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 Post subject: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 5:46 pm 
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Joined: Aug Tue 31, 2021 6:36 pm
Posts: 9
Location: Delaware, Ohio, USA
Guys,
Looking for a Variac and noticed that both Amazon and eBay have what appear to be inexpensive Chinese variacs. They carry different manufacturer names like "YaeCCC", "Mophorn", "Yonntech" & "Beleeb"... (I know, appear to be made-up names!).

In any case, have any of you tried any of these or do you look for old, use equipment made in the US? If so, what brands to look for?

Thanks!


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 7:59 pm 
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There are US made ones like Superior(I have) Genrad, Standard, etc. Buying used is a good way to save some money. They generally are used in applications where they don't "wear out" like a rheostat or a pot, as they are used in apps where they are not cycled much. So the only real wear point is the wiper contact and the exposed windings for that. On a used unit, look for damage, bad corrosion, or windings that look burnt or the enamel bubbled up from an overload. It is really good idea to have fuse protection for them. As for the new Chinese or Asian types, I cannot comment if they are good, really meet their specified ratings, etc. Someone else can chime in. But the American branded ones I have seen are high quality devices. You can also save money if you only buy one that is big enough to cover your appications. Like one rated at between 2-3 amps ought to cover testing most receivers you would ever encounter. And a high current model is large, heavy, and expensive. Just have a protective fuse in there somewhere. A circuit breaker might be rather slow to react to an overload and overheating. You can pick the fuse rating and speed so a shorted radio pops it right now and saves the variac and further damage to the radio.

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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 8:10 pm 
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OH BTW on used ones, be sure to read the fine print! Or look at the data plate in the pictures. There are variacs out there that were made for 400 cycles instead of 60. Those would be utterly worthless for your use. And perhaps for odd, or not 120V too.

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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 8:24 pm 
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Joined: Aug Tue 31, 2021 6:36 pm
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Location: Delaware, Ohio, USA
Hi,
I am looking to power up old radios from the 30's and 40's. The big Zeniths are likely power hungry.

Here's a used one on eBay, do you guys think this would be up to the task? It's 10A

https://www.ebay.com/itm/174949689694?_ ... %7Ciid%3A1

Thanks!

-john


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 8:27 pm 
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Joined: Jan Tue 10, 2012 8:39 am
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I don't know the current availability, but back in the day you could buy replacement brushes.

Another concern is mounting. Sometimes you have to get creative to make the unit useful. And of course be careful not to hurt yourself or start a fire or anything.


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 8:28 pm 
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That 10A one is nice, although a little too expensive.


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 9:23 pm 
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$130 for a 10amp is kinda pricey.

I bought both an 18amp and a 10amp variac over the last few months here in Florida. Got the 18 along with a really good tube tester (what I really came to buy), multimeter, oscilloscope, isolation transformer, tube caddy full of tubes and a lot more for about $300 and the 10 amp variac for $25. The guy with the 10amp one also threw in an stainless steel ultrasonic cleaner.

eBay is where there are almost always the highest prices. I found mine by looking at craigslist and facebook marketplace. I then found one that was very low priced, but too far away and told a local guy about it saying I was going to be driving there, but not for another two weeks, so I'd prefer his if he'd sell it at the same price. You can get away with that kind of thing when you find listings that have been up for 10+ weeks. They all think it's worth at lot more, but having it for sale for a dozen weeks tells them otherwise... it just takes time.

I find some people don't know it's known as a variac. So also search for Variable Transformer, Variable Autotransformer or simply Transformer to see if anything shows up.

Also consider posting in the classified section of this site and in the Vintage Test Equipment Facebook group since lots of people have more than one and seem to collect them whenever they see a good deal.

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Clearwater, FL


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 9:30 pm 
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I bought, on EBay, one of the larger red Chineseium ones to lower the voltage from 126 for my whole bench. It’s fused (came in handy twice already) and metered. Seems to be fine. If I remember next time I’m there I’ll update this with the make n model

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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 9:33 pm 
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Joined: Aug Tue 31, 2021 6:36 pm
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Location: Delaware, Ohio, USA
Thanks for the tip on searching... would something like this do the job?

https://pittsburgh.craigslist.org/ele/d ... 43973.html

Thanks!

-john


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 9:38 pm 
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jrmiller43015 wrote:
Hi,
I am looking to power up old radios from the 30's and 40's. The big Zeniths are likely power hungry.

I believe the 1000Z Stratosphere requires 350 watts. That's a little big for a 3 amp Variac, but 3 Amps would do fine for any smaller radio. 10 Amps is way overkill. Are you working on anything with more than 15 tubes?

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Last edited by stevebyan on Sep Mon 27, 2021 9:40 pm, edited 1 time in total.

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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 9:40 pm 
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Location: Dayton Ohio
The most power hungry Zenith from the 1930s would be the 1000Z Stratosphere.

It uses 360 watts. At 120 volts thats only 3 amps.

I have a 5 amp Variac on my bench and never needed anything bigger.

-Steve

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Consoles and floor models, the bigger, the better!


