100 Watt amp through 100 watt cab

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by sws1, May 19, 2003.


  1. sws1

    sws1 Member

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    Recognizing that a 100 watt amp can put out in excess of 100 watts, can I use an attentuator as a limiter to protect the speakers? If so, how far down (ie how many dB reduction) to cut the 100 watt amp down to 100 watt cab?

    If 50 watts of output power is only 3 dB quieter than 100 watts, is it safe to say that if I attenuate 3 dB, then I'm doing the equivalent of running a 50 watt amp through a 100 watt cab? (which is a safe thing to do)

    I would really like to have a 4x12 greenback cab for my 100 watt marshall.
     
  2. John Phillips

    John Phillips Member

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    Yes, absolutely right.

    In theory you can run a 100W amp into a 100W cab, if the way the speaker maker rates the speakers is for 'continuous RMS' not peak - Celestion do this, which is why many old Marshalls (eg 50W combos with two 25W speakers) seem to be in the 'danger area', although they actually seem to survive OK. But personally I don't like to do it, I like my old speakers and I don't want to risk them too much (particularly as the power handling may decrease as the glues etc age).

    There are three ways you can safely run a 100W amp into a 100W cab - the easiest is just to remove two power tubes (if it's a 4-tube amp). Pull either the inner or outer pair, and if the amp has selectable impedance outputs (like the Marshall), set the amp to half the cab's impedance - this will give the correct match and a spare pair of tubes too. I would do it this way.

    Or you can use an attenuator. There are two ways here - either 'in-line' between the amp and cab (with impedances set to match as normal); or simply as a dummy load in parallel with the cab, which (assuming both are the same impedance) will absorb half the power - and may give a slightly more natural sound than 'in-line'. If you do this you need to set the amp to half the impedance of either the cab or attenuator too, just as if the attenuator was another cab.
     
  3. 94prs22

    94prs22 Gold Supporting Member

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    Unquiet,
    Does using an atttenuator as a dummy load mean that you take one of the speaker outputs and run it to the speakers and take another speaker output and just run it into the attenuator with the attenuator sending signal to nothing? If so that's cool, I didn't realize that it could be done this way, I've always used an attenuator in line, but never really liked it.
    Brandon
     
  4. sws1

    sws1 Member

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    I don't think all attenuators do that.

    I suspect you would cause alot of damage if you hooked up, say, a Z Airbrake, in that manner.

    I think the Weber Mass might support this, but not sure.
     
  5. philster

    philster Member

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    I agree with the responses, but I have used a 100 watt Marshall with a greenback loaded 4x12 for years without ANY problems whatsoever.

    Pulling the tubes certainly works, but to me it changes that amps sound to a degree. I love the hugeness of a 100 watter, and I have never come close to getting it with a 50 watter, or pulling 2 power tubes.
     
  6. sws1

    sws1 Member

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    Really?

    Philster? How loud do you play? Channels jumpered?
     
  7. philster

    philster Member

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    I don't play that loud anymore, but I have had the thing dimed many times, especially in my soundroom. I'll now run it at around 10:30 to 11 o'clock on the amp, into both inputs and use pedals and the guitar volume knob.
     

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