considering a variac (found one) need help

[mark_enger]

im thinking of buying a variac, ive got alot of transformers that need testing and pairing, warming up old gear as i scrong up everything with tubes i can, what rating variac would be ideal for my basrment work shop, im a hobbiest not a pro i dont wanna buy twice ill spend the right money the first time. what is ideal and whats the cost? best place to order?     thanks  mark

{Edit: title fix - PRR}

[mackie2]

Mark--

Buyer-Be-Ware!   Variacs  run the gambit from very good to very bad.  I buy  good used American  made units-- I've got several and never had one to  give any trouble.  I bought one  new  Chinese variac--it was a dog!  I learn quickly.  I've had  some very old variacs, that cleaned up re-brushed,  that are like brand new.  There are a lot of  good ones on  Ebay--They run a little high--Ham
Fests and Surplus  sales, are better.  I have a 20Amp Powerstat that I'll always keep until It Is Declared Illegal.  You  may just need a 5 to 10 Amp unit.  If you turn the variac, and it feels rough--let that one go--Test it under load if you can.

Mackie2

[DummyLoad]

get a general radio, staco, or ohmite. 

[RicharD]

I run a 5 amp Staco with a 0-150 VAC meter and a 0-5 ACA meter both on the secondary.  I also have a 5A fast fuse on the secondary.  The current meter is probably more important than the volt meter.

[mark_enger]

 NEW 0.5K VA VARIABLE TRANSFORMER VARIAC 500VA 0-250V



Click to View Image Album
Click to View Image Album 
For sale is a brand new variable transformer, i.e., Variac. Input is single phase 110V AC, 60 Hz, output voltage can be adjusted from 0 to 250V AC. Maximum power rating is 500 VA, maximum current is 1A at 250V and 2.5A at 120V. Please email us with any question
HOW ABOU THIS ONE?  MARK                                       

[jjasilli]

You also have to decide on housing.  You can get anything from a a bare tranny to put in our own box or bench mount; a simple standalone unit; or one already housed in a box with meters, bells & whsitles.  I got a simple used, olsd, 7 amp standalone unit on eBay for about $100.  It's great.  I modded it to a 3-prong grounded outlet.

I love eBay if you have time to wait for a good deal.  Just buy from a reputable sller or dealer who has tested and gurantees the product.  

[RicharD]

2.5A is probably adequate for most amp work, but you really REALLY want an amp meter on the secondary.  Seriously.

[jjasilli]

Agreed.  If you just use the variac for as a safety device to slowly build-up voltage -- for new builds, or an old amp -- the exact secondary voltage doesn't much matter.  But it does matter for design purposes.  Two points in particular about the voltages set forth on the dial of the variac:  a) this assumes a certain input voltage, probably 115VAC.  If your wall voltage differs, your secondary voltage will niot match the dial.  b) the secondary voltage will vary with the actual load on the variac.  The variac will produce the stated output voltages only if the load is at or near the variac's load rating.  As the actual load gets smaller, the secondary voltage will get higher. 

[tubesornothing]

I have what buttery has - a variac with both a voltmeter and ammeter.  The ammeter is great.   'merican made.

[jjasilli]

You can use an ammeter, or "cheat" & put a 1 ohm resistor in series with your variac's output.  The voltage reading across the 1 ohm resistor = amperage of the circuit.  (Just like a 1 ohm "bias sense" resistor in the cathode of a power tube.) 

[RicharD]

My variac is built into my breadboard.  The amp meter (round one... totally pimp) is the one I watch to find out if something is amiss.  If that needle starts rising fast, I'm on the kill switch.  It's normal to see a little initial filament surge which drops after a couple of seconds, then as the circuit starts to conduct, one can see the normal operating current.  I only use the volt meter when dialing for full voltage.  My breadboard has a separate filament transformer that is not fed through the variac.  This is so that I can leave the tubes warm while changing out a part and also so that I don't necessarily have to bring the HT transformer all the way up to get the B+ I need.  Often times I use an excessive HT transformer for experiments and work out the power supply more towards the completion of the design.

[5150fan87]

this is a little off topic, but i just have to say that is the coolest and most awsome breadboard i have ever seen in my entire life. the stuff we worked on was NOTHING like that lol. great job on building it.

[RicharD]

Thank you very much for the complement.  Someday I will finish the control surface.  I did add a shelf to the right side to set transformers on and I attached a reverb tank to the left side.  Someday I'll add a rather large HT transformer inside fed through the variac and switch the variac output to the top like it is now so I can use either the internal HT or go topside for a separate transformer.  I'd also like to add 5VAC and a 5VDC supplies and maybe a 12VDC supply.  I was also thinking of adding a couple more analog meters and a regular bread board to the front panel.  Just one of many many things I intend to do.     It does serve it's purpose quite well.

Thanks again,

-Richard

[mackie2]

Butterlicious--

I've been a breadboarding a long time. I like your board!  I had lot of Mainframe back plane gold wirewrap terminals,
that I put in  spade lugs and crimped. I could wire-wrap instead of using so many spade lugs.  Also, I built a Var 500Ma Pwr Supply,
0-400v, Var Fil  0-6.3v DC and AC, And Bias Supply 0--60VDC.

