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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

    Dec 29, 2012
    United States
    peteb, In an amp with a two conductor power cord and no death cap the amplifier chassis is never connected to the hot wire. Neither the hot nor neutral wire is connected to the chassi so the chassis is never hot (unless there is a malfunction).

    So no death cap is safer than having a death cap installed. The death cap never adds to the safety of an amplifier. It is only a hum reduction device.
     

  2. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    Thanks Rob,

    I see where we are having a misunderstanding.


    In comparing no cap, to added cap.

    Side note: I read an old reciept I had from when an audio tech that did a checkup on my bassman head, he recommended going grounded wire and removing the 'ground reference cap'. I now consider it the ground cap.


    Back to cap vs no cap and which is safer.





    Your talking about removing the cap and the connection between the incoming power and the chassis. I never thought of it that way at all. Would that be a floating ground? I don't think anyone ever did it that way.

    When I talk about removing the cap, I mean to remove the cap but retain the electrical connection between the incoming power wire (one side or the other) and the chassis for ground reference purposes, and grounding in general. 2 prong amps were built this way, the power transformerless ones are the ones I know of, which are dangerous, ok if plugged in right , but very dangerous if plugged in reverse.


    I would say that in either case the ground cap is advantageous.



    My opinion this statement is false.


    I would rather play a stock condition 2 prong blackface fender amp with ground reference cap, than an amp with a completely floating ground, or an amp with one incoming power wire connected directly to the chassis. Edit:For safety reasons.



    Let's wrap this thread up. Let's emphasize what we can agree on instead of What we disagree on,
     
    Last edited: Mar 19, 2017

  3. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Show me some schematics or layouts where the power cord hot or neutral, or power transformer primary wires are connected to the chassis. This is the root of our disagreement. I've yet to see one but maybe I haven't looked hard enough.
     

  4. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    image.jpeg Thanks for reply Rob,

    Search 'all American five radio schematic" there is a lot of them, the first one that came up is here:


    But either way, would you feel comfortable playing an amp with a completely floating chassis?I'll play my amps plugged in reverse before I'd touch an amp with a completely floating chassis.

    None exist do they?
     

  5. dsutton24

    dsutton24 Poster Extraordinaire Gold Supporter

    Dec 29, 2010
    Illinois
    There's a world of difference between a guitar amp and a 5 tube radio. Those radios had plastic housings and knobs, there wasn't any way that you could come in contact with the chassis of an intact radio. Hot chassis were common in oscilloscopes, transmitters, and televisions. A guitar amp has a connection to the outside world, so safety was at least a modest concern. I don't remember ever running across a guitar amp that has one side of a two conductor line cord bonded to the chassis. I've looked at the likely suspects, Airline, Silvertone, Harmony, and the like and didn't find one.

    It's important to remember, though, that the internet isn't the Universal Arbiter of Truth. There are plenty of things that happened in the past that a Google search won't turn up.
     
    peteb likes this.

  6. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Yes, that widowmaker amp will electrify the chassis if it is plugged in backwards.

    A death cap only provides a small signal AC ground to prevent hum. The chassis is floating except for small AC signals. A death cap does not provide a high voltage AC or DC safety ground. With a death cap connected to the hot or neutral wire and a loose AC power or DC high voltage wire makes contact with the chassis the shock hazard is not reduced by the cap. When a player touches the chassis or guitar he will be shocked when he also touches a ground.

    Here are non-widowmaker amps with floating chassis. I found these by searching for "Gibson amplifier schematics" (it seems all old Fender amps always used a death cap). These are safer than amps with death caps because you don't have to worry about a shorting death cap to electrify the chassis:

    Gibson GA6

    [​IMG]


    [​IMG]

    [​IMG]

    [​IMG]

    peteb, I'm sorry if I'm coming across as combative but this is a good back-and-forth that's really fleshing out ground switch and death cap theory.
     
    Last edited: Mar 19, 2017
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  7. dsutton24

    dsutton24 Poster Extraordinaire Gold Supporter

    Dec 29, 2010
    Illinois
    This is true as far as it goes, but there are still plenty of hazards. For example leaky filter cap that isn't yet bad enough to blow fuses can result in a hot chassis condition.
     

