Variacs and Their Uses


What is a Variac? As the name suggests, it varies AC voltage. The descriptive term would be "variable AC transformer," but the trade name Variac was used by the General Radio company for many years, and eventually that became the generic name.

Here are four variacs on my workbench:

We'll look at each of them later in this article.

Why Use a Variac?

The line voltage in American homes is a nominal 120 volts AC, but a variac lets you adjust that voltage up or down (usually, down) for various purposes.

Use 1: Taking Accurate Voltage Measurements

If you restore vintage radios or TVs, you'll notice that schematic diagrams usually contain voltage readings. Often the key voltages are shown in the schematic itself, and some service manuals include a chart showing the correct voltage for every pin of every tube. The readings specify the voltage you'd expect to measure at various spots in the radio or TV when it is working normally.

For example, this schematic for a 1957 RCA television shows that you should measure about 200 volts DC at pin 5 of V7, the 6DT6 audio detector tube:

When diagnosing problems, it's useful to compare those ideal voltages to what you actually measure in your radio or TV chassis. If the schematic specifies 200 volts DC and your radio only produces 35 volts at that spot, something is wrong.

To optimize accuracy, it's desirable to set your line voltage to the same level as the guys who made all of those old measurements. Otherwise, you'll be comparing apples to oranges. Often, the schematic tells you to maintain the line voltage at 117 volts:

117 was standard when these old sets were made, but nowadays 120 volts is the norm, and your actual house voltage may differ, depending on variables such as how far you live from the power sub-station.

To test voltages accurately, you set the variac's output voltage to 117 VAC and then plug your radio or TV into the variac. This replicates the environment in which those old readings were taken.

Use 2: Gradual Power-Up

You can also use a variac when trying out an unrestored radio or TV for the first time, when its operating condition is unknown. The idea is to set the variac at a low level—say, 50 volts—and then gradually increase the voltage, carefully watching your set for any signs of trouble. This is safer than simply plugging your set into the wall and turning it on, which subjects everything to a sudden power surge.

If you do this test without a current meter, then your eyes and ears are the only measurement tools. Smoke, frying sounds, or strong smells are danger signs, of course.

Having a current meter in your variac allows a more scientific test. My variac in the clear Lucite case has an ammeter for checking a set's current draw. Here, I have plugged my RCA 14-S-7070G TV into the variac, which is set for 117 volts. The ammeter in the center shows that the TV draws about 1.4 amps.

What does a reading of 1.4 amps mean? Well, a label on the bottom of this TV says that it draws about 165 watts. To convert to watts, we multiply the amps times the line voltage. Multiplying 1.4 * 117 gives us 163.8 watts, very close to the specified value of 165. That's well within the right ballpark.

Similarly, let's say you want to power up a radio like my GE F-63. That set has six tubes and it draws about 70 watts according to its label. When I powered it on the variac at 117 volts, the ammeter read about .6 amps. This time, arithmetic tells us that .6 amps times 117 volts equals 70.2 watts, so that radio is also drawing a normal amount. (In hindsight, it would have been handier to build this variac with a wattmeter rather than an ammeter; then I could read the wattage directly without doing arithmetic.)

When you power up an unrestored set with a metered variac, the meter lets you watch for excessive current draw. If my metered variac showed that my GE radio was drawing, say, over 100 watts when I had increased the AC line voltage to only 95 volts, that would indicate a problem, most likely in the power supply.

Conversely, if the radio draws little or no current, even at full line voltage, that also indicates a problem. But a dead radio is something you can diagnose with your eyes and ears.

The reason to use a metered variac during power-up is to watch for excessive current draw. In that case, you should power down immediately and end the experiment before you damage the set by running it at full power with defective components.

If your set uses a tube rectifier and a power transformer, you may not notice much life until you increase the line voltage to roughly 80 or 90 volts. Below that point, the rectifier tube won't operate and the rest of the set (which depends on the rectifier) will draw little current. This is normal.

In a working TV or radio, you'll also notice that the current consumption spikes up to a higher-than-specified level during the first 30 seconds or so, and then settles back down. This is also normal. Tubes that are cold use more current, and their consumption eases down after they warm up.

