Ohms got me down

Discussion in 'Amps and Cabs' started by Geetar_Will, Jan 7, 2008.


  1. Geetar_Will

    Geetar_Will Supporting Member

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    Well I recently found out my '64 blackface tremolux is 4 ohms out and not 8 ohms as I thought previously. I admit to not being tech savvy in this area, but I shouldn't have believed the guy at the shop that told me it was 8 ohms out apparently.

    I run an avatar 8 ohm cab (that's two 16 ohm speakers), and have been for around 6 to 8 months with not a single issue from the amp. The odd thing is the amp performed like a champ every time I've used it. I've had compliments on my tone and I've run it up to just under half and had it stay clean pretty clean.

    What do you guys suggest I do? Is there an easy fix for mis-matched impedance? Should I do anything? Or should I put this in the closet (and let it gather value) and save up for a pre-assembled ceriatone head?

    Thanks
     
  2. FFTT

    FFTT Member

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    Are you talking the extension cab output or the primary speaker output
    being 4 Ohms?

    If the primary output is 8 Ohms, you can run a tap off that cable to your
    8 Ohm 2X12.
     
  3. webs

    webs Supporting Member

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    I'm sure someone will correct me if I'm wrong, but I believe it's almost always ok to have a 2:1 upwards impedance mismatch- so you should be ok running it as you were.
     
  4. plexirocker 68

    plexirocker 68 Member

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    Your O.k. with that mismatch, no biggie. Just remember it's like pouring water from one bucket into another. If your amp is 4 ohms going into a 8 ohm load it will handle all of it. If you were going from 8 to 4 then that's when it doesn't work so good.

    plexi
     
  5. teleamp

    teleamp Senior Member

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    Your OK, enjoy. The old Fenders used robust OT's. A 100% mismatch will not harm anything.

    MikeY
     
  6. Geetar_Will

    Geetar_Will Supporting Member

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    Thanks guys, I appreciate the info.
     
  7. Geetar_Will

    Geetar_Will Supporting Member

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    FFTT - I was told the primary speaker output is a dedicated 4 ohms. And a lot of literature online here confirmed it.
     
  8. JDandCoke

    JDandCoke Member

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    if the cab is 2x12 and each speaker is 16.

    then running it in parallel should give you the 4ohms that you need for the amp?

    are you just running the one speaker at the moment?
     
  9. dorianadams

    dorianadams Member

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    '
    I believe 2 16 in parallel yields 8 ohms.
     
  10. FFTT

    FFTT Member

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    Correct!
     
  11. John Phillips

    John Phillips Member

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    Yes, it is - but don't worry about it, it will handle 8 ohms with no problem at all. Just don't run it at 16.

    It will also handle a downward mismatch of the same relative amount, ie 2 ohms (many people are surprised by this), which is what happens when you plug in two 4-ohm cabs.

    Leo Fender was well aware that musicians weren't very knowledgeable about impedance matching, and designed his amps to be simple - no unnecessary selectors to fail or be set wrong - and sturdy enough to cope with any normal mismatch they would commonly be subjected to. (16-ohm cabs are relatively rare in North America, it's more of a British thing.)

    This is not always true with other amps though, so be careful.

    It's only a significant risk if you're cranking the amp into power-stage distortion anyway, with any mismatch.
     
  12. rockon1

    rockon1 Member

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    It all depends on how close the OT is to its limits whether the mismatch is acceptable. Generally speaking running a cab with an impedance higher(less load) than the amp is more dangerous to the OT and easier on the tubes. Running an impedance thats lower(more load) than the amp is easier on the OT and harder on the tubes.

    On an older classic amp I'd make sure the OT could handle it either way before I continued. Bob
     
  13. Geetar_Will

    Geetar_Will Supporting Member

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    How would one determine if the output transformer can handle it?
     
  14. pgissi

    pgissi Member

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    Theres fire...just kidding, not a good way to determine if your amp can handle it. Since you have been running it this way and your not experiencing any other issues, its fine.

    And on top of that your getting tone props from people and you may find that you may not like the amps tone or feel in the stock config, it will be different. With my amps stock impedance output, the amp sounded anemic, and in addition did not feel right, when driven hard sounded buzzy. Running a 100% mismatch smoothed it out, made it more responsive and balanced.

    Disclaimer- its not for every amp and/or speaker combination and
    results may vary

    I have been doing this for several years and find I like the tone in my R30 Rivera this way into a 2x12 much better than stock, turns this little sucker into a fire breathing beast. In the Rivera manual its recomemded as an alternative but running it at 4ohms is not, probably due to current handling limitations in output stage components since a mismatch to a lower imp cab means tubes are working harder, drawing more current.

    With a 100% mismatch in this direction you will find the amp puts out approx 1/3 less power at a given setting so you can turn it up more and get it cooking, this contributes to a tube amps tone significantly when you want OD, in fact its essential to really hearing a tube amps true tonal response. Additionally you will find the freq response is different, bottom and top are smoothed/flattened, my amps tone is now flatter overall and I find that it cuts through better and responds to eq changes more dramatically so I can run the tone stack pots slightly higher.

    In my opinion, the tone stack is a stage where you dont want to throw away too much gain to avoid anemia.

