4X12 8ohms and 4X12 16ohms question ?

Discussion in 'Cabinets & Speakers' started by Kelia, Sep 15, 2019.

  1. Kelia

    Kelia Well-Known Member

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    hi guys , need some input !

    I have two 4X12 cabs loaded with G12H 55hz Pre Rolas and I know the best world is to match the impedance but I just tried both with my 2555SL set to 8ohms on the amp and the sound is to die for !!

    Just did a quick test but can I play both without damaging anything and if yes , what ohm should I choose on the amp , 8 or 16 ?

    Tx
     
  2. JohnH

    JohnH Well-Known Member

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    Which amp do you have and what output sockets/switches does it have?
     
  3. Gene Ballzz

    Gene Ballzz Well-Known Member

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    The short answer here is NO, you should not do this! And the long answer is "NO, you should DEFINITELY NOT DO THIS!"

    The best solution here would be to get another 16Ω, 4x12 or another quad of 16Ω speakers to put into your 8Ω, 4x12!

    While I'm gonna bet that is NOT what you wanted to hear, but I believe it to be the absolute truth!

    You Asked & I Answered!
    Gene
     
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  4. john hammond

    john hammond Well-Known Member

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    look, it's like you're telling the guy he can't eat his favourite ice cream.
    At least you could tell him why.
     
  5. JohnH

    JohnH Well-Known Member

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    My question will lead to either, yes it can be done well if you have a full set of 5 output sockets, or if not but you want to avoid a load mismatch, a workaround with a couple of power resistors will equalize and balance the cabs.
     
  6. Kelia

    Kelia Well-Known Member

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    It's a Slash Jubilee with two speaker output and ohm selector.
     
  7. Kelia

    Kelia Well-Known Member

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    Glad I asked !!.......will not do it but man does it sound good !!
     
  8. Kelia

    Kelia Well-Known Member

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    Didn't know this resistor thing !
     
  9. Kelia

    Kelia Well-Known Member

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    By the way ,....completely forgot to say thanks guy's !!

    Just a little sad that those are rare and that I have an 8 and a 16 ,....oh well !!
     
  10. JohnH

    JohnH Well-Known Member

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    Ok. Apart from any risk to the amp of the ohm missmatch (I have no comment nor qualification enough to be valid about if it is ok or not), the other issue is that the 8ohm cab gets 2/3 of the power.

    So a first-stage workaround is a 3.3ohm 100W power resistor in series with the 8 ohm cab (wire it inside the cab, between speaker wire and jack tip). This will in combination, give an 11.3 ohm load. Then, plugged in with the 16ohm cab, the net load on the amp is nominally 6.7 Ohm. All speakers get about the same power and the mismatch is only 20% which Id say is ok with an 8 ohm tap. I have done that!
     
  11. Gene Ballzz

    Gene Ballzz Well-Known Member

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    @JohnH
    The amp the OP has (2555SL) has an impedance selector switch and two speaker jacks that I assume to be parallel. Here's a link to some pics in a Reverb ad:

    https://reverb.com/item/27215937-ma...n=6481783183&utm_medium=cpc&utm_source=google

    Now there have been discussions concerning some DSL amps and other Marshalls with the five jack output setups, along with claims from Marshall employees that multiple taps can be used simultaneously! I've been around a couple amps for a couple of minutes and I don't understand how/why this could be the case, although it does not pertain to this discussion, so I'll leave it at that.

    To address @john hammond 's comment, a 16Ω cab in parallel with an 8Ω cabinet comes out to 5.3-ishΩ. While impedance mismatches seem to be more easily tolerated as the amplifier wattage goes down, this 2555SL is a fire breathing monster, fully capable of pumping out 160+ WATTS and likely a bit more when being mercilessly flogged, which is how they sound their gloriously deafening best! When brutalizing tube amps in this manner, all "I-s" must be dotted and "T-s" crossed when it comes to anything connected to the output, with regards to impedance, power handling and robustness of construction! The danger in not following proper usage guidelines is a semi long list, starting with fried power tubes, output transformer, various other components and maybe even a power transformer!

    Did That Cover It John Hammond?
    Gene
     
  12. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Nice thought there, my friend. It may or may not work OK in this instance, but as alluded to in my previous post, as the watts go up, having everything "just right" becomes more and more critical. This can be somewhat likened to driving fast in a car. Up to a certain speed, any reasonable car can be sorta OK, within the driver's skill limitations, but when in a car built for really high speeds and used that way, every piece and every move had better be right on the money!

