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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. peteb

    peteb Friend of Leo's

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    I thought it was a good idea to move this topic from the conversion thread.


    Hum reduction and safety but how does it work?


    It passes hum to ground?


    If a two prong amp is plugged in with hot and neutral reversed, the chassis is isolated from the hot wire by the death cap.

    Some call it the life cap. I'd call it the ground cap.


    A typical value is .047 mF.

    The chassis is isolated from the hot wire by the cap.

    How much voltage and how much current gets thru?


    Does .047 mF pass or block 60 cycle power? IDK how does one find out?

    How much current gets thru? It gets limited by the cap. How is it found? Use ESR of the CAP at voltage? Is that hard to find?


    my examples pass the wall voltage entirely to the chassis when plugged in wrong. Is that right?


    The cap provides safety by limiting current, does it also limit voltage?
     
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  2. skydog6653

    skydog6653 Tele-Meister

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  3. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    The short version of a long story... The amp has a bunch of stuff 'grounded' to the chassis. That 'ground' was a virtual ground, and actually could float at some voltage above ground, and this could cause hum. You couldn't tie one leg of a two conductor line cord to the chassis to establish an earth ground because you didn't know whether you'd be tying your chassis to neutral or hot.

    What that switch does is connect a cap from the chassis to one leg or the other of the power cord. In theory that cap would couple the chassis to one leg or the other of the power cord. The idea was that it created an a.c. chassis ground that could be flipped from one side of the line to the other, and one relationship would be quieter than the other. Some polarity switches did this, plus add a center-off positon that opened the cap.

    The whacky ground relationship could be so bad that if you were hanging on to your guitar, and your lips would touch a microphone whose amp had a different ground relationship, you'd get shocked. That isn't at all fun. Sometimes no matter what you did something would still bite you.

    It was a cheesy way of doing things, but in the days before three wire line cords the ground polarity switch did help some. The problem is that it relied on the cap being in good shape. If it got leaky, you ended up with a hot chassis condition. But, if you look at any amp schematic you'll see dozens of things that if they fail will give you a hot chassis condition.

    The modern way of doing things is much safer. if all the things you expect to be grounded are grounded, the rig will be quiet and far safer. That's why a good three wire cord, a reliable ground at the power outlet, and a sensibly sized fuse are critical if you want a safe amp.
     
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  4. ebb soul

    ebb soul Poster Extraordinaire

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    Death cap for cutie.
     
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  5. peteb

    peteb Friend of Leo's

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    That video is interesting

    It said the quiet position is with the hot wire connected to the chassis, that surprises me, I'm going to look at mine, that does not seem right.


    Did he ever measure the voltage on the chassis when the plug was in the loud orientation, I don't think he did.


    That's my main question, when the plug or switch is in the loud position, hot and neutral are reversed, will there be AC on the chassis?


    Thanks dsutton for the explanation, do you know the answer to my question?
     
  6. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    Nope.

    Pass.

    How much current is available? Study basic electronics = that's where the answers are to basic calculations.

    Not sure what you said there. But basically, if you don't understand it DO NOT fool with anything that has a 2-prong plug and ground switch (and/or death cap). Not understanding basic electronics and opening a chassis to poke around at these things is a quick way to injure yourself - or die.
     
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  7. Edsel Presley

    Edsel Presley Tele-Holic

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    So that's the reason my old Supro Thunderbolt use to bite me once in a while. Good to know.
     
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  8. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    A good reason why 2-prong cords should be replaced - properly!

    And power sources checked at the wall outlets. I often encounter wall outlets that are wired with hot and neutral reversed.

    Speaking of Supro amps - some of the little ones (along with small Danelectros and similar) have no power transformer. The tubes will have first digits that add up to around 110-115 - meaning the filaments run on full wall voltage - and if the wall power is reversed the metal chassis is HOT!

    Unless these are run with an isolation transformer in place they are potentially little electric chairs. I won't even service them any longer and don't own a single one. Not worth the hassle.
     
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  9. peteb

    peteb Friend of Leo's

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    That's because of what? Because it is two prong, or because it is plugged in wrong?



    I've seen, played, fixed one of those. If they are plugged in right, I believe they can be played safely.

    Why does a transformerless amp have no death cap?
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    No, he didn't. If he did he would have seen wall voltage between the chassis and ground but the .05uF death cap will only pass 2.3 milliamps of AC current so the shock would be mild.

    Copied from the other thread:

    A .05uF 600v death cap has 53k of reactance at 60Hz so a max of 2.36 milliamps of AC current will flow through you if you are perfectly grounded and touch the amp or a guitar grounded to the amp with the ground switch connected to Hot.

    With no death cap the chassis will not have 120v on it with a two prong cord plugged in either way. The chassis will "float." The power transformer primary is not connected to the chassis.

    It's true the DC side of the amp is grounded to the chassis but there is a complete loop from the power transformer secondaries, through the circuit, back to the transformer so with a normally functioning amp there is no shock hazard. But there is no protection from a hot AC or DC wire coming loose and electrifying the chassis to create is a shock hazard. The now mandatory third safety ground wire mitigates this risk by shunting any stray current through the chassis straight to ground.

    The original purpose of the death cap was to provide an AC ground for the chassis for radio frequency shielding which it does when the neutral line is connected to the chassis through the death cap. This is why the ground switch cut's hum with a 2 prong cord.

