Bassman speaker bypass?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Bruster, Jul 8, 2005.

  1. Bruster

    Bruster Member

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    Please excuse what may be a dumb question, but here goes. I've got a 4 x 10 Bassman with one blown speaker. I don't want to put money into it because I'm amp-sitting it for a guy who went to Europe-- so I don't want to shell out to buy him a replacement speaker. If I just wire a bypass, it will create an impedence mismatch. Alternately I could wire in a resister, but that seems just as screwy.

    Anyone know how to disconnect the blown speaker so I can use the amp without placing the output stage at risk?
     
  2. 908SSP

    908SSP Supporting Member

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    You can wire a resistor in place of the speaker nothing screwy about that. It has to have at least the power rating of the other speakers and the same impedance. It will make a very slight difference in the tone as it will effect the impedance seen by the amp. The impedance of the speakers change at different frequencies the impedance of the resistor stays the same. But the difference in tone will be very slight and this is very safe for the amp.
     
  3. John Phillips

    John Phillips Member

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    What Alex said - you don't actually want an 8-ohm resistor because the true resistance of an "8-ohm" speaker isn't actually 8 ohms at most frequencies. The average over the audio range is more like double - 15 ohms or so. If you can't get a 15-ohm resistor, 10 ohms will do fine too, just don't go below that. You need a power handling of at least 20 watts, if the resistor isn't heatsinked - the amp can peak at well over the 45W rating when driven hard.
     
  4. TheAmpNerd

    TheAmpNerd Member

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    Hey John,

    So if were going to set up dummy loads for cabinets,
    I'd want to do somthing like the following:

    Cabinet Load Resistor

    2 ohm 3 - 4 ohm

    4 ohm 6.5 - 8 ohm

    8 ohm 10 - 15 ohm

    16 ohm 20 - 30 ohm


    Now would this be a better match when doing
    testing on am amp? That is, plugging into the
    dummy load and running a test signal through
    the amp?
     
  5. John Phillips

    John Phillips Member

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    It does depend on what frequency you're using. At low ones (400Hz and even 1K, which are the usual test frequencies) the impedance isn't too far away from the nominal - it may even be dead on at one of them. The big problem is that the impedance of a normal speaker rises so far at the upper end of the range - five times isn't unusual - that it shifts the average a long way when you look at the whole picture.

    True RMS power tests should actually be carried out at the frequency the amp was specified at, into the nominal impedance.

    If you want a simple resistor, I'd just pick the 'preferred value' just above the nominal impedance and call it close enough at that...

    2 ohms: 2.2 ohm
    4 ohms: 4.7 ohm
    8 ohms: 10 ohm
    16 ohms: 22 ohm

    As long as you don't want precise power measurements (which you'll need a scope to do anyway, since you need to clearly see the exact point of clipping) these will be safe and give you useful results. If an amp isn't working properly you'll get a much larger change in power output than the small error from using the wrong impedance.
     
  6. TheAmpNerd

    TheAmpNerd Member

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    John, That is what I was thinking. None of this is super
    exact any way. As you know, just that little bit extra
    on the volume knob of an amp with your test signal, will distort that wave form on the scope....There is enough
    error in there. Thinking close enough is fine for this
    purpose, because every amp even between models
    is slightly different.

    I like to ask because I'm also open to suggestions and
    new ideas...I try to keep an open mind.

    Ciao
     
  7. Bruster

    Bruster Member

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    Just wanted to say thanks! A 10 ohm it is, then!
     

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