    I also find my tone is more harmonicaly complex in OD, probably due to the OT working harder due to a higher reflected impedance its seeeing and in my case the Rivera is stock 8 ohms and I am using a 16 ohm 2x12 cab.

    The only negatives the clean tones are slightly more compressed and have reduced headroom (unless I run it with the Master on 10 and back off the pre-amp). This is probably due to the loss of efficiency in the OT from the mismatch but its minor and if anything its a little darker and jazzy. I use outboard processing or its eq to counter.

    Lastly, biasing the amp properly in this config is essential, dont overdo, 70% total dissipation territory works for me and hotter than that added more compression.


    I was never happy with the amp and by accident ended up with a 16 ohm 2x12, didn't know it and played for months until I checked it. Ultimately I realized rules are for fools within reason of course, meaning great tones can be found on occassion by abandoning them, rules that is.

    To keep within reason, never exceed 100% mismatch and maybe (imo) avoid doing this with high output amps, anything putting out more than 50 or 60 watts since there is some serious current/heat generated there and with the loss of efficiency a mismatch causes more heat is produced and not sinking that heat may sink the amp.

    Once again my opinion and imp mismatch is my trick to turn little amps into their larger fire breathing brethren tone wise.

    P.S. If your not sure a/b it with the stock ohm load using the same type of speakers of course.
     
  15. Geetar_Will

    Geetar_Will Supporting Member

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    Thanks a ton guys. I was really worried for a while there. This is great to learn as well.

    What brought me to find this information out was looking into an attenutor. You might've seen my thread a day or two ago.

    Having found this information out, is an attenutor still an option? Do i look for a 4 ohm out attenutor? Keep on learning me on this subject.
     
  16. pgissi

    pgissi Member

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    I personally do not like them, your not moving air and the buzzy characteristics they can render are less toneful imo. My strategy has been to stay with amps under 40 watts and drive them hard avoiding the need for an attenuator.

    In addition I would not recommend attenuation with imp mismatch.

    But there are many amps rated for 40 watts, yours amongst them correct?, that are loud as sheet at 40 watts, especially BF Fenders. I have found that 30 watts seems to be a good fit all around, enough umph for a larger stage using a 2x12 with a medium loud band or still toneful at a subdued volume. So if your thinking of attenuating your 40 watts I am assuming your being told your a little loud.

    If thats the case and knowing that your amp is a non master volume variety, your probably turning it up to "get there" and with 40 watts non master, it can be perceived as loud but to us its really not. So you need another approach as an alternative to attenuation if my assumptions are correct.

    Try these alternatives-

    To get OD or Sustain at lower volumes-

    do an extensive search for a pedal and use it to hit the amp harder in the front to dirty it up without too much gain, gain here reffering to input to output signal level ratio not amount of OD or distortion/clipping added. The fundamental problem here is that traditional OD refers to sending more signal into the first tube stage to then overload it and create overdrive (my personal fave) but this will also increase gain and subsequently volume. So for you I am thinking you need some soft clip on the floor while running the pedals output level at slightly above unity.

    I personally am loving the new Xotic AC Booster, a natural sounding OD pedal with on board EQ that gives you more felxibility which imo is absolutley necessary.


    To reduce your stage volume otherwise-

    work some acoustic magic, aim the amp sideways blowing across the stage if you want to not be as loud up front and/or use some plexi baffling in front of the cab to refect the sound up, very effective. Place the plexi sheet on a wooden base or some other mount about 10 to 12" from the cab, you can tilt it towards you to "paint" your position if need be.

    remember that guitar speakers are very directional, re-aim to stop whinning soundmen or band members and lastly trade in the wife if she whines and find one with a better set of ear muffs.

    Your goal here, if you and your fans are digging your tone, is to preserve the relationship you have going on now of guitar to amp to speakers, all working at X level to render Y amount of Headroom and Overdrive which ultimately speaks as it does due to the speakers working at Z spl.

    Change one thing and it goes away, especially using attenuation, your muzzling the voice of the amp, the speaker.

    One last trick is to use less efficient speakers and/or a combo of that with lower wattage speakers to get there sooner.
     
  17. John Phillips

    John Phillips Member

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    Ideally you want a 4-ohm attenuator. It's important to match the attenuator to the amp, not to the speaker, since the attenuator is what the amp actually sees unless it's bypassed. If you're attenuating by more than 3dB the speaker impedance doesn't really matter, since the attenuator is then the bulk of the load.

    Many 8-ohm attenuators will also work perfectly well though, because their impedance curve is flatter than that of a real speaker and they stay well within the safe mismatch range - but you need to know. One to avoid is the Marshall Powerbrake - not because it's a bad product, but because the impedance curve goes the other way and outside the safe range for a 4-ohm amp.
     
  18. cameron

    cameron Member

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    Your 8 Ohm cabinet is probably already slightly attenuating your sound. In most cases, you'd get maximum power output at the optimal matched impedance. If you switched to a 4 Ohm cabinet (assuming you get speakers with the same efficiency) you'd probably hear an increase in volume.

    The myth about upwards mismatches being OK and downwards mismatches being harmful is due to that actually being true of solid state amps. Tube amps have an optimal impedance, and can tolerate some mismatch in either direction. Solid state amps have a minimum impedance, at which they operate with maximum efficiency, but below which they fry.
     

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