Variac questions? Should I get one?

[Dumbmonkey]

Hi all!
I'm still pretty novice, but I've built a couple of amps from kits (5e3, and JTM45), and I'm currently in the process of building another 5E3 that I sourced the parts myself.  I've also repaired and modded a few amps along the way. 
Anyhow, I'm wondering if I might need to get a variac at some point.  I have a light bulb limiter, but I wonder if I need more than that.  I'm not looking to start a business, so I don't need high end.  What would someone recommend if I wanted to get something basic? 
Thanks in advance for your time!
cc

[mresistor]

https://www.ebay.com/itm/273503573156

{edit} item number is enough; don't need all eBay tracking --PRR

[shooter]

the bulb should be fine for the typical builder.
I have neither the bulb or the variac, but I do have 40yrs fixing broke stuff for a living.


once you understand how to find a short BEFORE power is applied, you're 80% there, the other 20% happens when you power-up, the lamp glows REAL bright, and you realize I soldered the socket wrong, or other simple errors

[HotBluePlates]

... I have a light bulb limiter, but I wonder if I need more than that.  ...

You don't need a variac unless you have old amps and need to dial-back the outlet voltage.  I have amps from the early-mid 60s and need to knock things down to 117v or 115v to avoid stressing filter caps.

https://www.ebay.com/itm/273503573156

I'm not impressed that the Chinese variacs that get promoted achieve their low price by shifting the burden of "over-rating for safety" to the end-user:

     "We recommend you purchase a transformer that is 100% higher than the wattage you need."

     "Remember to leave at least 100% slack for the transformer. Using them to maximum capacity will eventually burn the transformer out after prolonged use."

     "For these situations, you need to buy a transformer that is at least 3 times the item's capacity."

[PRR]

> buy a transformer that is at least 3 times the item's capacity

Yeahbut.... 1,000 Watts claimed, 333W derated, is still bigger than most "100 Watt output" tube amps suck.

I do wonder whenever low-price sellers claim "Copper" because it is so darn easy to slip Aluminum in there. The extra Ohms is minor but a sliding-contact like this won't last in Aluminum.

[CO_Hoya]

If you do pick up a variac, consider adding a panel meter as per Sluckey's suggested modded version in his Amp_Scrapbook. I did that for a used variac I picked up locally and I find the extra info quite helpful.

I also put in a fuse with lower current rating than what was spec'ed with the transformer, as the original seemed ambitious (and dangerous).

[Dumbmonkey]

Thanks for the responses everyone!  It sounds like I'm going to be ok with my light bulb limiter, which is nice!

[bmccowan]

Yeah but the other option is to go ahead and buy a variac and then buy more broken amps to fix in order to justify buying the variac. That's what I did, and it worked out fine.

[joesatch]

dont make the mistake i did and assume the variac on full would be 120V. i always thought they were used to limit voltage and didnt realize they can boost voltage

[Williamblake]

What has been said. Maybe you don't need a variac but once you have it you will frequently use it.

[Williamblake]

And there are isolating and nonisolating variacs, meaning they have or have not different windings for primary and secondary. For safety reasons you would want an isolating variac with two independed windings. And you would not connect more than one device to the secondary if you are not 200% sure what you are doing.

[sluckey]

I've never had a reason to use an "isolated" variac. If you have a need for isolation, a dedicated isolation transformer is a better choice.

[acheld]

When I got "the bug" (meaning, the amp building thing . . .) some years ago, I ordered up one of the red imported Variacs.    Honestly, I have not needed to use it much.

That said, using it to measure quiescent current of the amp (Sluckey's Variac mod) has made it much more useful.

[kagliostro]

Till now I've seen non isolated Variacs, 400Hz Variacs, 3 phase Variacs, low Wattage Variacs, but in my experience miss to see an insulated Variac with two separated windings, I'll be curios to see one, are there links to that tecnology ?

Doing a search this is the thing that is nearer I've find (Variac + insulation transformer)

https://www.conrad.com/p/thalheimer-tse-002-variac-1-x-230-v-368-va-160-a-516546

Franco

[RadioComm]

I seldom use my variac. I pull it out when I acquire an old amp that seems to not have been turned on in a long time.

Attached is a picture of my homebrew variac. Bough the core from a Hamfest for $3.00 USD, and rigged it with whatever I had hanging around.

ttfn

                                                                                                                                                                                 

[sluckey]

I like the can on that old Powerstat. Why not replace that duplex outlet for a switch/outlet?

[RadioComm]

I like the can on that old Powerstat. Why not replace that duplex outlet for a switch/outlet?

I rigged this one about thirty years ago. I'm used to it. I was tempted to add some MM jacks instead of using an outlet for measuring VAC, but it worked, so good enough...

[PRR]

> didnt realize they can boost voltage

There's usually (at least) two ways to wire them.

Rememeber they far pre-date old guitar amps. The real original point was the convention of speccing "line 115VAC" when Massachusetts Power could give you 108V or 122V or anything in between. So there is usually a 10% boost option.

You probably SHOULD be running a voltmeter with the Variac.

> there are isolating

I have never seen an isolating Variac. That includes many decades of reading catalogs and prowling surplus shops. I'm not saying they don't exist, but you won't run into one unexpectedly. I see Sluckey and Kagliostro give similar thoughts.

The Variac construction does not encourage an isolated winding on the same core.

Sure you could buy separate isolation and variacing and nail them to one board or box. I may have seen that in the 1950s. But for the maker it is more sense to make separate products so you can mix-and-match to your needs. (Or a made-up "matched bundle" as Kagliostro just posted.)