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Tube Amp Question

Discussion in 'Amps and Cabs [BG]' started by Lockout, Aug 14, 2003.

  1. Lockout


    Dec 24, 2002
    With the limited knowledge I have of tube power amps, I know that you're supposed to have a speaker cabinet connected to them at all times when running, correct?

    Now, what I'm wondering... is this always true?

    Say for example, if there is no preamp or anything that could be sending a signal into the power amp, and you've got the gain and everything turned all the way down... can it still damage the amp? Could someone explain in detail exactly how not having a speaker load damages the amp? I'd like to understand how it happens so that maybe I could answer my own question. :)

    The reason I'm asking this is because I'm looking into buying a used Mesa Stereo 20/20 tube power amp to use for guitar (*gasp* ;)) but I've only got one cabinet to use with it, and I'm wondering if it's safe to just not run a signal into the other side of the power amp.

    If it's not safe, do you know of any alternative solutions for this problem other than buying another speaker cabinet?
  2. Mcrelly


    Jun 16, 2003
    Minnesota, USA
    can it be bridged? meaning can left and right channels be "bridged" and the mono output of the amp is increased?

    I suppose if you can get a "power brake" to place on one channel of the amp or have someone make you a "dummy load" to attach to your unused channel.

    I don't know much, but I think heat build up (because of no load) in the tubes causes them to burn out faster???
  3. BryanB

    BryanB Moderator Staff Member Supporting Member

    Yes, you must never turn on a tube power amp without a load attached. I don't know all the technical details, but I know it will blow the output transformer. To run the amp without a speaker requires a dummy load. Just leaving the amp on standby is not enough.
  4. Big String

    Big String Supporting Member

    Apr 22, 2000
    Northwest Indiana
    BFunk is EXACTlY right.
    Must have a load or goodbye music.
  5. Actually you're all wrong and Lockout's question and assertion is correct. So long as there is no signal being amplified you don't need to have a load. It's not heat that damages tube amps with no load; it's the inductive voltage spike generated by the output transformer.

    The output transfromer normally takes the high voltage, low current flow of tubes, caused by their high output impedance, and "transforms" it into a low voltage, high current flow that can be directly connected to the relatively low impedance of a speaker. Without a load connected to its secondary coil, the output transformer acts as a voltage amplifier and causes the voltage of the tubes to rise dangerously until they or the output tranny or both burns up.

    If tubes had low output impedance like transitors, you wouldn't need an output tranny, nor would the amp require a load. Also, if the amp is left on standby, the high voltage supply to the tubes is not even engaged, so there is no voltage for the output tranny to step up and it doesn't matter whether there is a load or not.

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