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Attn electro dudes: cab/head impedance mismatch confusion

Discussion in 'Backline' started by ~BURNY~, Oct 20, 2016.

  1. ~BURNY~

    ~BURNY~ Member

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    I've always gone with the popular "cab must have an equal or greater impedance than the tube head" reasoning so far.

    A friend just sent me this article:

    https://robrobinette.com/How_Amps_Work.htm

    What about:


    Really confusing for a not so scientific mind like me. Any expert left on the forum who could explain what I am reading here with simple words and maybe coloring book drawings?
     
  2. Jormyn

    Jormyn Member

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    Disclaimer: I'm an electrician, so this explanation may not be 100% correct. Electronics and tubes are weird.

    - A better rule to work with, as he mentions in the article, is "amp and cab can be one 'step' different from each other" - i.e. an 8 ohm amp and a 16 ohm cab, or a 4 ohm cab. Both situations will still have the negative effects he discusses, but they'll be fairly mild.

    - Output Impedance can be thought of as how hard the amp is expecting to work; each setting is just taking a specific voltage from the output transformer, which provides an acceptable/safe amount of current when you give it the appropriate speaker.

    - If you hook a 4 ohm speaker up to an 8 ohm output, the amp is still providing enough power for an 8 ohm load, so you're pumping twice as much current through both your power tubes and the speaker. Current makes heat, heat wears things out.

    - Likewise, if you put a 16 ohm speaker on an 8 ohm output, and don't change the volume, the speaker is only seeing half as much current and your volume will drop. That's not so bad by itself, but because a speaker is an inductor (read: a coil of wire) it also pushes some current back toward the amp. This, in turn, goes back through the output transformer and hits your tubes.

    - This "flyback voltage" happens all the time in any amp, but they're built to handle it when your speaker impedance is correct. When the speaker has a higher impedance than the amp output, these voltage spikes can become much larger and damage the amp.

    - Keep in mind that all of these problems are reduced as you lower the volume. For bedroom playing, you could probably get away with a larger mismatch and not do any serious damage. I can't guarantee that, though.
     
    #2 Jormyn, Oct 20, 2016
    Last edited: Oct 20, 2016
    Korwent and ~BURNY~ like this.
  3. Korwent

    Korwent Member

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    ~BURNY~ and Jormyn like this.
  4. Trevoire520

    Trevoire520 Member

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    All I know is that I fried an output transformer in a few minutes because I'd accidentally plugged a 8 ohm cab into the 16 ohm output of the head when reamping. So yeah, don't do that.
     
  5. ~BURNY~

    ~BURNY~ Member

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    Ditto. But what's news to me is that the opposite (16 ohm cabinet, 8 ohm output for example) can potentially do as much if not more damages at high volume if you go beyond the margin of tolerance because of that flyback voltage.

    Worst thing I experienced myself in the past was fuse failure but maybe I was lucky.

    I once tried to use a "poorly rewired" marshall cab. Every speakers in series resulting in a 64 ohm total impedence!:eek: I started realizing something was wrong two or three fried fuses later.

    I even forgot to plug any cab at all in my 6505 a couple of times. luckily no consequences.
     
    #5 ~BURNY~, Oct 25, 2016
    Last edited: Oct 25, 2016

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