How do different ohms effect an amp??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by Laroosco!, Sep 22, 2009.

  1. Laroosco!

    Laroosco! Member

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    I'm playing a 50 watt Plexi clone that I can run at 4 or 8 ohms. My only cabinet is 8 so I have always run it that way.

    I am going to be replacing the speakers soon and am curious what going to a 4 ohm load would do to the amps response and tone.
     
  2. SatelliteAmps

    SatelliteAmps Member

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    There have been many threads about this, and they always turn into about the same discussion.

    From the amps standpoint, there is basically no audible difference between running an amp at 4, 8 or 16Ω as long as it is properly matched to a cabinet.

    From a speaker standpoint, there is no audible difference between two identical speakers at the same impedance. (Check the info sheets from the speaker manufacturers).

    A cabinet can be wired differently to get different sounds, and some amps can run mismatches in impedances which will also get different sounds.

    To me, the main thing is flexibility. If you have an 8Ω cabinet, I would get speakers that could be intermixed, or used as a second cabinet. (Run two 8Ω cab's as a 4Ω load total.)
     
  3. Stu Blue

    Stu Blue Member

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    Old Marshall superleads definitely produced more watts into 4ohms (120) than 16ohms (100), correctly matched tap/speaker of course.

    (Edit Since this old thread has been revived I wish to withdraw the above claim. I got those figures watching the techs at a big hire company... the odds are they were simply running into a 4ohm bench load regardless of which tap the amp was set at, since I never had to change the tap back to 16ohms when I sent the amps out with a 4x12 (or 8ohms for two). The guy who owned the company was a nasty piece of work and the techs adopted a pretty cavalier approach.... the "price" for replacing speakers or OTs on our own amps was a couple of beers, etc.....)

    For amps that can take mis-matches, putting a 16 ohm speaker on an 8ohm tap will lower the output and give a relatively bassier sound... a 4ohm speaker will work the amp harder and produce more watts than spec but a brighter harder tone. In theory, correctly matched, there should be no change in tone for different taps... but there always is a slight difference (at least to the bat-eared) in the same direction as the mis-match....
     
    Last edited: Feb 22, 2012
  4. SatelliteAmps

    SatelliteAmps Member

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    Never seen a Superlead that had a 20 watt difference in clean output power depending on which tap was used. Not sure where that info came from, but I don't believe it is correct.
     
  5. andrekp

    andrekp Member

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    Also, driving a speaker that is higher in impeadance, uses more of the transformer secondary. For example, in an amp with options for 4 & 8, the 8 will use the full secondary, while the 4 uses a portion of it.

    Lots of folks believe this makes a difference in tone. I don't know because I only ever use 16's, so I'm always using the full secondary anyway.
     
  6. VaughnC

    VaughnC Supporting Member

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    Lot of theories about tone...but the true test is one's ears. With some rigs I hear a difference between matched 4/4, 8/8, or 16/16...while others I don't. Even a one step mismatch is ok if you like that sound. However, with a tube amp, the major concern is having the speaker impedance more than one impedance step higher than the amp impedance setting....this can cause output transformer killing flyback voltage to be generated. For example, it's usually ok to connect an 8 ohm speaker to a 4 ohm amp, but its risky to connect a 16 ohm speaker to a 4 ohm amp.
     
  7. Stu Blue

    Stu Blue Member

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    You're right. Actually, with good mullards, the 120 into 4ohms is correct, but at 16ohms they did more than 100; more like 110. Back then in Europe they had to quote accurate RMS figures by law in their advertising (somewhere in the small print) . They produced 10% distortion at the clipping point; mostly second harmonic BTW. Back in 1972 I did a year with the biggest UK gear-hire company who had 20+ Marshalls of all types (six Fender Rhoads Two B3s and at least 6 twins and one of everything else). More to the point they had 3 full time techs, with all the gear, so I watched loads of them being tested and 10 watts difference was standard, sometimes 15.

    (See my edit above)
     
    Last edited: Feb 22, 2012
  8. SatelliteAmps

    SatelliteAmps Member

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    Again, I think this is some mis-information. I've got over 20 Marshalls myself.

    Wattage is somewhat dependent on what tubes are involved, but when checking between amps that are set up identically with identical inputs, there isn't a 10 watt swing between a 4Ω and a 16Ω tap.

    Mullards are great tubes, but they aren't doing any extra magic to add wattage at different taps.
     
  9. Stu Blue

    Stu Blue Member

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    I must bow to your superior wisdom in these matters, old memories are not 100% reliable. Of course that is the era when Marshall produced different amps for the states (valve wise at least) because the English ones ate EL34s in less than a year and you yanks got revalves under guarrantee which we Brits were too polite to claim. [​IMG]
     
  10. SatelliteAmps

    SatelliteAmps Member

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    I don't know if you had a mixture of tube types. There would be that much of a wattage swing between an EL34 and a 6L6 based Marshall.
     
  11. Stu Blue

    Stu Blue Member

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    Mmmm clever you. I did take advantage of all those "free" techs to try my own Marshall Superlead with RCA 6L6s (briefly), because I was fond of the JTM45....
     
  12. bunuel

    bunuel Member

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    Is this correct? Not doubting but'd like to be sure. I'd been running an 8 ohm spkr in ol' champtype airline that has a 4ohm transfrmr, but recently went back to a 4ohm because I didn't want to risk damaging the amp. I really liked the 8 ohm better, so I'd like to know...thanks in advance folks!
     
  13. SatelliteAmps

    SatelliteAmps Member

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    Some amps can take a one step mismatch and be fine. If you've been running your amp like that for a while, then chances are it is fine.
     
  14. Laroosco!

    Laroosco! Member

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    I ran my old Traynor YSR1(8ohms)into a 4 ohm cab for years and it sounded great and I never had one problem with the amp
     
  15. jamison162

    jamison162 Supporting Member

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    Running a 16 ohm 2x12 cab, my amp sounds pretty different when using the proper 16 ohm tap vs. the 8 ohm tap. The 8ohm is slightly louder, brighter and cleaner (more headroom/less compression).

    My question is: Why wouldn't an amps secondary taps sound different??? Isn't the signal being affected in a different way (more or less windings) depending on the tap used? What am I missing here....
     
  16. diagrammatiks

    diagrammatiks Member

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    science.
     
  17. SatelliteAmps

    SatelliteAmps Member

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    What amp? Secondary taps do sound different because they change the load on the tubes and change the overall ratio of the transformer. Its not a question of more or less windings but overall power transfer maximization.
     
  18. jamison162

    jamison162 Supporting Member

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    So what would be the physical difference in the secondary coil in relation to say an 8 ohm vs. 16 ohm tap??
     
  19. diagrammatiks

    diagrammatiks Member

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    the number of windings.
     
  20. Stu Blue

    Stu Blue Member

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    You're missing that you're mis-matching the amp......

    EDIT You ought to be hearing less volume and a slightly darker tone...... sure that cab is actually 16ohms?
     
    Last edited: Feb 22, 2012

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