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Thread: Trying to rig a variac switch to slowing raise the voltage on an old tube amp

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    DIY Senior Member CanOfWorms's Avatar
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    Default Trying to rig a variac switch to slowing raise the voltage on an old tube amp

    One person told me to put a 100w bulb in the series:
    Is this the correct layout?

    Does anyone know if I can just use a three pole dimmer switch.

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  2. #2
    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by CanOfWorms View Post
    One person told me to put a 100w bulb in the series:
    Is this the correct layout?

    Does anyone know if I can just use a three pole dimmer switch.

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    What you got here is not a Variac, and 100 watt light bulb will limit the final power to 100 watts.the output of a light dimmer switch is not intended to feed an electronic device, especially if it has a transformer in it.

    A real Variac can do what you want, but putting too low a voltage into the amplifier may damage (fry) it. Depends, and not the wearable type.
    I am definitely not a pro plumber, but I am a pro crastinator

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    DIY Senior Member CanOfWorms's Avatar
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    Well the pupose of this is to gradually start up an old tube amp that may have not been run in years.
    The idea and (what a few tube repair guys have told me) is that by gradually increasing the voltage the electrolytic capacitors will "reform" instead of blowing.
    The procedure using a variac is to run it for a minute at low voltage gradually increasing to full voltage over 5 minutes.

    There are a few types of dimmers. The one I think I need is the variable voltage one that you might use on a ceiling fan.
    Thank you for explaining what the light bulb would do.

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    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by CanOfWorms View Post
    Well the pupose of this is to gradually start up an old tube amp that may have not been run in years.
    The idea and (what a few tube repair guys have told me) is that by gradually increasing the voltage the electrolytic capacitors will "reform" instead of blowing.
    The procedure using a variac is to run it for a minute at low voltage gradually increasing to full voltage over 5 minutes.

    There are a few types of dimmers. The one I think I need is the variable voltage one that you might use on a ceiling fan.
    Thank you for explaining what the light bulb would do.
    I can certainly understand that you want to baby those electrolytics.
    I am definitely not a pro plumber, but I am a pro crastinator

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    DIY Senior Member CanOfWorms's Avatar
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    Thank you that made me smile, even If I didn't quite catch your drift.

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    DIY scratch-pad engineer leejosepho's Avatar
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    Putting a string of light bulbs (or any other resistance load) in series will reduce the voltage at the end...

    black: ---o---o---o---< put your multimeter leads
    white: ---------------< here to see what you get

    ...and changing bulbs with different wattages will affect the voltage at the end. So, set up three 100-watt bulbs is series and see what voltage you get at the end of the line, then experiment just a bit with more, fewer and/or different bulbs until you can get the voltages you want at the end...then put your old amp in place of the multimeter.
    Last edited by leejosepho; 05-07-2013 at 03:53 AM.
    "Great minds discuss ideas, average minds discuss events and small minds discuss people." --Eleanor Roosevelt

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    DIY Senior Member CanOfWorms's Avatar
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    Quote Originally Posted by leejosepho View Post
    Putting a string of light bulbs (or any other resistance load) in series will reduce the voltage at the end...

    black: ---o---o---o---< put your multimeter leads
    white: ---------------< here to see what you get
    .... until you can get the voltages you want at the end...then put your old amp in place of the multimeter.
    Thanks. I posted my question a few places and you gave the best and most understandable answer.

    Unfortunately and fortunately I did it the way I pictures(without the dimmer) and it worked fine. Since I did it in series I gather the lightbulb did nothing.
    I just need to clean up some frayed old wires and figure out how to properly hook up the speakers.

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    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by CanOfWorms View Post
    Thanks. I posted my question a few places and you gave the best and most understandable answer.

    Unfortunately and fortunately I did it the way I pictures(without the dimmer) and it worked fine. Since I did it in series I gather the lightbulb did nothing.
    I just need to clean up some frayed old wires and figure out how to properly hook up the speakers.
    In any case, I am glad you got your amp powered up. The bulb in series DID do something, it did limit the current to the amp and created a voltage drop across the bulb, which would lower the voltage to the amp by an amount dependant on the resistance of the bulb and of the amp. The resistance of a bulb changes tremendously from being cold to warming up to being hot. You would have had to measure it to see where it wound up, but you don't need to anymore. A 100 Watt light bulb would by itself, limit the max current (when fully heated up and bright) to less than 1 amp (approx 870 ma). Did the bulb light up at all when you tried it out?

