Disconcting the Polarity Switch

Discussion in 'The Workbench' started by stickyfinger, Jul 17, 2019.

  1. stickyfinger

    stickyfinger Active Member

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    The last one I did I completely disconnected all the wiring on the polarity as I repurposed the hole for a PPIMV but this time I want the easiest way and to keep the amp looking stock.

    Can I simply add the 3 pronged plug, clip the death cap from the polarity switch and all is good?
    The polarity will still be able to be switched. Is that a concern?

    Am I missing anything?
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Yes just clip out the death cap and you neutralize the problem.
     
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  3. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Ummm, if you clip the death capacitor then there is no more polarity to switch.
    The polarity switch should do nothing after that.
     
    Last edited: Jul 18, 2019
  4. stickyfinger

    stickyfinger Active Member

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    Polarity is weird. Not sure I understand what the death cap was supposed to do in the first place.

    No voltage on the death cap (switch side) is present in either position. So in what situation will the cap see voltage?

    Another thing I noticed is the mains makes 117v and 127v at different polarity's. 127v being the quietest side with live through the fuse.
     
  5. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    What make and model amplifier?

    Long, long ago most building wiring and appliance wiring had no GROUND/EARTH provisions for the most part.
    So in an attempt to rid more noise from a musical amplifier the user was given a choice for AC grounding chassis to NEUTRAL (through the capacitor/switch circuit) in some amplifiers. The switch was used to find NEUTRAL since it was a two wire supply thingamabob. It had to be one wire or the other.

    So the POLARITY switch is on the AC side of the amplifier power transformer. You have to measure VAC.

    The reason why they call it a death cap is because if the capacitor shorts it could place the AC at chassis. Yes your chassis could be at 110/120VAC or whatever. You no longer need a knife to stick in the wall socket. All you have to do is touch the amplifier chassis or anything connected to that (which could include a guitar).

    upload_2019-7-18_13-31-19.jpeg
     
    Last edited: Jul 18, 2019
  6. netlocal

    netlocal Well-Known Member

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    Ahh... Those were the days... First nightclub show I ever played, rockin' my two prong Kustom amp, step up to the microphone during the opening number and BAM! Got blown backwards. Friends said there was a flash of light.:hbang:

    I learned to always check the polarity. ...To this day, once everything is powered up, I place my hands on the guitar strings and gingerly tap the microphone with my other hand to make sure I won't get shocked. That everything is actually grounded.

    Even in recent years I've carried three-to-two prong adapters in my gig bag, because we'd roll into a venue with old wiring and only find two prong outlets on the walls by the stage. In old buildings even if there's three prong outlets I won't assume they are actually grounded. I always check. I want to keep my lips on my face.

    I wonder if we were safer with two prong outlets and polarity switches?
     
  7. Filipe Soares

    Filipe Soares Well-Known Member

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    today the best route is going wireless... literally it's a life saver.
     
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  8. stickyfinger

    stickyfinger Active Member

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    Gotcha so the death cap is simply snuffing AC noise ?

    And by removing the cap the amp will be safer but potentially be noisier?
     
  9. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Not safer at all. The polarity plug merely was a noise detector for trying to establish that the Neutral wire, (the grounded wire) was connecting to the chassis---- through chassis earthed grounds. That means the chassis was not grounded at all, just at a lower potential if the Neutral was polarity correct.

    The only way to make the amp safer is to install a 3 wire power cord connection, with the green/ grounding wire (not a grounded wire) to a separate grounding point in the amp, not a transformer bolt. The green/grounding wire is the path back to trip your circuit breaker in your electrical panel should you develop a fault current.

    Fault current is what kills you. You die when your body (a resistor) becomes a better path to a grounded source and all of that the fault current flows through your body, instead of all that fault current flowing through that green grounding wire, racing to the breaker to make the breaker trip.
     
    Last edited: Jul 18, 2019
  10. stickyfinger

    stickyfinger Active Member

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    I meant with adding a grounded cord and disconnecting the death cap but leaving the polarity switch working.

    "And by removing the cap the amp will be safer but potentially be noisier?"


    In a modern power transformer wire scheme the neutral isn't grounded. So why is it grounded in the old polarity days? The cap is there to reduce AC noise??
     
