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Thread: Dummy coil experiment... A Pot for testing.

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    Dummy coil experiment... A Pot for testing.

    Hi!

    I'm having a problem just testing a few old coils to see which is good for a dummy.

    I'm also winding an antenna coil.

    I wanted a pot involved so I can match the hum cancel from various coils - I used on with the SC ground/earth to the wiper, along with the dummy hot (and on through the coil to ground).
    ... then an earth connection at one end/lug so I can bypass to ground and take the coil out of circuit gradually...

    But there's so much noise I can't really hear much going on yet. Is that the best way to wire a pot for this or have I missed something?

    I just want to eliminate this quickly or I would just test like crazy... the positioning of the dummy, coil orientation, coil/hum sensing mismatch, ambient hum, guitar orientation... so many variables it's confusing me right now as I can't just hear a drop in hum at any setting.

    Thx in advance!

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    Please post a circuit diagram.

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    It sounds like the other end of your dummy coil isn't connected to ground. You may have better luck with a dual ganged blend pot here if your dummy coil doesn't have as many turns as the pickup coil. Start by just connecting the two coils in parallel and switching the dummy's polarity until you're sure you have the windings going in opposite directions then add a volume pot to the dummy for fine tuning.

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    OK, schemo:

    Click image for larger version. 

Name:	Dummy Coil Schemo.png 
Views:	374 
Size:	28.8 KB 
ID:	39054

    Please excuse the playroom quality, my tablet is being very weird...
    I'm not an EE, but I figured this would bleed progressively more to ground, bypassing the dummy as the wiper approached the grounded terminal.
    I commandeered an existing tone pot for this as it was just convenient, im sure a much smaller pot would be right.

    OK, David...
    The coil was defo grounded.

    I have no dual-gangs, but I can cobble something together for testing... maybe two trimmers?
    The coils should be correct relative polarity now... I experimented and wired them so they humbucked, but the config lost half the volume the volume of the sensing pickup.

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    Last edited by Plectrum; 05-09-2016 at 04:56 PM.

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    Your volume was halved when the dummy was added in? That seems rather dramatic and not what I would have expected. Also I'm not sure your circuit is doing what you think it's doing. I'm not sure it's doing much of anything. You want to connect the pot as a voltage divider.

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    OK, my amateur thinking was:

    The SC ground was joined to the dummy "in"... With no pot, the two coils would be in series, like a regular HB. Add the Pot - When the pot wiper was at the "top" in the diagram, there would be 500kΩ between the SC "out" and the ground, so the path would still be through the dummy. When the pot was opened up the signal would bypass the dummy and go straight to ground...

    The "halved output" scenario was actually another wiring which I have since changed... not the one in the digram.

    Thanks for being patient! I know I'm a hack.

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    Quote Originally Posted by Plectrum View Post
    OK, schemo:

    Click image for larger version. 

Name:	Dummy Coil Schemo.png 
Views:	374 
Size:	28.8 KB 
ID:	39054

    Please excuse the playroom quality, my tablet is being very weird...
    I'm not an EE, but I figured this would bleed progressively more to ground, bypassing the dummy as the wiper approached the grounded terminal.
    I commandeered an existing tone pot for this as it was just convenient, im sure a much smaller pot would be right.
    I see the problem. I would do it differently: This assumes that the dummy coil is a bit hotter than the active coil.

    Put a 50 Kohm pot across the dummy coil. Connect one end of the 50K pot to one end of the active coil and the pot wiper to the output, the other output wire being the other end of the active coil. IF things are phased correctly, there will be a pot setting where full cancellation happens. If not phased correctly, there will be no point at which hum cancellation happens, so reverse the active coil.

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  8. #8
    Supporting Member Steve A.'s Avatar
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    You might look into the Chiliachki patent for ideas. Rather than trying to match the sensing coil he uses a thicker wire for the dummy coil which has a much larger circumference than the sensing coil. For a 7,000k ohm strat with 42GA wire you might have a dummy coil with 38GA wire measuring 300 ohms.

    http://www.blueguitar.org/new/misc/p...chiliachki.pdf



    Suhr puts a smaller dummy coil on the strat spring cover while Ilitch uses a larger one which requires a rout under the pickguard.

    Steve Ahola

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