Wiring a Variac for a tube amplifier

Thread Starter


Joined Apr 23, 2015
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!


Joined Jun 22, 2012
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.


Joined Dec 13, 2013
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.

Thread Starter


Joined Apr 23, 2015
I dont know what the other wires are for, is it a socket, can you draw out what you're doing?
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
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.