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The death cap, what's it for, what's it do?

Discussion in 'Amp Tech Center' started by peteb, Mar 7, 2017.

  1. robrob

    robrob Poster Extraordinaire Ad Free Member

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    How could a cap that can only pass 2.4mv of AC and zero DC have saved you from a heavy shock? The death cap provides absolutely no safety but does add the substantial risk of a shorted cap with wall voltage AC on the chassis.

    The light shocks you received from amps with death caps were actually caused by the death cap because it passes AC onto the chassis if the cap is connected to the hot wire instead of the neutral. The death cap didn't save you from heavy shock, it just gave you a light shock. If the amps that gave you a light shock didn't have a death cap you wouldn't have gotten any shock at all.
     
  2. peteb

    peteb Friend of Leo's

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    Rob, you know the answer to that question, you wrote this in you website:

    " The typical .05uF 600v death cap will pass only 2.4 milliamps of AC current so any shock received by touching the chassis or guitar will be mild, but of course in modern times no shock is acceptable."


    We agree what it is and what it does, it's only we look a it differently.





    You're splitting hairs. Too many conditionals in that statement for me.




    The subject is the death cap as used in all fender amps, 1948-1969. I'm talking about the cap that IS there. It serves more than one purpose.
     
    Last edited: Mar 21, 2017
  3. chris m.

    chris m. Poster Extraordinaire

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    I have to agree w Rob. You cant have it both ways. Cap or not, electricity will take the path of least resistance. That might be through you. I can also see how a failing cap would create even greater risk of shock. The old way would never get UL certification, for good reason. There's no cap like this on my toaster, either.
     
  4. peteb

    peteb Friend of Leo's

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    I think that applies more to Rob and you than to me.


    All I am saying is that the death cap includes a safety function, and that safety function appears to be accepted by most, if you plug the two prong amp in backwards, and you ground yourself, the death cap limits current to (thru) you to a sub lethal amount.

    the counter argument:

    It is accepted that the death cap as it was designed to function limits current and shocks to the user if the amp is plugged in backwards and the user is grounded. this is then followed with the statement: the death cap has NO safety purpose.

    To me the two statements taken together, 'limiting current, limiting shocks', and 'the cap has no safety purpose' don't jibe together, its logic that I cant follow.
     
    Last edited: Mar 21, 2017
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    This is what's tripping you up peteb, in an amp with no death cap you can plug it in either way and you are safe because the power cord neutral wire is not connected to the chassis (aptly named widowmaker amps are the exception). The chassis will not have a voltage on it either way. On the other hand an amp with a death cap connected to the hot wire will put 120v on the chassis and it will give you a light shock if you ground the amp or guitar with something like a microphone. The death cap is causing the light shock, not preventing a heavy shock.
     
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  6. peteb

    peteb Friend of Leo's

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    thanks Rob.
    point taken. (although I don't consider it a superior design, it was not widely used as the fender design was)



    Will you accept the point that I'm not talking about amps with completely floating chassis? Im strictly talking about the fender design.
     
  7. peteb

    peteb Friend of Leo's

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    this is where we are viewing the subject from two different perspectives.


    why did the egg cross the road?
     
  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    "Will you accept the point that I'm not talking about amps with completely floating chassis? Im strictly talking about the fender design."

    In the Fender design with a death cap the cap was only there to create an RFI prevention ground, not a safety ground--the death cap does not provide a safety ground because it cannot pass any DC and only 2.4ma of AC, but it does add a light shock and full up electrocution hazard. Think of the Fender design as a floating chassis with an AC signal ground. The chassis does not have an AC power or DC ground at all.

    "The death cap is causing the light shock, not preventing a heavy shock."

    "this is where we are viewing the subject from two different perspectives."


    If there were no death cap there would be no shock at all. It's not differing perspectives, it's differing understanding of the circuit.
     
    Last edited: Mar 21, 2017
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  9. chris m.

    chris m. Poster Extraordinaire

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    Exactly. If the cap can only pass 2.4 ma of AC then in the event the chassis becomes hot the rest of the current passes through your body. Hence my point that the current is going to make its way to ground one way or the other. In this case, through your body. Much better to have a grounded chassis so it can never be hot. If a wire comes loose and makes the chassis hot then the current goes straight to ground through a big fat wire (the ground wire) with zero resistance. And thus no current passes through your body. I'm not an amp tech, barely understand the concept of hot/neutral/ground, but this seems eminently obvious after a few times around the merry go round on this thread.

    It also makes eminent sense that if the cap fails then it has the potential to actually make the chassis totally hot if the two-prong cord is plugged in the wrong way, the polarity switch is flipped the wrong way, or the house wiring has hot and neutral reversed. Absent a ground wire on the chassis then that hot signal passes through your body to ground.
     
