Have I done permanent damage to my plexi?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jezzzz2003, Jan 20, 2006.


  1. jezzzz2003

    jezzzz2003 Member

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    Hi
    I was doing a show last night.
    Went to turn on, struck a chord and ..hey.... no sound??
    Check all connections.. seems cool,
    Check my speaker cable...
    Idiot.. I forgot to plug my cabinet in. :jo
    Took me about 15 seconds to find that out.
    I am just wanting to know if these stupid mishaps do cause damage to the transformer or if its just a case of blown or not blown with these components.
    Can it weaken components??
    Thanks for listening.
     
  2. Scott_F

    Scott_F Gold Supporting Member

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    Most qualtiy transformers can handle this stuff in small doses. It's the cheap crappy ones that I worry about.
     
  3. John Phillips

    John Phillips Member

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    It could in theory 'weaken' a transformer if an arc occured and broke through the insulation but didn't cause a permanent short, but if it did that the HT fuse would almost certainly have blown (assuming you DO have the correct value in). If the amp appeared to work perfectly afterwards, chances are you've done no damage.

    A lot of people do this and get away with it, especially if the amp wasn't really cranked.
     
  4. michael patrick

    michael patrick Supporting Member

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    If I had a nickel for everytime I'd done that, I'd be able to afford a nice NOS Mullard ECC83.... Like John said if you didn't blow a fuse and it seems to work properly you are probably ok.
     
  5. billdurham

    billdurham Member

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    I find it kinda strange that Fender uses a shorting jack on the output, meaning that with no plug in the jack, the output transformer is shorted to ground. Seems to me that this would just about ensure that you smoke the transformer if you didn't have a speaker plugged in. I know that they are tough enough to take the occasional occurance of not connecting, but in the bigger picture, why would they do that?...if having the transformer into an open circuit wouldn't cause the current draw that it being shorted to ground would? Just curious

    BD
     
  6. John Phillips

    John Phillips Member

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    It's the exact opposite - the shorting jack is to protect the transformer. (It's rather hard on the tubes, but they are considered expendable compared to the transformer.) The danger to the transformer comes from internal arcing caused by the very high voltages developed when there is no load - the power from the tubes has to go somewhere, and with no current able to be drawn if the output circuit is disconnected, a very high voltage is created instead - power = voltage x current, so if current = 0 then voltage = infinite, theoretically. Obviously the voltage isn't infinite in practice but it can be several times what it normally should be. If you're unlucky it will arc through the wiring insulation in the transformer and burn it out; if you're more lucky it will arc in a tube (possibly not causing any serious harm). If you don't have the amp cranked up, the voltage may not be enough to cause trouble at all.

    If you short the output, you do increase the current, although the efficiency is so much reduced because the tubes aren't seeing the right load, that it may not rise all that much. It's certainly not good for the tubes, but at least it keeps the voltages low. If you keep it up long enough and the tubes don't fail it may cause the transformer to overheat and melt the insulation instead, so it's not a perfect protection - but the danger from an open-circuit is more instantaneous, so the lesser of the two evils is to short it.

    Fender used the shorting method, and Marshall didn't. This may be part of the reason why Marshalls seem to suffer from blown OTs a lot more than Fenders.
     
  7. hunter

    hunter Supporting Member

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    Since this topic comes up now and then I thought I'd bring up the follow-up question.

    When this happens is it better to turn the amp to either Standby or Off and connect the speaker or is it better to just reconnect the speaker with the amp On as is?

    FWIW I read a Gerald Weber article that suggested it was better to leave the amp On and plug in the speaker since switching to Standby/Off with the speaker unplugged subjected the amp to all of the most damaging bad flyback voltages.

    I guess I'd follow Webers advice if I had the occasion to need to but I thought I'd see if anyone else has dealt with this question.

    hunter
     

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