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Thread: Help with bias please

  1. #1
    Senior Member vchizzle's Avatar
    Join Date
    Apr 2012

    Help with bias please

    I want to check the bias on my Archon to see where it's at. I'm considering changing power tubes. Step by step instructions would be cool. Want to see if it's biased on the cold side. Seems a hair quiet. I leave my levels the same all the time but seems a bit quiet from yesterday to today. Maybe I need new power tubes.

  2. #2
    deus ex machina
    Join Date
    Apr 2012
    Power tubes can last for years, if not decades in a properly-biased class AB amp. What you are more than likely experiencing is AC supply voltage fluctuation, especially if your metro area and surrounding states have experienced a spike in usage during the last few days. Unlike solid-state amps that have regulated power supplies, tube amps are sensitive to AC voltage fluctuations. If you own a multimeter, take wall voltage readings for several days in a row (set the meter to measure AC voltage). I bet that the wall voltage is lower when the amp sounds quieter. Head room is a function of plate voltage, and plate voltage is a function of wall voltage.

    By the way, biasing a class AB amp colder increases output power because the amp runs closer to class B, which is a more efficient class of operation. Biasing a class AB amp hotter, pushes it closer to class A operation, which is a very inefficient class of operation. A lot of power is wasted as heat when an amp is biased hot.
    Last edited by Em7; 05-14-2014 at 11:05 AM.

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