Buying a Variac

WhoJamFan

Member
Messages
2,557
I've been looking to get a Variac used, is there anything I should look out for?
Is there a way to test it without using an amp as the guinea pig?
Any help would be appreciated.
 

guitar007

Member
Messages
403
Get one at around 8 amps if you're going to use it on 100 watt amps. Although not necessary, it's nice if it comes with a built in volt meter so you can see the output voltage. To test one, use a VOM and track the voltage as you sweep the full range of voltage to see if there are any issues.
 
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PremiumPlus

Member
Messages
1,106
If you don't have a VOM, just plug in an old incandescent table lamp and use the variac to dim the light.
 

twangbanger

Member
Messages
1,493
Are you going to use it as a testing and amp repair tool or to brown out an amp? Mine has a meter that you can set for wattage or voltage. A nice feature to have.
 

WhoJamFan

Member
Messages
2,557
I would like to use for both, although repair/mods etc will be it's main use. I found a well used one that supposedly works fine but I just want t be sure.
So 8 or more amps is the main thing to look for?
 

zenas

Member
Messages
8,762
Mine's a big old one found in an antique shop. Of course no test equipment in my pocket, 300 miles from home.
Solution ?
Plugged a vacuum cleaner into it, turned it on then used it for a speed control. It worked.
Also paid cash so there was no way to track me if the vacuum was damaged. :)
 

Vanyu

Member
Messages
738
I'm curious too, I'm in the market for one also. Are there any specific features to look for or avoid in a variac? How would one like this work?
 

AXEnGEAR4J

Member
Messages
5,901
I use a Staco 10 amp variable with my vintage amps that like 105-110 volts and bringing up old amps slow. Very sturdy and reliable, picked it up off the bay cheap as it had felt markings on it I cleaned off and looks brand new
 

WhoJamFan

Member
Messages
2,557
I'm not looking to perform with it, I'd like to take my limited repair skills to the next level and am looking to put together a workspace so I can start off building kits and going beyond the simple component replacing I haven't managed to kill myself doing yet-haha.
I gather the 2 pieces of technical equipment I need to get are a Variac and an Oscilloscope to join my DMM and soldering station.
I know very little about either, in terms of not overspending on something I don't need, or buying something that's either junk or not able to do what is required.
I don't need to build the Enterprise, just simple tube circuits and such.
I'm no EE, but am fairly handy with an iron and would like to learn more doing hands on projects. I'm wanting to learn how this technology translates to what I hear in my head and how to apply it, and be able to fix and/or mod my own amps.
I saw a Variac on Craigslist from a guy clearing out his clutter for Holiday Cash, and thought this might work for me.
No idea if he's ever gonna call me back like he said he would, so who knows.
Been doing guitar rewires and simple amp stuff for years, but want to up my skill set mainly for my own enjoyment.
 

swiveltung

Member
Messages
14,492
From experience, my advice would be to get one with a digital voltage readout. Something to read easily without getting out the VOM etc in the middle of performing, breaks etc etc.
 

MKB

Gold Supporting Member
Messages
8,380
You have to be careful with variacs, they are primarily meant as test equipment for techs that know about how to deal with line voltages and connections (as has been mentioned earlier). Since they are autotransformers and do not isolate its output from the input (line connection), you have to think about your voltage/current path in every case and determine if the setup is safe. The variac output is essentially at the same safety level as plugging directly into a wall socket. It should never be considered a consumer level device like a power strip or UPS.

You can get some strange situations from a variac such as if you have an older amp with the neutral connected to chassis, and the amp is plugged in the variac so that the neutral is on the high (adjusted) side of the variac. You will then have a voltage differential between the neutral connection on the amp and the line neutral connection, which can place a dangerous voltage potential on the chassis. Older amps with death caps/ground switches can have issues from this as well. You really have to give the entire setup some thought and checkout for safety before throwing the switch.

I borrowed one from a friend once that he had internally rewired so 0v on the knob was actually 140v and vice versa. This is very easy to do, as many variacs are a single standalone component in a case. Another friend (a EE) blew up an amp with this same variac. I tend to measure the voltage out of a variac instead of trusting the knob setting.

A very good compliment to a variac is an isolation transformer. While working on amps I almost always use an isolation transformer on the output of the variac just to be completely safe. With such a setup you do not have to be worried about much more than the voltage level coming out of the transformer.
 

darkfenriz

Member
Messages
207
Most toroidal variacs will nuisance trip home circuit breakers due to inrush.
I advice using an NTC thermistor in series, not sure if anyone offers an integrated NTC but if they do, it's a buy.
 

71strat

Member
Messages
9,285
I have a Parts Express Variac I used to use, and it worked with my Metro GMP45, but will trip the breaker with my 69 Dual Showman.
 

WhoJamFan

Member
Messages
2,557
I looked at the one I was talking about. It was in a very large grey metal box and the guy said it was 20 amps.
I passed, knowing as little as I do about them, it kinda seemed like setting myself up for a very bad experience, and way overkill for my needs.
I'll be doing a lot more research on this, varifying any findings with some great tech friends I have before pulling the trigger on anything.
The words of Aspen Pittman kept ringing in my head-"The best way to use a variac is not to."
Obviously targeted at people like myself who at this time know very little and whose "learning curve" could include catastrophy..
Thanks for all the advice, need to do some more homework on this one.
 

Tomm Williams

Member
Messages
964
I looked at the one I was talking about. It was in a very large grey metal box and the guy said it was 20 amps.
I passed, knowing as little as I do about them, it kinda seemed like setting myself up for a very bad experience, and way overkill for my needs.
I'll be doing a lot more research on this, varifying any findings with some great tech friends I have before pulling the trigger on anything.
The words of Aspen Pittman kept ringing in my head-"The best way to use a variac is not to."
Obviously targeted at people like myself who at this time know very little and whose "learning curve" could include catastrophy..
Thanks for all the advice, need to do some more homework on this one.
I bought a used Superior Electric at the suggestion of Ben Fargen to use with my vintage amps. You might contact him if you'd like to know more.
 




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