Use 3: Running at Reduced Line Voltage

If you own a valuable restored set, such as my RCA CT-100 color television, you may always want to power it up using a variac. Gradually increasing the line voltage gives the set a softer start than suddenly turning it on a full power.

Some old TVs even had built-in "soft start" mechanisms. My DuMont RA-103 has a circuit with a tube and a relay, to delay the rise of B+ current for a while. My RCA CTC-7 has a simpler B+ delay, and others like the CTC-11 achieve a similar result by putting a thermistor on the power line. I can't think of any radios with soft-start circuits, however.

After the set has come on, you can leave it to play at a reduced voltage such as 117 VAC. Some people believe this extends the life of tubes and other components. If you experiment a little, you'll find that many radios and TVs work just fine at a lower voltage, even as low as 110 volts.

In some vintage TVs, especially early color sets, it's vital to limit the current drawn by the horizontal output tube, to prevent damage to the "unobtanium" flyback transformer. Operating at slightly reduced line voltage provides a more comfortable environment for these TVs.

For a permanent installation, there are other ways to reduce line voltage, such as building a humbucking transformer, but that's a topic for another day.

Use 4: Reforming Electrolytic Capacitors

Another use for a metered variac is to "reform" an old electrolytic capacitor to restore it to working condition. The idea is to increase the voltage gradually—even over a period of hours—with an eye on the meter, until the capacitor operates normally and it becomes safe to use again.

I honestly can't recommend this procedure with capacitors that are several decades old. I have experimented with it several times. In some cases, the capacitor never reformed—it was simply too deteriorated inside. In others, the capacitor seemed to reform at first, but it inevitably failed before long.

New capacitors are cheap and easy to replace. My recapping article explains various ways to replace electrolytics. including methods that preserve the authentic appearance of your TV or radio. Don't risk damaging your set by relying on a reformed capacitor.

Types of Variac

Let's take a closer look at these variacs. Earlier, we saw one which a friend of mine built into a Lucite case, along with a voltmeter and ammeter. The big black knob on the right is the rotary voltage control.

Behind that voltage knob is a General Radio Corp. (GRC) Model W-2 variac, rated for 2.4 amps. Its appearance is very similar to this larger, unwired unit: a Model W-5 rated for 6 amps:


The amperage rating of the variac is important. You need one rated higher than the maximum current consumption of the devices you plan to test. The 2.4-amp-rated variac in my Lucite model is sufficient for nearly all radios and many black and white TVs. But it's too wimpy for my early color TVs, which can burn more than 300 watts. For blowtorches like that, you need a beefier variac.

I bought the bare variac at a radio swap meet for $20, a bargain price. I plan to mount it in a case along with meters, like the first one.

Here's a nice little Calrad Model VC-6 in a metal case. It's a 5-amp variac with two outlets, each rated for 2.5 amps:


The Calrad has a voltage output meter, but no consumption meter.

I saved the manliest variac for last. This Powerstat Model 116 has no meter, only a rotating voltage control, on/off switch, and an output plug. It is rated for a whopping 7.5 amps:

Using a Kill-A-Watt to Meter Your Variac

If you own an unmetered variac like the Powerstat unit just shown, you can plug in a handy little device called a Kill-a-Watt to serve as your voltage indicator, wattmeter, or ammeter. Here's what it looks like:

The Kill-a-Watt is designed to help monitor electrical consumption of household devices. Here, it's set to measure the line voltage (which happens to be 120.2), but it can also measure amps and watts. You push a button on the front to change what it measures.

To use it, plug the Kill-a-Watt into your variac, and then plug your radio and TV into the Kill-a-Watt. Leave it at the voltage setting while you set your variac's voltage, then switch to the watts setting to measure the wattage used by your radio or TV.

Voila! You now have a metered variac. The Kill-a-Watt is widely available. I bought mine for about $20 at Home Depot.

Wiring a Variac

If you have a suitable enclosure and a bare variac, wiring it up is simple. These photos show the schematic used to build my Lucite-cased unit, and its variac backplate with terminals numbered as in the schematic:


As soon as I find a suitable enclosure, I'll build my unwired variac like the Lucite one. This time, I'll include a wattmeter rather than an ammeter. That will save me doing the arithmetic to convert an amperage reading to wattage.

©1995-2017 Philip I. Nelson, all rights reserved