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Questions about Aguilar DB728

Discussion in 'Amps and Cabs [BG]' started by Lockout, Jun 25, 2003.

  1. jokerjkny


    Jan 19, 2002
    NY / NJ / PHL
    woah, learn something new everyday.

    coming from solid state power amp experience, we're told to set it for 4 ohms.
  2. This is not right! The impedance selector on the amp needs to be set for an equal or HIGHER impedance than that of the connected cab/s. Driving into twice the rated load causes a voltage spike in the plates of the output tubes that can be potentially catastrophic. Many an SVT met its death being cranked into an 8 or 16 ohm load.

    You will lose a bit of power if your speaker is not a match to the selected impedance and is lower, but it will not hurt the amp (unless you ridiculously low, like half an ohm, and even then all you're liable to do is blow your HV fuse.)

    Here's the source of the confusion:

    An "open" and a "short" are two completely different and separate (matter of fact, opposite) things. An "open" is NO load presented to the amplifier. It is the absolute worst case of having too HIGH a load. An open circuit has an infinite impedance. Since the infinite impedance is reflected back across the output transformer at the ratio of an expected load, it causes a (theoretically) infinite rise in plate voltage which will blow the tubes, damage the rest of the power supply and probably toast the output transfomer in the process.

    A "short," while not a good thing for the amp, is far less dangerous. It is an almost zero impedance which means current is flowing directly to ground. Old Fender tube amps had shorting jacks which diverted to ground when no plug was insterted as a means of protection from no load. It is possible that a short COULD damage the output transformer from heat, but amp power supplies don't store anywhere near enough current to do it.

    A "closed" short (remember, there is NO such thing as an "open" short) would be a speaker cable shorted to itself and wouldn't damage the amp. BFunk's own experience shows this.

    Had it been an "open," two minutes would have been more than enough time for the amp to destroy itself.
  3. BryanB

    BryanB Moderator Staff Member Supporting Member

    Thanks for clarifying that Psycho Bass Guy. I hate to be the one to spread mis-information.
  4. jokerjkny


    Jan 19, 2002
    NY / NJ / PHL

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