8 ohm output into a 4 ohm cabinet?

Discussion in 'Amps and Cabs' started by jtw, Oct 4, 2006.

  1. jtw

    jtw Member

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    The Pro Junior thread got me thinking about messing around with some different extension cabinets for larger gigs. My question is can I run the Pro Junior, which is an 8 ohm output, into a 4 ohm 2x12 cabinet without damaging the Pro Junior? I can't remember which way you can go with mis-matched impedance. Thanks for any direction here guys.
     
  2. RevelationAmps

    RevelationAmps Member

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    Hopefully someone with specific PJ knowledge will pipe in. However the "rule" you are thinking of is that it's ok to plug a 16 ohm speaker into the 8 ohm tap, but not the other way around. This is generally true but bear in mind, the tone might stink if you mismatch it. Depends on the amp.
     
  3. Blue Strat

    Blue Strat Member

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    Actually, with tube amps, it's the opposite.

    Note how many Fender amps have shorting jacks for speaker jacks in case you accidentally run the amp with no load. Hence, lower (in this case, zero ohms) is safter than higher.

    On the other hand, you can usually go 1/2 or 2X the nominal load without trouble in Fender amps. You'll loose maximum power though.
     
  4. hasserl

    hasserl Member

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    Mike is right, if you're going to run an impedance mismatch with a tube amp it is better to go lower than higher. With a solid state amp that is not correct, but with tube amps it is. A higher impedance load will reflect higher voltage spikes back to the output transformer and tubes, which can lead to damage to the OT or flashing at the tubes/sockets.
     
  5. michael.e

    michael.e Supporting Member

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    I cannot believe it, though I am pretty smart guy, I can never retain this info...

    When you say "it is better to go lower", you mean:
    It is better to go 8 in to 16, correct?

    When one refers to "higher", they mean:
    8 in to 4, correct?

    Does this mean that it is not a good idea to go:
    8 ohm output in to a 4 ohm cab?

    Or typically, one step "up/higher [?]" is not a major biggie..


    Thanks for once again entertaining my further questioning..


    I don't understand why ohms/impedance is such a mystery for me??

    M.E.
     
  6. Blue Strat

    Blue Strat Member

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    I'm not sure what you just said, but here's what it is.

    8 ohm amp into a 4 ohm cab is better than 8 ohm amp into a 16 ohm cabinet.
     
  7. B Vance

    B Vance Member

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    4 ohm amp can run a 4 ohm, 8 ohm and 16 ohm cab.
    8 ohm amp can run an 8 ohm and a 16 ohm cab.
    16 ohm amp can run a 16 ohm cab.

    If you violate this, there is a good chance you could seriously damage the output transformer. The power tubes will most likely run ultar hot as well. You could get some burning smell.
     
  8. Blue Strat

    Blue Strat Member

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    Back to my first post.
     
  9. Gary Brennan

    Gary Brennan Old cavorting member Gold Supporting Member

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    So Mike says feeding into the same or a lower load is ok, and BVance says the same or higher is ok?

    gb
     
  10. jamison162

    jamison162 Member

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    According to my knowledge and what I have learned, B Vance is correct.

    Your amps output transformer (OT) must see at least the impedance or "ohms" it is rated for or higher. If you plug into a lower rated cab, your OT is not seeing enough resistance and can overheat and burn up. That's another reason they say to always have speakers or a "load" hooked up to your amp, and not operate it without or you will definitely cause damage to your amps OT.

    Hope this makes since.
     
  11. Blue Strat

    Blue Strat Member

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    Last post from me on this, I promise. :)

    Actually, the only part that makes sense is the part about running an amp without no load. The rest of it is incorrect for TUBE amps.

    Back to my first post.

    Mike Kropotkin (BSEE)
    KCA NOS Tubes & Amp Repair/modifications
    www.kcanostubes.com
     
  12. trazan

    trazan Member

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    So, the conclusion is that it doesn't matter what you write in a forum, because nobody pays attention anyway?

    :dude
     
  13. cameron

    cameron Member

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    Mike "Blue Strat" Kropotkin is correct. There's a basic difference between tube amps and solid state amps in this regard. Solid state amps have a minimum impedance. Plugging a solid state amp into a load lower than its minimum impedance is very risky. This is the origin of the (in this context) false information that others have posted in this thread. Solid state amps operate at maximum efficiency at the minimum safe impedance, that's why you'll often see solid state amps' spec sheets list different power ratings at different loads.

    Tube amps, however, have an optimal impedance. They will generally tolerate a 100% mismatch either way, but in general it's safer to go with a load lower than the optimum than to go higher than the optimum. You lose power either way, and get maximum efficiency at the optimal impedance.
     
  14. hasserl

    hasserl Member

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    Mike and Cameron are correct. B Vance and jamison162 are wrong on this. As MIke and Cameron have posted, and I tried to earlier, it's typically considered safe for a 100% mismatch either way. I say generally because there seem to be some amps that are more sensitive to this, it is not a univeral rule, just a general one. But if you are to run a mismatched load it is better that the load be lower than the amp is designed for, i.e. a 4 ohm load for an 8 ohm amp is safer than a 16 ohm load for the same 8 ohm amp. To explain why is a long process, just simply put a higher load will reflect higher voltage spikes back thru the circuit which can damage the OT or tubes and/or sockets. A lower load will cause the tubes to work harder, not a big deal. There will also be increased current thru the OT, which should not be sufficient to cause any harm, at least not as likely as the high reflected voltage spikes a higher load will cause.
     
  15. Blue Strat

    Blue Strat Member

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    What the previous 2 posters REALLY mean is that a 1:2 or 2:1 mismatch is ok.

    A 100% mismatch in the negative direction would be 0 ohms. This is not only confusing but incorrect.
     
  16. Gretschman

    Gretschman Member

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    I own 16 tube amps and about 21 speaker cabinets . It is my understanding and my practice to never go over double the transformer load or one half the transformer load . So , if I have a 4 ohm transformer in my amp , I can either add another 4 ohm cabinet to it { 2 Ohm load }
    or unplug the 4 ohm speakers and plug an 8 ohm cabinet into it .
    { 8 ohm load } It is safe and has worked for me for years . Anything else is not good for the amp .

    Hope this helps ,
     
  17. RevelationAmps

    RevelationAmps Member

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    a 1:2 or 2:1 mismatch are generally ok for many amps but I don't know about the PJ in particular. Mike K I was not disagreeing with you, just making a suggestion not knowing the particulars of the amp.

    I would not recommend running a tube amp (or any amp) into a short circuit. It might be preferable to an open circuit, but neither is healthy for a guitar amp.
     
  18. Blue Strat

    Blue Strat Member

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    Mind boggling!:cool:
     

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