  11. RickyLee

    RickyLee Well-Known Member VIP Member

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    The power in to supply your power transformer is just two wires: The Hot and the Neutral wires. That polarity switch with that capacitor cut out will now either reverse the Hot/Neutral order going in to your power transformer primary, or it might not do nothing at all now. Depends on the amp design. That is why some guys were asking for that info.

    Is the mains fuse being fed from the black/Hot or the white/Neutral?
     
  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    If you were to connect a 3-wire ground chord and established ground to chassis while still utilizing the POLARITY circuit and the death cap actually shorted then it would blow the MAINS fuse.

    Once you have a 3-wire ground chord and established ground to chassis the noise follows that ground path so the POLARITY circuit would then be obsolete.
     
  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    He still did not tell us which amplifier.

    By the way here is some knowledge which may twist or confuse some minds.
    Your ground is also connected to the neutral.
     
    Last edited: Jul 18, 2019
  14. stickyfinger

    stickyfinger Active Member

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    Amp is a 72 Marshall. Amp is quietest with voltage passing the fuse.

    Still, I don't really understand why the polarity switch ever needed (or used) a cap to ground in the first place?

    The "death cap" if working properly serves what purpose??
     
  15. RickyLee

    RickyLee Well-Known Member VIP Member

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    Basically as the others mentioned before, it is just a noise or interference shielding trick they used. Just like the shielded cables we have in our guitar amp going from input to the first gain stage or beyond, that help block or kill off noise.

    But remember, not only were the two bladed wall plugs on these old amps without a ground as the guys mentioned, but the blades were the same size for the longest time. That means you could reverse the orientation of how it was plugged into the wall, therefore reversing the polarity.

    Now when you look at a wall receptacle, you see three terminals. Besides the ground, the Hot is the smaller blade and the Neutral is the larger blade. You cannot reverse the polarity on a cord/plug with these type blades. BUT . . . . you could still plug an old school amp or an old lamp ect. with the old 2 bladed plugs either way/reverse polarity.

    It is AC. Alternating current. Now DC, that is a whole different story and you have to abide by the polarity rules.

    :D
     
  16. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Here you go. Marshall was terrible at schematics and pretty much did not show the POLARITY switch on most all diagrams. Even in this schematic it is drawn wrong. The "E" represents earth connection but in Marshall's case it is just the connection to chassis ground which is actually the thick black line from left to right in the middle of the page.

    I eluded to and Ricky pointed out that we had two cords/plug prongs both of which were the same so LIVE and NEUTRAL could have been on either side.

    Notice the capacitor is in series with the switch. The circuit goes from AC wire to switch through the capacitor and then to chassis ground which kind of looks like this.

    [​IMG]
    [​IMG]


    Grounds should look more like this.

    [​IMG]
     
    Last edited: Jul 19, 2019
  17. stickyfinger

    stickyfinger Active Member

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    Its hard to say what schematic out there was the official Marshall schematic.

    USA electrical standards were way behind in modern standards. This schematic represents the UK (and others) with a 3 prong plug with ground. The AC still had a death cap.
     
  18. stickyfinger

    stickyfinger Active Member

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    Even with the polarity switch still hooked up? You can still flip the switch? Right? Polarity should change.

    I ask because I've only ever completely removed the switch. I would assume it does no harm with the amp in accidental reverse polarity and no (ac noise snuffer) removed?? This is my main concern.
     
  19. RickyLee

    RickyLee Well-Known Member VIP Member

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    It should not matter on that old power transformer which way you orientate the primary in regards to hot/neutral. And just make sure you have the fuse on the hot side. I just remove all that old setup, switch too.
     
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  20. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    The ground is only connected at one point ever to the neutral, and that is in your service panel. It is designed to pull the neutral down to the ground potential of the structure, made either by grounding rod or grounding ring, and also bonded to/if metal plumbing pipes are used.

    The only common use of any utilization equipment now days (table lamps excluded) comes as either a three wire grounded cord, or the device is in general, insulated to prevent chance of shock.

    And yes it used to be common practice to wire stuff with a two prong cord, same blade size cord. Sadly changes to the national electrical code are only generally made when there are recognized large losses of property or life.
     
    Last edited: Jul 19, 2019
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