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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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    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.
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    DIY Junior Member John in herndon's Avatar
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    Default Using a Variac® to gradually raise the voltage in a tube amp

    I used to be in the audio repair business and used this technique all the time on both tube and solid state amps. It prevents doing a lot of damage if you didn't find all the blown components.

    That said, you have to use a VARIAC® or variable autoformer to do this because this preserves the sine wave of the AC power. Solid state dimmers do not do this. They use SCRs and TRIACs to vary the voltage by delaying the turn on point of each individual cycle of the AC waveform. The result is a non sinusoidal waveform which would not work to power an amplifier or other electronic device.

    If it doesn't weigh 10 pounds or more it is not a transformer and won't work.

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by John in herndon View Post
    I used to be in the audio repair business and used this technique all the time on both tube and solid state amps. It prevents doing a lot of damage if you didn't find all the blown components.

    That said, you have to use a VARIAC® or variable autoformer to do this because this preserves the sine wave of the AC power. Solid state dimmers do not do this. They use SCRs and TRIACs to vary the voltage by delaying the turn on point of each individual cycle of the AC waveform. The result is a non sinusoidal waveform which would not work to power an amplifier or other electronic device.

    If it doesn't weigh 10 pounds or more it is not a transformer and won't work.


    You can use a variable high power resistor also, If you do not need isolation from the mains using a transformer.

    I never have seen a DC transformer that lasted very long. They do not like chopped DC, unless they are designed for it.

    I use a light bulb in series with the AC line and it gives a good indication of your situation.


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    Sprinkler Guy Wet_Boots's Avatar
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    Besides seconding the "use a genuine variable transformer" advice, there is also a caution for some circuitry, where it can actually be advisable to construct a replacement for a rectifier tube, using solid-state components and a tube-base connector. I don't have more details at my fingertips, but I know the idea behind the temporary replacement was that the initial low voltage applied in a series of several steps was not going to be enough to make a rectifier tube functional, and the solid-state replacement would function with any input voltage applied to the tube device's power transformer.

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