    The death cap serves no safety purpose. If a short does occur to the chassis (with 2 prong cord) with the death cap selected correctly to the neutral line, the cap will limit the AC flowing through it to neutral to 2.36ma, the rest of the current will flow through you when you ground the chassis with your body. Of course no high voltage DC will flow through the death cap to ground so it will flow through you when you ground the chassis.

    If you put a 3 prong cord with safety ground on an amp with a death cap as long as the safety ground is secure there is no shock danger from the death cap. If the cap shorts out it will blow an amp fuse or circuit breaker. The ground switch with a safety grounded chassis becomes a noise filter switch. When the hot wire is selected the death cap will sink line noise to ground so the ground switch can have a justifiable purpose.

    Widowmaker amps powered directly by the mains do not put wall voltage on the chassis unless there is a circuit malfunction or loose wire. The problem is when there is a malfunction or short you get the full 15 to 20 amps of 120v applied to your body.
     
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  11. clintj

    clintj Friend of Leo's

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    Neutral should be bonded to ground even in old buildings. That's the reference point for zero in two conductor residential wiring, so you have 0 and 110V on the outlets. The death cap and ground switch changes which wire the chassis references ground to - hot or neutral. Align it to neutral, and the chassis should be effectively grounded - not necessarily grounded as far as safety goes though.
     
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  12. peteb

    peteb Friend of Leo's

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    please explain the 53K, where it comes from


    is it the equation for the reactance of a capacitor?


    Xc=1/wc=1/(2)(pie)(freq)(C)=1/(2)(pie)(60)(0.000000047)

    = 1.2X10EXP-11?

    I made a mistake

    = 53K ohms


    thanks
     
    Last edited: Mar 8, 2017
  13. peteb

    peteb Friend of Leo's

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    then theres the question about understanding what frequencies a certain cap passes and which it blocks.


    Look at a tone stack, the treble pass capacitor is really small and passes really high freq and blocks all others. the bass cap, is much larger and passes lower freq, but still blocks even lower freq? then the mid cap passes middle and higher freq, but blocks lower freq like the bass freq. the mid cap is closest in value to the death cap.
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I used the reactance of a capacitor formula and came up with 53k. There are online calculators.

    The death cap isn't being used as an RC or CR filter. The cap's reactance determines the impedance or AC resistance and therefore the AC current through the cap.
     
  15. peteb

    peteb Friend of Leo's

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    Thanks, I just found my mistake in the calc, edited above.

    V=IR

    120VAC = I * 53K

    I = 2.26-2.35 mA, check
     
  16. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    What Rob said re - why no death cap. or simply because it would serve no purpose.

    If they are plugged in with the correct orientation AND the wall outlet is wired correctly AND the are no loose wires or circuit malfunctions (as Rob said) they can be played safely.

    That's a lot of "ands".

    My issue is that these "Widowmakers" are usually made very cheaply, with true point-to-point wiring - no eyelet boards or terminal strips and often fail ylong lengths of exposed (bare) wire and/or rated insulation - very, very often on the power cord itself, most commonly where it enters the chassis. This wear is sometimes just inside where it can't be seen, and I've seen several where bare AC cord wire was touching the chassis.

    That can make the chassis hot depending on which wire is touching - and if the neutral wire is touching with the wall outlet wired incorrectly - it's hot. Incorrectly wired wall outlets are extremely common - I check them every place I play or practice, and about 25% of the outlets in small clubs have been wrong - even more in homes. OF 4 different houses I've owned every one had a few reversed outlets that I had to correct, plus several nonexistent ground - yet 3-prong outlets.

    I learned that decades ago, and it's why I check outlets every single place I plug in. A $6 outlet tester is great life insurance.

    Yes, "WIndowmakers" can be fine as long as all the possible problems are ruled out, but there are many safer small amps out there that sound far better than the bulk of them. Granted, there are some that sound good - but I still would not mess with one without eliminating all problems - and I'd still use isolation transformer as insurance.
     
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  17. peteb

    peteb Friend of Leo's

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    SF, I need to read your post later.


    the death cap limits current to 2-3 mAmps

    100-200 mAmps are lethal

    so isn't the deathcap keeping the current below lethal levels

    Rob,

    I still don't get the current bypassing the cap, going thru the person to ground

    thanks
     
  18. FenderLover

    FenderLover Poster Extraordinaire

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    A 0.047uF cap has a reactance (ohms) of:
    1/(2*pi*f*C)
    At 60 Hz, that's ~56K ohms.
    At 120V RMS that's a little over 2mA RMS
    120V RMS = 170V PK, yielding about 3mA PK.

    A GOOD death cap keeps the current at 'safe' levels IF it is used correctly. It's still a bad idea.

    Regarding "current bypassing the cap" just means the cap has no part of the equation. If one chassis is hot, and another is not, a person becomes the conduit. ZAP!

    Pay your insurance, update your power cables.
     
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  19. peteb

    peteb Friend of Leo's

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    agreed, they did their job in their time.

    agreed
     
  20. peteb

    peteb Friend of Leo's

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    my last question, I think

    the cap has an equivalent resistance of 56K ohms, that's quite a bit,

    wouldn't you expect a voltage drop across that resistance?

    If I make my chassis hot by reversing the plug, there is no voltage drop across the cap, the full wall voltage shows up on the chassis.


    How can that be?
     
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