Amp running without cab...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by firewater, May 14, 2004.


  1. firewater

    firewater Guest

    Will a poweramp with no preamp connected (so no input signal) be damaged if you run it also without speakers?
     
  2. VaughnC

    VaughnC Supporting Member

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    Depends....is it a solid state or tube power amp? Generally speaking, there will be no damage to either one unloaded with no input signal. However, if an unloaded tube amp would happen to develop an internally generated oscillation or hum, theoretically that could cause a flyback voltage to be generated in the output transformer, which could fry it or arc the tube sockets. With a solid state amp, having no load generally won't hurt them.
     
  3. firewater

    firewater Guest

    I`m interested in what happen in tube amps. Can the oscillation generate harm and ruin some capacitor or transformer even if it`s a 150w with nothing on the input?
     
  4. VaughnC

    VaughnC Supporting Member

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    When a signal of sufficient level is applied to the input of a tube amp (with no load attached), the energy in the output transformer has nowhere to go, so the voltage can be stepped up sufficiently (via the transformer itself) to cause a voltage arc between the transformer windings and its case and/or across the output tube sockets themselves. This is called a flyback voltage. However, with no input signal applied to a tube amp, said flyback voltage can't be produced....UNLESS there's a malfunction in the amp that would cause an internally generated signal (oscillation, hum, etc.) to appear at the amps output. So, as long as a tube amp is functioning properly, there shouldn't be any problems with a no input signal/no load condition....BUT one never knows when an internally generated signal, due to a malfunction, will occurr.

    It's also important to note that many tube amps have shorting type speaker jacks which shunt the output voltage to ground if there's no plug inserted. While this might be hard on the output tubes (with an input signal applied), it does protect the output transformer from flyback voltages.
     
  5. John Phillips

    John Phillips Member

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    What Vaughn says.

    In general it's safe to run a tube amp with no load if there's no input signal. But just the other day, I came across an example which makes me very wary of saying it's safe - I was working on an old Vox AC50, and powered it up with no load (as I often do), just to check everything was OK. As it warmed up it suddenly began to make a loud whistling noise - coming directly from the amp itself. I shut it off very quickly - that's a serious self-oscillation producing a lot of power. Luckily, no damage happened, but it could easily have done - basically the amp was running at full power with no load for a second or two.

    This is rare, but it's not the first time it's happened, and I ought to be more careful... the potential consequences are as bad as a fried OT. There's nothing actually 'wrong' with this amp either, BTW - with a speaker connected it runs perfectly. There is a warning label on the back that cautions against operating without a speaker connected too! Quite standard, but in this case it really means it.
     

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