Variac?

Tom Higgins · From: New Jersey, USA

I have some analog recording gear that hasn`t been used for a couple of years.Not having,or having ever seen a Variac,I`m planning to plug into a rheostat-controlled light socket w/an ac adapter installed. Is this ok,and if so,without any frame of reference,as far as voltage,would a gradual ramp-up from off to full-on over a period of,say,60 seconds,be a safe way to get started?Thanks for any help. Tom

Richard Damron · From: Gallatin, Tennessee, USA (deceased)

Tom -

If - by analog - you mean things like tape decks, amplifiers etc then forget the Variac. If your equipment has a cord with a plug on it then just plug it into the wall. Be advised that some electronic gear does not like being operated at highly reduced "line" voltages. The fact that your gear has been sitting around for some time is of no consequence. I've had gear that has sat around for 12 to 15 years and worked just fine when plugged in. Not to worry!

Jerry Gleason · From: Eugene, Oregon, USA

A light dimmer that fits into a light switch receptacle is not a substitute for a variac. NEVER plug a piece of electronic gear in to dimmer-controlled receptacle. A variac is a large, wire wound variable transformer. If you looked at the AC output on a scope, it would always maintain a sine wave, but change in amplitude as you turn it up. Most of those little dimmers just chop off the top and bottom of the wave, which is why they create line noise in nearby equipment.

If you plug a tape recorder into an outlet controlled by one of those, you'll get a loud, ugly hum, and it may damage the equipment.

There are differing opinions and schools of thought about using a variac to gently bring up the voltage on electronic equipment, but I'd say than unless it's old tube gear, I wouldn't do it, or worry about it.

Steve Norman · From: Seattle Washington, USA

If any of the gear has large waxy caps you should variac it, if the caps look like the wax is melted you should replace them
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Jim Sliff · From: Lawndale California, USA

Ken Fox · From: Nashville GA USA

i am not ready to throw out my Variac just yet! I use mine with a series light bulb and a current meter when testing a newly repaired output stage in transistor amp. This will save a new set of power transistors if there is still a problem in the circuit.

I have re-formed caps in old amps using a Variac. However, my final test is using an ESR (equivalent series resistance) meter to test electrolytic caps. I prefer new caps as well in older amps!!!

I have seen many a time a tech replaced a can style cap with individual caps and there was nothing wrong with the can style cap! Another fine example of a tech "shot-gunning" parts when he did not have or understander how to use proper test equipment.

Lately the old Variac has been a useful tool with Marshall and Musicman amps I have been repairing. These amps had bias circuit issues and bringing them up slow while watching the Bias King meter revealed high bias current was developing.

A great tool if used wisely.

Now for thatlight dimmer. I don't think so! It is chopped AC and a low current device at best. Use the proper tool for the job!!

Tom Higgins · From: New Jersey, USA

Thanks,guys,I guess a better idea will be to get things serviced by a pro.Now if I can find one in North Jersey..... Tom.

Jim Sliff · From: Lawndale California, USA

Ken - those are all good uses. I was avoiding those things because the average player (not tech) is not going to use one to watch for transistor problems or understand bias issues may exist - they're going to uselessly bring up caps slowly because Gerald Weber said so, or try to get the Van Halen "Brown Sound" via voltage drop...and they're going to get into trouble.

When I do a major restoration I almost always bring the amp up slowly to watch for problems...but again, most players have a totally different motive for variac use.

I've reformed caps with mine as well - but also tossed about half the ones I was trying to reform as they didn't meet spec, or there was something that just didn't "feel" right. I don't do it anymore. Changing filter caps is an accepted process, even on vintage amps, and IMO there is no real reason to try to salvage old caps - if they are past their service life, replace them.
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Casey Lowmiller · From: Kansas

I have a few amps that I haven't turned on for about 2-3 years.

Some of them haven't been turned around for probably 5-6 years.

How long can an amp sit and then be turned on without damaging them?

Quite a few of them are solidstates from the 60s, 70s and 80s. A couple of them are tube amps...Fenders from the 60s, 80s and 90s.

I was told that I should get a variac...but some of the more tech savvy folks that I know say that I don't need one.

Anyone have some good theories/answers???

Hopefully, Ken Fox, Brad Sarno, Jim Sliff or some of you other electronics folks can chime in here and help me out.

Any help is much appreciated!!!

Casey

P.s. Sorry to hi-jack the thread...I thought I was starting a new one.

Blake Hawkins · From: Florida

You have to be beware of variacs. Unless you have
calibrated it, you could put over voltage on the
equipment.
Most variacs have a scale of 0 to 100. But, they reach full line voltage of 120 volts at 80.
If you go all the way to 100 you will have a voltage
of 140+volts.
This is just based on the one's I've used over the years. There may be other autotransformers that
are calibrated differently.
As Ken and the other techs have noted, it is a fine
tool in the hands of an experienced technician.
Blake

Dave Beaty · From: Mesa, Arizona, USA

Please be very, very careful with "Variac's".
Common Variac's are indeed an autoformer, with one side of the AC line connected directly to what is considered common AC ground, which 'may' be connected to the metal frame of the device!

For proper usage, the Variac should be mounted in an insulated box/housing with a switch and a safety fuse on the input side, and an isolation transformer with an adequate current rating on the output side (which can also be fused).

For testing U.S. equipment the isolation transformer can be a 1:1 type. For testing Euro gear a 2:1 step-up transformer can be used, allowing the output voltage to range from 0-240 VAC. Having a AC voltmeter mounted in the box which is connected across the output is also a very helpful tool. An AC current meter in series with the output will monitor the load current as the Variac is used to increase the voltage to the isolation transformer and the device load under test.
Open frame Variac's are among the most dangerous items one can use. I have seen a number of them which were literally welded to metal benches/frames by inexperienced technicians over my 50 years in the aerospace electronics industry. Scary stuff indeed.
Taking precautions with AC "mains" voltage may allow you live to pick another day.
Dave

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