Staco Variable AUTOTRANSFORMER

[plexi50]

Some one gave this to me this afternoon. They have a ton of goodies. Is this a variac? Is this useless here in the states?  The knob looks like bakelite

[jjasilli]

Looks to me like you could put 120VAC in and get 0 - 140VAC out, just like a "standard" variac.  I suspect, but don't quote me, that VA would double to 7 amps.  If so, that might upset the 4 amp fuse.  But if that fuse stays happy, I can't see the harm.

[plexi50]

What are the Line jacks used for? Line out?

[RicharD]

That is a variac and it should work fine wired for 120V.  I don't think I'd try to pull 7 amps through it.  Yes that's how the math works out but I'd question the wire gauge of the coils.  Goodness knows 3.5A is sufficient for most of the stuff we do.  The banana jacks are for a volt meter.  For the record, a variac is much more useful with a current meter.  Nice freebie.

-Richard

[plexi50]

Gee Wally thats great! Do you think dad will get mad if i try it out on our TV?

Thats great news all. I have another Fluke Meter i can plug into it to read the volts

Guys some times i asked the stupidest questions even though there not. It just feels that way some times

I would much rather get scolded by any of you than not learn all about what we do with amps and our tools

Isnt biasing the amp another topic using a Variac?

[jjasilli]

Isnt biasing the amp another topic using a Variac?  Biasing is not the problem.   :alien: Plate voltage will change with bias and stay in sync well enough.  As plate voltage is lowered, the tube needs to be biased hotter -- i.e. more current.  But the Variac drops the bias voltage too, thereby increasing the cathode current automatically.  Free lunch!

What are the problems:  a) turning down the Variac too low thereby drawing excessive B+ current; b) insufficient heater voltage, below say 5.5V.  If your heater voltage reads 6.3 volts normally, then:  5.5/6.3 = ~87% = a 13% drop.  So drop only 10% to be safe.  That matches the general rule not to drop the Variac below 90 on the dial when feeding it to an amp.  But, don't put blind faith in your Variac's dial.  Test it under load to see where 100% and 90% really are, and mark those positions with dots on the dial.

[plexi50]

I need to change the plug ends for outlet & input usage.  Good info to know jjasilli. I have never used one so this will be an interesting tool.

[FYL]

It's a US 240V variac, check if it has been designed for *two phase* mains (2 x 120V) with two identical windings. If so, the third wire/connection isn't a ground but a neutral, a common return conductor, something you should take into account for your safety.

[plexi50]

Thankyou FYL. From what i can find on the internet it says this Staco unit 3PN1020 is a single phase

[PRR]

It is a 1,000 VA core with a <300V winding and <=3.5A gauge/contact.

Using it at 120V means "you paid too much". If you ordered the 120V version it would have fewer turns of fatter wire, still 1,000 VA and ~~7 Amp limit. Or if you wanted 120V 3.5A it would be smaller, lighter, and cheaper.

> gave this to me

Then you didn't pay too much. And the "excess" weight is probably not a problem.

Core losses at 240V are probably under 2%. This drops at 120V but not enough to count.

The wire gauge and sliding contact were scaled for 3.5 Amps. You can't significantly raise this by running 120V. Any reduction of losses will be quite minor.

Call it 120V 3.5A.

And frankly: why would you need more? 420 VA will support any tube amp to 150 Watts.

An amplifier with a 3.5A fuse, in overload the fuse will pop in a minute. This Variac in overload will pop (smoke) in a half-hour. You are probably fine testing amplifiers with 5A fuses.

How will you use it?

For starting-up sick amps, at low "line" voltage the load current will tend to be lower than normal. And you probably won't run this way for very long. Many minutes, not hours.

If starting a HI-power tube amp: these generally idle much cooler than their full-roar demand. I would not fret about starting a idling 200W tube amp on this, even though the full-roar load would exceed the rating.

And the rating is conservative. These things get used in 24/7 industrial controls. I'd be comfortable pulling 1.5 times the rating for a quick one-minute output-power test. I'd expect it to carry 2X rating for a short time. Yes, you could probably clip-test a 300W SVT, though if you get in the habit you might eventually kill it. (But compared to the cost of owning an SVT, buying a proper Variac is teeny.)

DO: use a lamp limiter after re-wiring. Variac connections can be confusing; you can wire a dead-short which would be bad.

Ponder what you want. For sick-amp fixing, you "never" want the voltage-boost mode. Either wire it for 0-100% (the end terminals) or at least put a big mark where "100%" happens and don't go past that carelessly. (I guess the 280V/240V switch does that; re-mark as 140V/120V.) OTOH in my 108V office I could have used a boost to get to 120V nominal.

[FYL]

Call it 120V 3.5A.

Exactly.

"You can use the 240 volt unit for 120 volt as long as you do not exceed the amps of the standard 240 volt unit. You will get 0 to 120 volt with new use.

Paul Heiligenberg
Sales Operations Manager
Staco Energy Products Company"

From http://forum.doityourself.com/archive/index.php/t-388701.html

[plexi50]

Have it workin along with my other meter. I decided to use the Beckman meter instead of my Fluke meter. Great addition /