Wiring a Variac for a tube amplifier

Discussion in 'General Electronics Chat' started by Toddstar71, Nov 24, 2015.

  1. Toddstar71

    Thread Starter New Member

    Apr 23, 2015
    13
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    Hi everyone

    I have a voltage regulator I am trying to use as a variac for a tube amplifier. It is in a chassis (see pic) but its Chinese and the voltage meter was poor so I decided to remove the front cover and wire in a box with plugs, an on off switch and a new meter.

    Problem is, there are three red wires coming from the regulator and I am unsure of what to connect the plugin live leads to. Ground is identified by the chassis but there are three red wires that are unidentified. If someone could guide me how to connect the power plug to the unit I would be greatly appreciative!
    IMG_20151124_105232.jpg
    Thanks!!
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    Its just a large variable wire wound resistor, one wire will be the wiper, the other two will be the ends of the coil, get an ohm meter and connect across any two red wires, then turn the dial fully clockwise or fully anticlockwise, if the meter doesn't alter your across the ends of the coil, if it varies one of the wires is the wiper.
     
  3. Toddstar71

    Thread Starter New Member

    Apr 23, 2015
    13
    1
    Ok I got it! Thank you...so what would the wiper be connected to? The hot black wire?
     
    Last edited: Nov 24, 2015
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I dont know what the other wires are for, is it a socket, can you draw out what you're doing?
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    is there any resistance between the red wires? if there is, turn the knob all the way off, and the two with the leastare one end and the wiper of the variac, the other is the other end. then turn it back all the way up, and the red wires with the least resistance are the other end and wiper. ther you have the wires identified.
     
  6. Toddstar71

    Thread Starter New Member

    Apr 23, 2015
    13
    1
    So here is a diagram - the regulator has 3 red wires coming from it. The top one is the meter, the bottom left one is the neutral and the bottom right is the hot. This I have figured out.

    I need to connect these wires to a fuse holder, lamp and dual socket receptacle as outlined in the picture. I the 120vac put into the regulator and have regulate the amplifier I will plug into the receptacle. I am not sure how to wire these up in a circuit. IMG_20151124_153339(1).jpg
     
  7. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,143
    202
    Look at pic #2 here: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0ahUKEwiZ7MXH-KnJAhVJcz4KHTpbAoEQFggyMAI&url=http://instrumentation-central.com/Staco/PDFCutSheets/VT%20Schematic.pdf&usg=AFQjCNGrfCoMidRYE2Ara3FuP0KfQKADtQ&sig2=4r6nieilUu2jwM1MBoJe7A&cad=rja

    This is likely what you have.

    With Variacs, the WIPER or output, is always fused. Why? The winding current is rated at some value and when you reduce the voltage, the max current is from the original designed value.

    The wiper usually has a carbon brush attached to it. If you get the other ends wrong then CCW will be max output instead of CW.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    i think this is what you have , diagram below...




    variac.png
     
  9. Toddstar71

    Thread Starter New Member

    Apr 23, 2015
    13
    1
    Thats very clear! Thank you.
     
    Dodgydave likes this.
  10. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,143
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    My pic was basically the same, only smaller. Win some, loose some.
     
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