OT damage ??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hamfist, Jan 8, 2008.


  1. hamfist

    hamfist Member

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    Well, basically I'm a doofus ! :crazy At regular intervals I accidentally keep switching on my tube heads with no speaker load. I seem to do this for all sorts of bizarre and varied reasons. I guess I'm probably not alone in this !

    Today, I was attaching a new Hotplate (I haven't used these before), and after a number of attempts at switching on, and getting silence, I finally realised that I had my cab attached into the Hotplate's "line out" rather than the "speaker out". This would have have definately been a "no load" situation for the amp. Each time it probably lasted no more than 10 seconds and the volumes were low.
    My question is this - As the amp seems to be fine, have I "got away with it" this time, or do lots of little cumulative bits of "no load" do long term, cumulative damage to the amp (OT, I'm expecting) ??

    Experts please apply wisdom here ......
     
  2. PRNDL

    PRNDL Member

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    The risk of no load on a tube amp is high current. The tubes usually start sparking. Transformer damage usually occurs when the current oscillates and "runs wild", which doesn't always occur. It takes a fairly large current and time to burn a transformer, so your probably OK, although your tubes are taking a beating.
     
  3. Nolatone Ampworks

    Nolatone Ampworks Member

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    Something I'm wondering:

    Are there physical indicators of a burned out transformer? How do you check. Just measure resistence across the windings? Shouldn't read open or short. Anything else?
     
  4. John Phillips

    John Phillips Member

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    No, it's the exact opposite. With no load, no current can flow in the secondary circuit, so the energy in the transformer's magnetic field is turned into a high voltage instead, which can arc through the winding insulation.

    Sometimes, but not always. If they do, there's a reasonable (but not certain) chance that they'll protect the OT by absorbing the voltage spikes.

    That's definitely true if the amp doesn't actually have a signal going through it - damage will only occur if the amp self-oscillates. Most amps aren't actually unstable with no load, so there's usually only a risk if you were actually playing through it, and at substantial power, too - running an amp at low power will not damage it even with no load.

    Not at all - if the amp is already producing high power and the load suddenly goes open (ie if the speaker cable gets pulled out or something like that) the damage can occur in a fraction of a second. It's the initial voltage spike as the transformer's magnetic field collapses and the stored energy has to go somewhere that is the biggest risk. (This is exactly how a car ignition circuit works BTW :).)

    If the amp is self-oscillating, definitely.

    If it reads open or short, it's definitely dead. But they don't normally do that... the normal failure is a partial short on the primary side. This can be difficult to tell by measurement because the difference between the correct resistance and the damaged reading can be quite small - even a single shorted turn is enough to stop the transformer working properly. The two sides of the primary are normally slightly different resistances anyway, so comparing the two doesn't always help much either.

    Really the only ways to be completely sure are to temporarily patch in a new one, or to calculate the power input to the transformer (from the voltage swing across the power tube plates and the average tube current) and measure the power output. If there's a drastic difference - a healthy transformer should be about 90% efficient - the transformer is dead.
     
  5. hamfist

    hamfist Member

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    John, I know you're the "main man" around these parts, and thanks for stopping in on my thread. But, could you have a quick peek at my initial posting. I really would like to know if I'm damaging my transformer. Tubes, I don't care, they can be replaced cheap, but transformers - not so easy.
    My apologies if you answered my question in your post, I just didn't really understand a lot of it, to be honest. :jo

    If it helps at all, what I'm generally doing is a bit of light strumming at lowish volumes each time I turn on with no load ..... yes I know, I'm a doofus !!
     
  6. John Phillips

    John Phillips Member

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    It's kind of 'hidden' in all that stuff I wrote... :)

    If the amp is not unstable into an open circuit, and there was no significant signal going through the amp, there is no danger.

    It's unlikely that any 'cumulative' damage has occured. It's possible that the winding insulation could be partly damaged but not make a permanent short, but unless the amp was putting out a lot of power this cannot have happened either. There needs to be enough power being developed to create a high voltage in the OT.


    It's not a Mesa Dual Rectifier or Trem-o-verb though is it? ;)

    (These are unstable with no load.)

    Anyway, try not to do it. While probably nothing bad has happened, it really isn't a good idea :p.
     
  7. hamfist

    hamfist Member

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    Thanks John.

    BTW, it's with a brand new AC30HH. I really, really don't want to knacker that one !

    BTW2, Just a tip for others - I've now put tape over the Hotplate's "Line out" to make sure I don't accidentally plug in my cab there in future.
     

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