Ohm vs. Volume

Discussion in 'Amps and Cabs' started by GasMask, Jun 24, 2005.

  1. GasMask

    GasMask Member

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    All other things being equal, how much difference in volume is there between a 4 ohm cab and an 8 ohm cab and a 16 ohm cab?
     
  2. leofenderbender

    leofenderbender Supporting Member

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    I believe it depends on the output impedance of the amp to a large degree.

    If there is a match in input and output impedances, there is no discernable difference in volume (8ohm into 8ohm, 16ohm into 16ohm.) Mismatch impedances and the amp will either have to work harder to push the same volume or it will work hard enough to blow your speakers (16ohm into 8ohm, 16ohm into 4ohm.)

    That's as I understand it.
     
  3. John Phillips

    John Phillips Member

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    If the cab is correctly matched to the amp there is no difference in volume as leofenderbender says.

    If it's mismatched (which you should only do by up to a factor of two in either direction with a tube amp), you will typically lose about 20 to 40% of the power no matter which direction the mismatch is in, depending on various factors in the circuit and output transformer.

    With a solid-state amp, you can't use a cab which is lower impedance than the amp, but you can use any which is higher, and you always get less power - typically 20 to 40% lower with double the minimum impedance, and 50 to 70% lower with four times, again depending on the amp design (power supply regulation etc).

    This can be useful in order to protect the speakers (and the amp) if they're a bit close to the edge. Just don't run a solid-state amp below its minimum impedance... you'll get a lot of power for a short time, and then something (amp, and possibly speakers as well) will blow.
     
  4. GasMask

    GasMask Member

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    Thanks for the replies!

    If I intentionally mismatch with a tube amp- say use the 8 ohm output into a 16 ohm cab, I understand there will be a power loss. Is this an acceptable way of reducing volume? If I do this for many years, will there be any additional wear on the components of the amp?
     
  5. VaughnC

    VaughnC Supporting Member

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    While it's more risky running a tube amp into a higher than normal impedence than lower, with most tube amps you're generally safe one impedence step up or down. However, in a mismatched situation you may actually notice more loss in clean headroom than volume per se. Also, with a one step impedence mismatch, while it's doubtful that you'll hurt a modern tube amp, the output tubes may not last as long as in a matched situation. Tubes are inherently current limited so the danger of pulling too much current through other componants (output transformer) is practically nil....but the flyback voltage generated from too high impedence load can be a transformer and/or tube socket killer.
     
  6. John Phillips

    John Phillips Member

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    It should be fine, providing you don't mismatch by more than a factor of two, as you suggested. In the mid 80s, when I started repairing amps, a 'trick' to get more distortion and less volume out of your Marshall amp was to run it at 4 ohms into a 16-ohm speaker. It does work - until the OT blows.

    An 8-ohm amp into a 16-ohm cab will likely be perfectly safe for the amp, and will actually reduce wear on the tubes.

    Be aware that the volume reduction isn't huge (not more than 2dB) and it will change the tone too, which you may want, or not. Deliberate mismatching is usually done more for tone reasons, since it doesn't reduce the volume much.

    With solid-state amps, the volume reduction is also not huge, and the tone change is usually even less. In fact, I often deliberately do this with a SS amp with a blown speaker - replace it with one of double the impedance, as well as greater power handling. It protects both the amp and the speaker, since neither is having to handle as much power.
     
  7. jaimz

    jaimz Member

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    Does the same apply in reverse order as well? as far as damaging the tubes or transformer that is.

    I recently noticed (by accident) that if I have my marshall amp switched to 16ohm running into an 8ohm cab that It seemed to have a richer, meatier tone. Is that possible or was I imagining it. I called a local music store and asked about it, and he said running that way all of the time was a very bad idea, that It would destroy my OT.
     
  8. VaughnC

    VaughnC Supporting Member

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    Usually what kills an output transformer is an unusually high impedence load or no load on the amp. This can generate a high flyback voltage that can arc between the transformers windings and its case, which breaks down the insulation....causing a funny smell & smoke!.

    However, in your scenario, running a lower than normal impedence load will stress the output tubes but, because the tubes are self current limiting, they can't generate enough current to actually hurt the output transformer....and there's no flyback voltage generated to worry about. So, you're safe running your 16 ohm amp into an 8 ohm cab.

    Also, when you changed the impedence setting on your amp, you also changed the inductance ratio between the transformer & speaker which, theoretically, should change the frequency response, and thus the tone too....along with lowering the available output power.
     
  9. jaimz

    jaimz Member

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    Thanks for the info!!

    So is it possible by doing this that I will burn through tubes faster than normal? this is probably a stupid question, but if so could I lower the bias a little (still out of crossover range) to lessen the stress on the tubes?

    Also, is doing this bad for the speakers?

    I am using a 50w jcm800 running into a 2x12 celestion cab (v30s)
     
  10. leofenderbender

    leofenderbender Supporting Member

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    Whenever I have mismatched impedances, it was by accident - it isn't likely I'll do it on purpose since it never sounded that good to me. The amp sounding bad is how I determined I had plugged into the wrong jack.

    If I ever wanted tone that isn't as good as it could be AND degraded tube life, I would mismatch impedances on purpose.
     
  11. John Phillips

    John Phillips Member

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    I prefer a match too, but jaimz actually prefers the tone he gets with a mismatch... it's just taste. The trick is to know how to do it safely.

    I don't think running the tubes cooler would help much with tube life under that kind of mismatch, although it wouldn't hurt to try it and see if you still get the tone you like.


    Using an attenuator with the amp cranked also may shorten tube life (contrary to what some attenuator makers say) - some attenuators are worse than others too - since they don't all match the impedance of a real speaker closely.


    Mismatching won't harm the speakers, since the power output of the amp is always reduced.

    (With a solid-state amp, running below the minimum impedance increases power and may blow the speakers as well as the output transistors.)
     
  12. veldt666

    veldt666 Member

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    John, have you ever used, or experienced the product "Speakermate"? A loadbox of sorts, allowing you to connect up mismatched cabinets to one amp.
    I just wondered your opinions on such a device?
     
  13. soupbone

    soupbone Member

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    So it would be ok to run my Traynor 8Ohm output into my Germino 2x12 cab 16Ohm?
     
  14. kannibul

    kannibul Member

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    Tube amps? no
    SS amps - yes.
     
  15. VaughnC

    VaughnC Supporting Member

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    While it's more risky running a tube amp into a higher than normal impedence load, most tube amps will be fine with one impedence step up or down. So, even though you would be stressing the output tubes a bit and you will loose output power, an 8 ohm amp into a 4 or 16 ohm cab will be ok. However, a 4 ohm tube amp into a 16 ohm cab would make me nervous at gigging volume.
     
  16. fr8_trane

    fr8_trane Member

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    Sure..but....I have a SF bassman whose output inpedance is the odd 4 ohms (why 4 Leo????). I have a perfectly great 8 ohm Marshall 4x10 that I have been using with this head for years. No problems because the old Fenders trannies were overbuilt and can easily handle a one step up or down mismatch from what I have come to learn. The amp through this cab is punchy and LOUD!!! I don't think there would be any noticeable increase in volume running through a matched cab but the headroom would probably increase slightly but who would want that? Its already loud enough on 4 to play outdoor gigs.:dude
     

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