have either a sealed or vented cab, so they take a conservative route. That's not a bad idea for users as well.
     
  9. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    I guess I need to say this again. Under 2:1 mismatch conditions, strictly maintained, there is 11% reflected power from the load back into the amplifier. This is dissipated mainly in the output tubes and output transformer, and 11% of that is reflected back to the load (speaker). The amp will also produce less power into the mismatched load.

    If the voltage or current limits of the output tubes or output transformer are not exceeded by this condition, nothing will fail.

    And +1 to Bill's clarification of what real world loads can look like, above.

    EDIT: Bill, it might be fair to call that Kappa Pro 10 cab an amp killer or pathological case. :D Those resonances have Qs on the order of 25.
     
  10. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    :eek:
    I'm probably a n00b to seeing raw speaker data like that, but I didn't know there was that much swing in those. Holy crap.
    Can you expand on the sealed versus ported bit? Even in PM would be OK.

    -Ray
     
  11. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    Biggest difference is one hump versus two as the vented box contributes to the impedance curve above and below box tuning, the rest is up to the driver and box size in question.
     
  12. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    So the x-axis is freq. then?
     
  13. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    True, and whether the box has damping material to lower the Q on its resonances. You guys can feel free to correct me, but that Kappa 10 curve looks to me like a rigid, undamped box.
     
  14. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    It's an optimal alignment, actually, and is typical of low Q drivers.
     
  15. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    Well oiled beef hooked. Thanks, Bill.
     
  16. GrowlerBox

    GrowlerBox

    Feb 10, 2010
    Nude Zealand
    I don't think it's sufficient to describe fully any real situation; like all physical laws, it describes an idealised version of reality, and serves as a useful approximation of the behaviour of a system operating within certain limits, defined in a rather circular fashion, as the range within which the system's behaviour is more-or-less linear (i.e. can be descibed by said law). Clearly this range is different for tube amps than it is for solid state (both smaller, and no doubt "skewed" differently), and the physical components that make up the system will respond more or less dramatically as well.

    Sorry, just pontificating. I'm not sure we're really arguing :).
     
  17. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    Ohms law works fine. You just need more data points when you describe a non-static situation.

    Typical of bass players who discuss "tone" as if it were steady-state rather than noting the amplitude and tone envelopes ; }
     
  18. GrowlerBox

    GrowlerBox

    Feb 10, 2010
    Nude Zealand
    I wasn't aware that I did that, but I'll try and do better ;).
     
  19. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    Not you, man. Never. You a real clown. Real clowns don't do dat. I know. And here's a picture of "tone" just to prove bright colors are good for more than just balloon animals:

    BassNote.png
     
  20. Rockmusician

    Rockmusician Inactive

    You can connect an 8 ohm cab to a 4 ohm head. Tube or SS does not matter. The only difference is that in order to get the maximum wattage output that the head can safely operate at, you would need a 4 ohm cab. Generally, if the head is 300 watts at 4 ohms it will run at around 150 to 200 watts at 8 ohms.
     
  21. 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.

     
    Aug 5, 2021

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.