    Just curious, as I don't fool around like that anymore, but it still does interest me.
    I am definitely not a pro plumber, but I am a pro crastinator

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    DIY Senior Member CanOfWorms's Avatar
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    Well, the 40 watt and the 60 watt bulb both lit up what seemed fully. I'm not certain, but it did seem that the tubes glowed a little less with the 40 watt bulb than the 60 watt.
    The bulb lit up right away. I started it with the tubes in.

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    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by CanOfWorms View Post
    Well, the 40 watt and the 60 watt bulb both lit up what seemed fully. I'm not certain, but it did seem that the tubes glowed a little less with the 40 watt bulb than the 60 watt.
    The bulb lit up right away. I started it with the tubes in.
    Cool; thanks.
    I am definitely not a pro plumber, but I am a pro crastinator

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by CanOfWorms View Post
    Well, the 40 watt and the 60 watt bulb both lit up what seemed fully. I'm not certain, but it did seem that the tubes glowed a little less with the 40 watt bulb than the 60 watt.
    The bulb lit up right away. I started it with the tubes in.

    You need more than a 100 watt lamp to get much voltage out of the secondary of the transformer.

    A 200 Watt or two 100s in parallel would even be very low voltage.

    I would not run a transformer on a dimmer, it will most likely let some smoke out.


    Have Fun.
    Last edited by DonL; 05-07-2013 at 07:43 PM. Reason: Op error
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    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by DonL View Post
    You need more than a 100 watt lamp to get much voltage out of the secondary of the transformer.

    A 200 Watt or two 100s in parallel would even be very low voltage.

    I would not run a transformer on a dimmer, it will most likely let some smoke out.


    Have Fun.
    yup, kind of what I said in post #2.
    I am definitely not a pro plumber, but I am a pro crastinator

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by CanOfWorms View Post
    Well, the 40 watt and the 60 watt bulb both lit up what seemed fully. I'm not certain, but it did seem that the tubes glowed a little less with the 40 watt bulb than the 60 watt.
    The bulb lit up right away. I started it with the tubes in.
    Your graphic in your first post shows the bulb and amp wired in parallel, not in series. So, the bulb and the amp each got full voltage...and that is why "the 40 watt and the 60 watt bulb both lit up what seemed fully." Glad you got your amp running, but you had not done anything that would have warmed it up any more slowly than usual unless the wiring you used was too small to carry the combined load and worked as a resistance "load" that put some of the energy off as heat.
    Last edited by leejosepho; 05-08-2013 at 05:12 AM.
    "Great minds discuss ideas, average minds discuss events and small minds discuss people." --Eleanor Roosevelt

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    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by leejosepho View Post
    Your graphic in your first post shows the bulb and amp wired in parallel, not in series. So, the bulb and the amp each got full voltage...and that is why "the 40 watt and the 60 watt bulb both lit up what seemed fully." Glad you got your amp running, but you had not done anything that would have warmed it up any more slowly than usual unless the wiring you used was too small to carry the combined load and worked as a resistance "load" that put some of the energy off as heat.
    Actually Lee, they are electrically in series: The power hot goes into (through) the dimmer, then through the bulb, then through the amp, and returns to the Neutral to simplify it. (ignoring the wire colors the OP drew and being 115 volt crcuit). The bulb and the amp are definitely in series. If that were a true Variac (brand name) variable transformer, it would be reducing the original voltage and supplying another voltage to the real load (the bulb and the amp in series, as drawn).
    Last edited by BobL43; 05-08-2013 at 07:07 AM.
    I am definitely not a pro plumber, but I am a pro crastinator

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    Jack of all trades DonL's Avatar
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    They did look in parallel from the drawing.

    If someone is dumb enough to hook them in parallel then they should not be playing with electricity.

    To "reform" a cap is when you take a NOS and go to use it.

    If it is dried out and has been in use for many years, then it needs to be replaced.


    The only way to get her going again without a quick pop, is to add some KY and bring her up slow.
    Theory only works perfect in a vacuum.

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