  10. peteb

    peteb Friend of Leo's

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    thanks for your reply Rob,






    I don't know, were you there? Fender did not document the function of the death cap., so all we can do is look at what it does.


    I am with you on that. It has shortcomings in the role of the traditional chassis safety ground, blocking DC and only passing small amounts of AC to ground.


    But that is not what I have been saying anyway.


    what the death cap brings to the fender amp in terms of safety, is that it allows the user to plug the amp in either way, get grounded, and not receive a significant shock. You cant say that Fender was not aware of this and did not appreciate this function. I believe 100% that they were well aware of this benefit and its role in the overall design.





    I think we have our misunderstanding completely ironed out. I understand your position and I think you understand mine.
     
  11. peteb

    peteb Friend of Leo's

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    understood,

    if the amp is plugged in backwards, and the cap shorts, and you ground yourself, then you get a big shock.
     
  12. reddesert

    reddesert Tele-Meister

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    I don't agree that the death cap does anything for safety.

    If the amp is functioning properly, then no death cap means the chassis is totally isolated and you don't get a shock. If there's a death cap and and it's not shorted, you might get a mild shock from the 2.4 milliamps, depending on whether the cap is connecting hot to chassis or neutral to chassis. But no danger yet. The cap is so small that it only lets a non-dangerous current through (if it isn't damaged).

    If the amp is not functioning properly and some short puts high DC or AC voltage onto the chassis - this is the key situation. If there is DC on the chassis, it can't short through the death cap because the capacitor will block DC. If there is 60 Hz AC on the chassis, 2.4 milliamps will leak from the chassis through the death cap to the hot or neutral wire. But that isn't enough to blow a fuse. The cap is simply too small to let enough current through. The hazardous voltage stays on the chassis.

    If your body provides a path from chassis to earth, then the high voltage will go through your body rather than the cap. In this case, the cap hasn't done anything bad to you (it's some other fault in the amp that electrocutes you), but it hasn't offered any protection, either.
     
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  13. peteb

    peteb Friend of Leo's

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    red, looks oK,




    Rob,


    Do you accept this:

    The cap is there for RF reduction in the first place.
    The cap introduces a shock hazard.
    the cap also mitigates the shock hazard it introduced by limiting current.
     
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  14. peteb

    peteb Friend of Leo's

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    this is the most objective and complete information about rf suppression caps I find on the web. Its about radios but they say the same applies to guitar amps:

    http://www.justradios.com/safetytips.html


    what they say about the purpose of the caps:

    • Your vintage tube radio needs proper line filter capacitors to sound good and to operate reliably and safely. These important capacitors are called line filter capacitors because they are used to filter-out/remove RF (radio frequency) line disturbance and interference picked up by your home power lines.


    • Line filter capacitors also help to keep you safe from nasty (possible fatal) electrical shocks, by isolating the “hot” side of your 120 volt power line from your radios chassis.
     
  15. peteb

    peteb Friend of Leo's

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    the discussion left off with do the caps cause the shock or prevent it?


    When design elements are added and subtracted one must look how the added or subtracted elements affect other parts/systems, it would be the unresponsible designer that would add an element and not consider grounding/safety considerations.


    This is what I picture happened at Fender, and I could be wrong:

    They decided to add the rf suppression cap, and not go with the completely floating chassis, for amp performance reasons, less noise.

    Now they have added the possibility of charging the chassis to wall voltage. That's now a concern they have to deal with. I would think that cap value was chosen to minimize noise, and maintain function and safety.

    they chose the best overall solution considering safety, function and best overall sound quality.
     
  16. chris m.

    chris m. Poster Extraordinaire

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    peteb- I agree with your three statements except would add a fourth:

    the cap only mitigates the shock hazard it introduced as long as it is working correctly. If it fails it can pass full current. Hence the name "death cap": it can cause your death if it fails.
     
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  17. robrob

    robrob Poster Extraordinaire Ad Free Member

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    chris, I totally agree with this but peteb and I are discussing an amp with a two prong power cord with and without a death cap. As I have said many times I recommend everyone with a two prong cord to install a three prong cord.
     
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  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes I do but like Chris I would add that the cap introduces a severe shock hazard if it fails as a short.

    We have plenty of modern electronics powered by non-polarized two prong power cords that are safe because the manufacturer doesn't try to connect the chassis to neutral to cut hum and noise, they let the chassis float.
     
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  19. scooter_trasher

    scooter_trasher Tele-Meister

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  20. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Thanks scooter for the link.

    The Bassman 100 has the same setup:

    [​IMG]
     
    Last edited: Jul 3, 2017
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