  8. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Yes, I totally agree and strongly recommend anyone with a two conductor power cord on their amp to upgrade to a three conductor with a dedicated chassis safety ground.

    peteb and I are discussing which is safer in an amp with a two conductor power cord, an amp with a death cap or without.
     
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  9. dsutton24

    dsutton24 Poster Extraordinaire Gold Supporter

    Dec 29, 2010
    Illinois
    I know what the discussion was about. The 'death cap' thing gets a lot of attention, but electrolytics are far more likely to become leaky over time and cause an issue. That's important for everyone to understand, in my opinion, anyway.

    I won't intrude further.
     

  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    dsutton24 you're not intruding, I appreciate your input.
     

  11. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    Thanks for the circuit example and discussion.
     

  12. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    I cant find a guitar amp with the chassis connected direct to the power wire either. I thought I had seen it before, but not sure. searching transformerless tube guitar amps doesn't come up with much.


    are those floating chassis amps safe? I wouldn't think so.

    I am surpised to see the reverb unit with a floating chassis. how is that OK by fender standards? because you don't touch it or play it, kind of like the radio.
     

  13. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Modern standards do not allow non-grounded metal chassis because we have three conductor power cords with the safety ground wire. Before three wire cords most metal chassis were floating, non-grounded because you couldn't be sure which wire would carry the hot and which one the neutral. Two prong polarized plugs (one narrow, one wide) were better than nothing but it's common for wall plugs to be wired backwards so it wasn't safe to tie the chassis to the neutral wire.

    I don't know why Fender didn't use a death cap for RFI reduction in the reverb units, you would think they would benefit more than amplifiers. Maybe they were worried about creating a ground loop issue with the reverb plugged into a different wall socket than the amp.
     
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  14. chris m.

    chris m. Friend of Leo's

    I remember getting bit on the lips by a microphone many times back in the days of two-prong outlets. If you switched the polarity then you didn't get shocked anymore.

    Here's a question. I have a wall outlet tester. What happens if I get a reading that says the outlet isn't wired properly? I'm at a gig, need to play, can't find any other outlets. It's not like they're going to re-wire it for me. Am I risking death by electric shock when I go ahead and plug in anyway? I think back on all the gigs I've played and somehow didn't die....and that was back in the days of two prong cords.
     

  15. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    thanks Rob, good info.

    out of the three methods to wire a two prong amp which is safest? And which would you feel the safest playing a guitar thru?

    I would take the fender style with the ground reference cap over the other two. for obvious reasons the power wire attached directly to the chassis is out, not sure if it was ever done this way.

    The floating chassis with no ground connection to the chassis at all is a bad deal. I would feel uncomfortable playing on of those. if charge were to build up on the chassis there is no path to ground.


    this is my opinion and this is why I see safety value in the ground reference cap in the two prong situation. I would feel less safe without it.
     

  16. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    there are a lot of two prong outlets out there and probably a lot of three prong amps being plugged into them, which wont be grounded. The question isn't just one for nostalgia sake.
     

  17. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Could always run a clip on lead from the chassis to a grounded object, or get one of the three-to-two prong adapters that attaches to the outlet cover plate screw.

    As far as a stage outlet goes, I absolutely would cancel a gig over that, and tell the owner exactly why. My health and well-being are not worth gig money.
     

  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    The "lip bite" was caused by the death cap allowing 115v at 2 milliamps to flow onto the chassis.

    As long as the outlet's ground wire is correct and you're using a three prong power cord you are safe. If the ground is missing then I'd probably still perform but I'd just be very wary of touching anything metal while wearing the guitar--unless I'm running the guitar wireless, then of course it'd be safe. If a wire comes loose in the amp the chassis could become hot and kill you if your body grounds the hot chassis.
     
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  19. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    With a two prong power cord I would take a completely floating chassis over one with a death cap. The death cap cannot send more than 2 milliamps of 125v to ground and it cannot pass any DC to ground so the death cap is no help when you have a loose hot wire in the chassis, but the death cap is a hazard on its own because if it fails as a short it can pass 125v at 20 amps onto the chassis. The death cap is simply a small AC signal ground to prevent hum, it serves no safety purpose.
     
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  20. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    Thanks, that's a good and accurate answer except for the no safety purpose part.

    The cap saved me and many others from heavy shocks.

    That's safety, no matter how you want to call it.


    That's all I got.
     

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