I  have an old wire-wrap gun I still Use.

Mackie2

[RicharD]

Our barrier strips have wire clamp washers so there's no need for spades.  ISO has one of these too. 

[mark_enger]

i found a staco type 251 at flea market wasnt sure if it was a variac but it was only 5$  and it looks new no solder so i got it, anyways should i mount this in a coffee can also what type of leads do i need i cant post a pix camera screwed. so i hook power cord to leads or terms 1&2 that gives me up to 120v where do i ground to the houseing coffee can? where the shaft from pot goes throu? need help setting it up   mark

[tubesornothing]

Congratulations.  Not sure of the config of that variac, but definitely ground your power plug to the coffee can then route it to the ground on the output plug.

[mark_enger]

i need help on how to set this thing up   mark

[tubesornothing]

Can you post a pic?  Did it come with any information?

[samwheat]

I just bought a Sencore PR570 from the factory for 601 shipped. Lifetime warranty. They normally sell for 785 + shipping.

[RicharD]

I read your name as Sam w/ heat.   

[tubesornothing]

I just bought a Sencore PR570 from the factory for 601 shipped. Lifetime warranty. They normally sell for 785 + shipping.

Very nice!  wow.

[mark_enger]

i cant get my camera to post on here tubesornothing email me and ill send you a pix i do need help with this     markenger@ymail.com

[RicharD]

Hey Mark,

Does it look similar to my Staco pictured below?  If yes then wire it like this:

Pin 5 is the hot input.  (from the wall power)
Pin 4 is the neutral.  (parallel connect your wall supply and output receptacle neutrals)
Pin 3 is the variable hot output.  (it's very wise to put a fuse on the output)
Ground the chassis.

I've also attached my breadboard's PSU.  Don't let it confuse you.  There are 3 separate transformers (variac, 6.3V, & 12.6V).

[mark_enger]

yes it looks alot like that one, mine only got 4 tabs i talked to the guy on ebay david riddle that sells all the test gear and variac, he gave me alot of insight on it said he could sell me a housing for it but id be better off buying a box from home depot. said it would need a fuse and basicly how to set it up, seems like a big project ill get around to it i cant seem to post a pix illl try again

[tubesornothing]

Here we go...

[tubesornothing]

Here is what I would do:

Input cord:

Green ground right to coffee can.
White to pin 1
Black to pin 4

For output
Green from ground lug on coffee can
White from pin 1
Black from pin 3

[mark_enger]

so i run the power cord in and hook the output to a wall outlet plug right?  in between i put in a power switch, a fuse where would the fuse go and what size?  what about building the lightbulb limiter in the same box id just get a box from home depot, i just blew up a small tube amp id found by plugging it right in. i'll have to work on this ill post a pix if i can befor i plug it in make sure it looks right  thanks tubesornothing ill post when i make some progress   mark

[tubesornothing]

Yes, for my old variac I just cut up an old extension cord.  In your case, it might look like this:

Power cord
into Enclosure
Green wire to chassis ground
switch black wire
fuse black wire
White to pin 1
Black to pin 4

then output of wire:
Green from ground lug on coffee can
White from pin 1
Black from pin 3

In my case, I also had a cheap meter wired permanently to the output so I could set the voltage.

The light bulb would be put in series with the input black wire - just like the fuse and the switch - however I am not sure if you could have it always in circuit or would require a switch to switch it out of circuit.  I have never played with a lightbulb current limiter so I don't really know how much current the truly limit.

[bnwitt]

I really don't see much use for a Variac in tube amp work.  They are indispensable for solid state amp work however.  The light bulb limiter is crucial for all amp work.  You initial post mentioned testing and pairing transformers.  You don't need a variac for that.  If you are going to buy one for solid state work then the volt and ammeter on the secondary are important.  My choice would be a Sencore PR57:

http://cgi.ebay.com/SENCORE-AC-POWER-SUPPLY-MODEL-PR57-VARIAC_W0QQitemZ230362810966QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item35a2b18a56&_trksid=p3286.c0.m14

[tubesornothing]

I can see how a one time amp builder might not get much use out of it, but I use mine all the time for a number of things.  I measure and record voltages over time so I need to have a consistent voltage source. I also check the voltage when replacing tubes so I can get a level of consistency amongst each build of an amp.  I also use mine in place of a lightbulb limiter to check for shorts at low voltage when starting up a new build.  I also use it for experimenting with power control.  I also use it as a quick AC power supply for testing things like bias circuits, AC supply to switching circuits etc.

[RicharD]

Mine is indispensable for all the reasons ToN listed + it's such an integral part of my prototyping setup.  The filament transformers are not controlled by the variac so I am in standby when I hit the main switch.  I can dial in HT to get my desired B+ and worry about calculating out the final PSU later. 

[mark_enger]

i got this for very little money and am planning to set it up this winter,and
 i find a lot of uses for it,  and i thank all you guys for your replys.   mark