Variac differences and use

I bought one on ebay for about 75 bucks, as I remember there were a few out there. There's not much to them, buy one that works, from a reputable seller, and you'll probably have no problems.

Variac is a variable autotransformer; you can step AC voltage down to near zero, & some can even step up above input line voltage ~10% or so. Unlike a balanced transformer, it does not provide line isloation. You'll need one with reasonable current capacity; several amps seems typical (500 watts or more). Ebay seems like a good place to find one at reasonable cost.

I assume you want to bring up the input voltage slowly in order to re-form the power supply electrolytic capacitors without risking a short. Tubes should be installed. Speakers should not be connected. Signal input (drive) should not be connected, so if this is a receiver, do not set the input to 'tuner'. Starting from zero volts, raise very slowly to around 5V for several (10) minutes at least, then step up ~5 volts at a time. The whole process can take quite some time of course. Do not ever exceed 120VAC max.

I have never actually done this; perhaps someone who has will chime in later. A knowledgable member who has experience with this actually recommended taking several days to raise voltage VERY slowly, only a few volts per step & leaving it there for many hours at a time. Not sure if it's necessary to go that slowly, but you get the idea. That post might yet be in the forum archives; was posted by Sean many years ago. Try a forum search on "Variac" if desired.

A unit like this(10A) would suit your purpose nicely: (http://cgi.ebay.com/PowerStat-3PN116B-Variac-Variable-Transformer_W0QQitemZ220451775080QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3353f32268&_trksid=p3286.c0.m14&_trkparms=65%3A12%7C66%3A2%7C39%3A1%7C72%3A1205%7C293%3A1%7C294%3A50) I've always powered equipment up, and monitored power supply voltages without the tubes in the circuits(whether new construction or old gear), to be certain everything was working as intended. If a cap is ready to blow at operational voltage; it will(whether you take three days to bring it up, or three minutes).

It may be overkill for your purposes, especially at $539, but I use an earlier version of this B&K Precision variable ac power supply.

Note though that its output current capability is 3A continuous, 4A intermittent, which makes it unsuitable for some power amps.

I like the fact that it provides metering for current as well as voltage, and it also provides isolation, which is important for safety if you are doing troubleshooting while the equipment is powered up.

I disagree with the suggestion to run without a speaker connected. With many tube designs that can raise internal voltages to greater than normal values, possibly overstressing capacitors and other components. I use a $10 Radio Shack speaker driver as a load when I'm checking out or bringing up the ac voltage on equipment that is in unknown condition.

Re powering up without tubes inserted, that is a good idea to do initially on new construction or old gear in unknown condition, but keep in mind that if the design utilizes a tube rectifier you won't be reforming the electrolytics (or verifying a lot of other things) until the rectifier tube is installed.

Regards,
-- Al

thanks for your observations Al I wouldn't want to post bad info. of course. Not sure why a transformer coupled device would behave that way though?

I suppose a better choice of words would be "do not connect any speakers that you care about being blown up" because that certainly could happen if something goes wrong. A cheap pair of car speakers, yes.

FYI: I once energized a tube receiver without one of the tubes inserted & an electrolytic blew it's top. It was working decently up until that point but had a hum I was attempting to diagnose. Bad day.