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Thread: A 4 ohm head with an 8 ohm cabinet

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    357mag is offline Senior Member
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    A 4 ohm head with an 8 ohm cabinet

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    I was considering picking up a Marshall Valvestate 150 watt guitar head, and using it with my 8 ohm Marshall cabinet. The minimum ohm rating of the head is 4 ohms. I know it's safe to do so, since one fellow at Marshall already told me that's okay, plus in the manual they discuss which jack to use if you are going to be playing an 8 ohm cabinet.

    Thing is...what about wattage loss and volume loss? If I use a 4 ohm head on top of an 8 ohm cabinet, is the amp effectively only putting out 75 watts then? And then I will lose some punch and volume?

    Is there a reason hybrid amps and all solid-state amps don't have impedance switches like tube amps do?

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    Dani Pace's Avatar
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    A 4 ohm output head to a 8 ohm speaker cab should pose no problems, might even improve your sound. You should notice only a slight volume drop and should notice better clean tones. The volume drop may be so slight that you wont notice it.
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    toobalicious is offline Dedicated Member
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    Quote Originally Posted by Dani Pace View Post
    A 4 ohm output head THAT IS SOLID STATE to a 8 ohm speaker cab should pose no problems, might even improve your sound. You should notice only a slight volume drop and should notice better clean tones. The volume drop may be so slight that you wont notice it.
    fixed.

    while most tube amps can also tolerate impedance mismatches, there are a few that cannot, and said mismatch may cause OT to release magic smoke.

    a

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    toobalicious is offline Dedicated Member
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    Quote Originally Posted by 357mag View Post
    And then I will lose some punch and volume?
    ......
    Is there a reason hybrid amps and all solid-state amps don't have impedance switches like tube amps do?
    yes. you will lose volume, as a higher impedance load wont be drawing as much current. in this example, this reduced current should result in lowered volume (speaker dependent). OTOH, speaker efficiency can (a) vary wildly, and (b) can contribute to SPL more than sheer amp power.

    for example, (and it isnt a great one, but it proves the principle) i happen to know someone who went to a "real" sub in his car. he had been using an efficient peavey black widow that he had "inherited" (which i was able to snag from his "upgrade"). the kicker sub he installed had sensitivity of like 87dB, while the BW is more like 98dB. guess what? the "old-assed" peavey was much louder, and truly, played almost as low as the sub in his box. i am still using that speaker today.

    also, since as impedance increases, current is reduced, and therefore will actually cause your solid-state amp to run cooler. SS stuff only struggles when you decrease impedance below the minimum. actually, lots of amps can handle that for a while---- but the increased current demand puts more stress on the output section, and eventually will cause it to fail (sometimes sooner rather than later). on that note, i used an old big-knobbed peavey CS-800 at 2 ohms stereo fro literally years with not a single hiccup (rated @ 4 ohms min).


    a
    Last edited by toobalicious; 05-07-2008 at 06:31.

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    Oancient1 is offline Dedicated Member
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    Quote Originally Posted by 357mag View Post
    since one fellow at Marshall already told me that's okay, plus in the manual they discuss which jack to use if you are going to be playing an 8 ohm cabinet.
    The volume will not necessarily be reduced significantly. If the head has separate jacks for 8 and 4 ohms and uses an output transformer at the end of the power amp stage, then the 8 ohm output could be of a separate tap on the transformer's secondary side that outputs a higher voltage to compensate for the higher impedance. Such multi-tap arrangments were common on older tube amps -- I don't know about today's stuff.

    Tom

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    Farview's Avatar
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    If it's a tube amp, plug the cabinet into the 8 ohm output. That is the way it was designed to work. The only way you would notice a drop in volume is if you generally run the thing flat out into a 4 ohm cabinet. The difference would be (at most) 3db spl.

    If it's a solid state amp, you have no worries. 4 ohms in the minimum load, meaning that is the limit, don't go below that. Anything above will work just fine.
    Jay Walsh
    Farview Recording and HERE!!!!

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    toobalicious is offline Dedicated Member
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    The only way you would notice a drop in volume is if you generally run the thing flat out into a 4 ohm cabinet.
    wait, what?

    the only scenario where this would be true is running a 4 ohm cab off of the 8 ohm tap of a tube amp. with a solid-state amp, the 4 ohm load would draw approximately 30% more current, and all other things equal, will be significantly louder than an 8 ohm load. 3dB is a lot; all things considered.

    his valvestate amp is definitely solid-state.

    a

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    Quote Originally Posted by toobalicious View Post
    wait, what?
    I didn't finish the sentence.
    If you you are used to the volume of a 4 ohm cabinet with the amp flat out, and 8 ohm cabinet will be about 3db quieter with the same amp flat out.
    Jay Walsh
    Farview Recording and HERE!!!!

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    Dani Pace's Avatar
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    If you are still concerned, open up the speaker cab and reconfigure the wiring (series/parallel) to make it a 4 ohm cab. Perceived volume is often more a matter of speaker efficiency rather than the power handling rating. If your amp has multiple speaker outs, use the one which matches your speaker cab impedance.

    Another alternative would be to plug another (8 ohm) speaker cab into the parallel speaker jack on the speaker cab, thereby making it a 4 ohm system. If you don't need the extra cab for your up front sound, use it for a monitor so the drummer or bass player can hear you better.
    The more I learn, the more I realize how much I don't know. [url]http://www.soundclick.com/sixfeetover[/url]

  10. #10
    toobalicious is offline Dedicated Member
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    Quote Originally Posted by Dani Pace View Post
    If you are still concerned, open up the speaker cab and reconfigure the wiring (series/parallel) to make it a 4 ohm cab.
    cant.

    no way to wire an 8 ohm 4x12 into 4 ohms (at least not without something unorthodox like a big 8 ohm resistor). 2, 8 , or 32.

    a

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