    Just My $.02,
    Gene
     
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  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    1X 16 ohm + 1X 8 ohm
    The answer is: 5.4 ohms, you must set the amp on 4 ohms.
    If the amp does not have 4 ohm out then it is a mismatch.
    You could re-wire the cabinet possibly...

    For example 8 ohm cabinet can be rewired to 32 ohms.

    Formula
    Speaker 1 X Speaker 2
    ----------------------------- = total impedance
    Speaker 1 + speaker 2

    Speaker 1= 16 ohm X speaker 2 =32 ohm = 512
    -------------------------------------------------- 512 / 48 = 10.6 ohms total (use 8 ohm amp out)
    Speaker 1 = 16 ohm + Speaker 2= 32 ohm = 48
     
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  14. South Park

    South Park Well-Known Member

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    Go ahead and hook it up when you burn up the output transformer we can tell you how to replace it. It not that that hard
     
  15. john hammond

    john hammond Well-Known Member

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    -no, you didnt even start to cover it.

    crossing t's and dotting i's doesnt do it for me.

    -'this 2555SL is a fire breathing monster, fully capable of pumping out 160+ WATTS'
    - bullshit. that power transformer provides .5 of an amp of 360v for the H.T taps.
    under sag, youd take the voltage reading and multiply that by that 500milliamp rating.
    lets estimate a ten percent drop in voltage under full load
    320v x .5A = 160 watts
    take half that off for losses ( you lose energy through the output transformer, it leaks like a sieve) and you lose energy through the output tubes.
    = 80 watts.

    the issue of mismatching comes up a fair bit.
    this amp doesnt utilize voltage doubling therefore no need to match loads to amps.

    there are people on this forum who have mismatched for 20 years with no problems, I know because they post here and I read it.

    i mismatch exclusively on all amps except marshall 100 watt jmp/jcm800 style amps that utilise voltage doubling.

    the OP mismatches as well, and he loves it too
    his amp hasn't mysteriously blown up, my amps haven't, and im STILL waiting on other threads for people to put their hand up and say " i mismatched on my amp and it blew up".

    that 50 percent ( or less ) mismatch on that amp just means good tone, nothing else.
     
    Last edited: Sep 16, 2019
  16. john hammond

    john hammond Well-Known Member

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    tell us, South Park, about when you blew up your output transformer.

    God I'm lucky, i've got some vintage amps here , several of them approaching sixty years old and they still have their original o/t's. I always mismatch them, always, and i don't pussyfoot around with volume either, i run these amps at around 8 on the volume.
    Its very common for fender users to mismatch, it just sounds better 4---> 8 or 8---->16.
    The reason why it sounds better is because you're 'shifting' the tube on its published curve, you're getting more mids and highs, amp sounds sweeter, but with a bit less body.
    The reason why the marshall guys are cautious is because if you mismatch on a 68+ plexi or a plexi derivative ie jcm800, you're placing the secondary windings under massive strain, coating wise. They short out slightly, snap crackle and then pop as they permanently short and burn.
    People arent bothered to try and understand the differences in power supplies,and so the myth that all marshall amps are sensitive to mismatch lingers.
     
  17. South Park

    South Park Well-Known Member

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    The speaker is a power limiter to the out put transformer. The drivers feed the transformer.you can bigger on the speaker ohms but not smaller.you can do lot of things with this stuff.some of it works some of dosent
     
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  18. john hammond

    john hammond Well-Known Member

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    simple test. run as a matched load, feel output transformer for heat with your hand.
    then mismatch 50 percent, feel output transformer for heat with your hand,
    there will be zero difference.
    If iron, either output or power is running in an application its not suited for, it will get physically hot a long time before there is a short.

    edit, dont do this on any amp that utilizes voltage doubling.
     
    Last edited: Sep 16, 2019
  19. john hammond

    john hammond Well-Known Member

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    another, better test.

    flog your amp for an hour or whatever, feel how hot that power transformer is.

    consider that power and output transformers are the same thing, just copper wrapped around an iron core.

    if the power transformer can get that hot and not be considered at risk..then why the fuck would the output transformer be considered a risk if it remains cool?

    Does your output transformer get hot under any circumstances?
    I doubt it.
     
  20. South Park

    South Park Well-Known Member

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    Getting creative is great . Power matching electronics helps things last . Checking for heat is good
     

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