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Will a 400+ do 3 ohms and 6 ohms?

Discussion in 'Amps and Cabs [BG]' started by thirdeye, Apr 27, 2003.

  1. This cab >terminator,has switchable fullrange impedence of 3 ohms and 6 ohms.

    The Mesa has 2 ohm-4 ohm and 8 ohm outputs,Is it safe to run the 400+ at 3/6 ohms?What output jack would I hook up to?

  2. ChenNuts44


    Nov 18, 2001
    Davenport, IA
    you'll probably hear conflicting arguments, but i'd hook the 3 ohm to the 4 ohm tap, and the 6 ohm to the 8 ohm tap. tube amps are a bit different than solid state in the respect that it's better to go a little under the impedance rating of the tap than over.

    see if you can get psycho bass guy in on this thread to help you out. he could probably explain this a thousand times better than i can. ;)

    good luck
  3. PICK


    Jan 27, 2002
    Sydney, Australia
    I would do the opposite. Hook the 3 ohm up to the 2 ohm jack, and the 6 ohm up to the 4 ohm jack. Better to be safe than sorry IMO. Ive been running my Ampeg SVT2pro at 8ohms for about a year, its only switchable between 2 and 4 ohms I havent had any problems.
  4. Phone Mesa. I once used both a 4 ohm and an 8 ohm cab and they told me which jacks to plug into to balance out the levels. i don't remeber which ones of course...that was over 10 years ago.
  5. 'Safe' for soild state is NOT safe for tubes. Tube amps run at higher impedances than they are set for will "back up" plate voltage and possibly arc and destroy themselves. The louder you play, the greater the risk. Most amps will tolerarte a 100% mismatch, ie. 8 ohms from 4 ohm tap, BUT the louder you play, the greater the risk. You can run lower impedances all day long, and all it will do is give you a reduction in power (unless it's an outrageously low load run for a long period of time,in which case you might toast your output tranny). Some vintage Fender amps shorted their outputs to ground when no load was attached to protect the output tubes, can't get any lower impedance than that.

    ChenNuts44 covered it nicely. Go lower, NOT higher. This is NOT a solid state amp.
  6. This leads to the question "how do tube amps do with real impedance loads?"

    Guitar players often run open back cabinets, and the impedance rise at the speaker resonance can go upwards of 100 ohms.

    Using the Eminence Legend GB12 as an example, in free air this driver resonates at 86 Hz, which is low F on a 6-string guitar. The impedance at 86 Hz is 192 ohms. This is a far cry from 3 or 6 ohms set in the output transformer circuitry.
  7. A resonant transient in a guitar amp is countered by a negative feedack voltage from the amp. It acts against the impedance rise by decreasing the voltage produced by the EMF of the coil and causes the amp to damp the speaker better. As long as the speaker load presented to the secondary of the OT is at rating or lower, there is sufficient NFB from the amp itself to counter speaker impedance spikes and damp the speaker, but if you go above that rating, the amp loses damping and the output tubes will see a dangerous rise in plate voltage. This is true of most "regular" class A/B amps.

    Class A amps and A/B1 don't usually utilize NFB that way. With A/B 1 (ultralinear), the grids of the tubes are connected to the feedback line from the speakers. As they generate more EMF, the grid voltage rises, keeping plate voltage in check, like momentarily turning the bias WAY up. This lets the amp generate much larger amounts of power and stay cleaner longer, i.e.- the Ampeg SVT.

    In class A amps in most cases, there is NO NFB and they're famous for being unstable and for blowing speakers and tubes. Their output trannies are usually much heftier than the OT for an A/B amp of equivalent rating and are designed to absorb negative voltage spikes caused by impedance spikes. That's part of why VOX AC30's eat tubes.
  8. BTW, Bruce how did you derive an impedance of 192 ohms at 86 Hz? That doesn't sound right. A resonant frequency would still have its impedance paralleled against whatever low frequency content the driver is producing and a real-world value of 192 ohms wouldn't happen.
  9. Thanks Psycho.

    So if I go 3 ohm into the 4 ohm output jack
    -or if I go 6 ohm into the 8 ohm output jack,

    Will they both put out the same power?

    And will that power be less than what the amp is normally capable of?


    Also on a different note,

    There are 2 jacks for each output impedance,if for instance I had 2-8 ohm cabs and hook 2 cables from each cab to the 2 separate 8 ohm output jacks,would this end up at 4 ohms?
    Or in other words is this a correct hook up?

    Sorry if this is confusing.........I'm confused by the mesas jack layout

    Thanks very much for spending time with this.:cool:
  10. It is not derived, it is measured.

    Speakers running in sealed cabs or free air have a high impedance peak at resonance. Those in vented cabinets who two resonsances, one below and the second above the cabinet tuning frequency.

    When measured, both resonance points are MUCH higher than DCR. I chose the Legend GB12 as an example of a free air resonance frequency (open back cabinet) within the operating range of the instrument (guitar).

    Eminence publishes the measured impedance peak for all their drivers operating in free air.
  11. HOW is the resonance measured? A mechanical resonance can be induced without feeding the speaker signal, and the corresponding impedance spike can be measured absent amplifier damping which will make it MUCH higher than the output on an amplifer, tube or s/s will ever see. Ratings are an average impedance derived over the whole range of the speaker's operation, including resonances. A real-world impedance of 192 ohms would have a myriad of other factors influencing it that would significantly lower what the amplifier "sees".
  12. Petebass


    Dec 22, 2002
    QLD Australia
    Can I ask a question on behalf of us mere mortals:- what exactly is "amplifier damping" and how does it work?
  13. Damping is the ability of an amplifier to control the speaker and is a function of slew rate, measured in volts/microsecond. High slew rate is good to point, BTW, but if it's too high, it can make the output section unstable and induce oscillations in a tube or s/s amp. Basically damping is how quick the amp can stop the speaker once it has been set into motion.
  14. Phat Ham

    Phat Ham

    Feb 13, 2000
    I thought the value for damping was derived from the output impedance of the amp. i.e. 1/Rout = damping factor. But don't take my word for it.
  15. Output impedance determines slew rate; higher Z= lower slew rate.

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