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if a filter cap shorts, does that put high voltage DC on the chassis?

Discussion in 'Amp Tech Center' started by peteb, Jan 12, 2018 at 2:11 PM.

  1. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    I think it would

    this is just a question, not something that happened to me.




    that would be a short to the chassis.


    shorts to the chassis I thought were rare, but filter caps could basically fail at any time, new or old, old is more likely of course.
     

  2. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Usually the end result is an exploded cap, and possible damaged components up to and including a fried PT.

    If the reservoir cap shorts, the available current is limited by the capacity of the rectifier (tube or diode) and HT winding capacity (ampacity). Either the mains fuse will blow, or one of those components will quit working.

    A later cap that shorts becomes limited by the dropping resistors, and depending on the value/power rating etc one may burn up and open, stopping the fault.

    There's two safety features every amp should have these days that will keep the guitarist safe. The shorted cap may draw enough current to trip the mains fuse (and/or B+ fuse if equipped), and with a proper grounded plug the ground conductor can easily handle the fault current, preventing the chassis from building up a voltage.
     
    peteb likes this.

  3. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Short answer, No.

    A shorting cap can disable an amp without catastrophic failure and without a hot chassis.

    Long answer, the resistance of a failing component (including the dielectric integrity of a capacitor) will always drop the applied voltage across it as current rises, up to the instance of failure. So no, the chassis is always ground if the safety ground is connected. That's the purpose of the safety ground.

    The caveat is Yes, in that it is the mechanism that happens at ultimate failure, but it is instantaneous. This may be the answer you are looking for, even though the chassis is still always at ground potential.
     
    Last edited: Jan 13, 2018 at 9:46 AM
    peteb likes this.

  4. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Great answers both, thanks.



    The subject is plausible amp catastrophes. Ones that are known to happen.


    You got the tube failure which often results in excessive current draw, fuse failure, and the need for new tube(s) and a fuse. No tube amp is immune from this.


    Then you have the shorted filter cap. No amp is immune from this either, but fresh caps will mostly prevent this. The shorted filter cap will provide a direct path from the high voltage DC supply to the chassis. This has to be one of the very worst things that can happen to an amp.




    I was looking for a general answer. I think you guys have answered it well.





    Now consider a stock and mostly original fender amp from the 60s.



    (The filter caps have been renewed on 3 of my 4 amps. The 4th I consider as a test case)






    Another tdpri member just had a shorted filter cap, it happens.



    A two prong fender with 50 year old filter caps is one shorted high voltage cap from a scenario in which 400 VDC could suddenly appear on the guitar strings. If the player provides a path to ground, the amps 1amp or one half amp fuse could be out of the circuit (on the neutral line) and the 15 amp circuit breaker is the lone remaining safety feature at this point.





    The 3 prong grounded cord, fresh filter caps, and even moving the fuse to the hot line could could all be life savers.






    This could be the educatin that forces my hand to do some long overdue upgrades.
     
    Last edited: Jan 13, 2018 at 12:47 PM

  5. BobbyZ

    BobbyZ Doctor of Teleocity

    Jan 12, 2011
    Snellman MN
    The fuse is still in circuit regardless of how you plug in an old two pronger cord. With any luck it'll blow instead of saving itself while the power transformer does it's imitation of a fuse. That's slower, smokey and stinky.
     
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  6. dsutton24

    dsutton24 Poster Extraordinaire Gold Supporter

    Dec 29, 2010
    Illinois
    There are shorts, and there are shorts. If you get something close to a zero ohm short, and there aren't any other paths to ground, the fuse will blow and everybody survives. Worst case scenario is a two wire line cord, and a cap that is shorted to the degree that significant current could flow to the chassis but doesn't blow the fuse. You then have a live chassis and the potential for death or injury.

    There are any number of things in an amp that can create this situation, including tubes or a leaky Death Cap™. This is why three conductor cords are essential if you don't want to kill yourself or anyone you care about. A properly installed three wire cord plugged into a properly wired three conductor outlet eliminates this danger. With a three wire cord plugged into a good three prong outlet, the chassis is always going to be grounded, and therefore, safe.
     
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  7. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    You aren't going to see DC on the chassis or strings. Voltage potentials are developed with respect to ground. If you short the B+ to ground, you have zero dc volts on the chassis.
     
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  8. Dacious

    Dacious Friend of Leo's

    Mar 16, 2003
    Godzone
    The general answer is if you walk outside an airliner could fall on your head. It's more likely you spill a fluid on a live point just as you're plugging in....

    It the cap earth's, that is where the current goes.

    Yes you should put fuses on a HT leg of your transformer. More to save it. 3 wire cord is easy safety measure.

    People who've been killed by amps is nearly the result of bad house or power point wiring.
     
    Last edited: Jan 13, 2018 at 6:55 PM
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  9. Hobs

    Hobs TDPRI Member

    18
    Sep 16, 2011
    Nashville, TN
    RLee77 has it right. A shorted filter cap will not put HV DC on the chassis, because the DC potential ground is referenced to that same point. There may be some oddball amp in which that is not true, but that's the case with all of the common designs. Shocks from amps occur because of line voltage AC being present on the chassis.
     
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  10. Hobs

    Hobs TDPRI Member

    18
    Sep 16, 2011
    Nashville, TN
    As far as fuses: a fuse provides some protection against fire, and some protection against catastrophic damage to equipment. Fuses do not protect against shock. 20mA can kill you. Even a 1/2A fuse will not even begin to blow at that level.
     
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  11. dsutton24

    dsutton24 Poster Extraordinaire Gold Supporter

    Dec 29, 2010
    Illinois
    Not true. Those voltages are referenced to the chassis. if the chassis is floating, you most certainly can have a hot chassis situation. Witness the lip biting amps of yesteryear, or the thousands of transformerless two wire electronic devices built from the forties through the sixties. Or, for that matter the bad stuff that happens when outlets get wired wrong.

    I had an old National HRO radio that had a leaky bypass cap that did exactly what I described above.
     
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  12. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Yes true. I'm referring to the OP question, which was about DC, not AC. And if a chassis is floating, all bets are off. The assumption was for normal amps.
     
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  13. 24 track

    24 track Poster Extraordinaire Silver Supporter

    Nov 6, 2014
    kamloops bc
    isnt that the premixe to the old widow maker amps of the 60's , no power transformers etc like my pepco?

    P1011571.JPG P1011566.JPG
     
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  14. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Thanks for all of the excellent replies.



    Gerald weber wrote, if a filter cap shorts, the fuse blows.


    This is a general answer for vintage amps. G weber does not discern 2 prong vs 3 prong, and does not discriminate against 2 prong. He also does not say this is dependent upon the fuse being on the hot leg.
     

  15. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    More


    I agree with the assertion that the fuse will blow on either leg



    Because



    Nothing of significance good or bad is going to come out of the other end of the PT unless both legs of the primary are in service.
     
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