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 9:43 pm 
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jrmiller43015 wrote:
Thanks for the tip on searching... would something like this do the job?

https://pittsburgh.craigslist.org/ele/d ... 43973.html

If the brushes and track are in good condition it would be more than sufficient. In my 40+ years of experience with variable autotransformers I rank Powerstat second only to General Radio in quality.

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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 9:47 pm 
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Joined: Aug Tue 31, 2021 6:36 pm
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Location: Delaware, Ohio, USA
The biggest set I have is this Zenith:

https://www.radiomuseum.org/r/zenith_9h ... h8c21.html

8 Tubes... and a record player! no doubt, 78 RPM

I attached the schematic from Radio Museum.

-john


Attachments:
File comment: Zenith 9H081 Schematic
radiomuseum-zenith-radio-corp-9h081-conventional-491642.pdf [296.99 KiB]
Downloaded 11 times
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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 10:31 pm 
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jrmiller43015 wrote:
The biggest set I have is this Zenith:

https://www.radiomuseum.org/r/zenith_9h ... 8c21.htmln

According to the schematic, it's only 70 Watts. A 1 Amp Variac is plenty for that set.

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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 10:57 pm 
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If you use a variac with an AC ammeter, you can monitor the current as you increase the voltage.
If the current climbs too much, stop there and find the problem. No need to wait until you smell smoke or blow the fuse!

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It's not the Destination, It's the Journey.


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 11:44 pm 
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One comment about variacs. The current rating isn't so clear cut. For instance, when putting out 120V the brush is almost exactly on the input line and you could draw a lot of current without problem, as long as the brush can handle it.

The allowable current drops sharply as you turn the knob from there. It's not a simple function of power, rather a complicated formula that depends on the load and knob position. At half voltage, the current to the load is provided through the brush from both the upper and lower parts of the winding. So the winding current is not the load current.


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Mon 27, 2021 11:59 pm 
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I'm not overly versed in variacs since I just bought my first one within the last month or two, so I'll let others comment on that craigslist find.

It certainly is rated for enough amps.

If it were me, I'd offer him $50 and mention that you don't need one rated for more than 3-4 amps, so the seller knows you're really looking for something smaller and therefor a lower price might be warranted for your situation.

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Clearwater, FL


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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Tue 28, 2021 3:22 pm 
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I got a small Superior that I got somewhere for free. I put it in a recycled enclosure with a voltmeter, not enough room for the ammeter. It is rated at 270VA and so far I have not had a radio receiver strain it at all. Although I did try using it as a motor speed control once and that turned out to be a bit much for it. I have it fused with a 2A fuse, I could even go safer 1 or 1.5 amp fuse and still run just about any radio. trying to keep it a fast blow type. 2A is just under it's max rating. I have a Keithley DMM and a matching clamp on amp probe and HV probe for it. I merely took an extension cord I have and opened up a space for about 6 in. between the zip cord conductors so I could put the amp clamp on that to watch the current. And it still works as a regular extension cord anyway. I guess you could use a high current variac for other stuff like slowing down motors and for that day you get a steal bargain on the Zenith Strat, keep your fingers crossed.

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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Tue 28, 2021 3:29 pm 
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I've collected and repaired many "Variacs". I can offer a few basic but important tips.
Don't try to clean the windings along the brush track. If there is a black substance on them, it's a special metallic coating applied to enhance conductivity (and brush longevity).
Variable autotransformers are difficult to measure with an ohmmeter. The multicontacts of the brush need to pass a small amount of current for them to all contact the windings properly.
The flat area of the winding where the brush contacts it is milled and lapped to a high degree of precision.
Unless it is damaged from overheating and/or arcing, and the only option with which to attempt restoring functionality is to use an abrasive on this area, it is best left alone.
To test for winding contact and brush connectivity I would use one or more 120 volt incandescent bulbs as the load, and with a voltmeter connected across them slowly rotate the knob back and forth through its range to look for dropouts.
There is typically some range of brush tension adjustment by loosening the setscrews on the "live plate" that carries the brush and moving it closer to the windings.
If one fuse only is used, it needs to be connected to the output. This is critical because in certain conditions the winding can be destroyed if it is fused only on the input. The fuse will not necessarily blow.
There is at least one company that rebuilds worn out and obsolete brushes for a lot less cost than having to buy a replacement brush.
There is a lot of good information about many aspects regarding variacs in this web search through Duckduckgo:
https://duckduckgo.com/?q=rebuilding+va ... =h_&ia=web

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 Post subject: Re: New Variac vs. Old Used Variac
PostPosted: Sep Tue 28, 2021 9:09 pm 
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Location: Long Island NY
Quote:
One comment about variacs. The current rating isn't so clear cut. For instance, when putting out 120V the brush is almost exactly on the input line and you could draw a lot of current without problem, as long as the brush can handle it.

The allowable current drops sharply as you turn the knob from there. It's not a simple function of power, rather a complicated formula that depends on the load and knob position. At half voltage, the current to the load is provided through the brush from both the upper and lower parts of the winding. So the winding current is not the load current.


If the output current rating of a variable autotransformer is given, it's the simplest thing in the world. The current rating is based on the gauge of the wire in the winding. It's a physical characteristic that does not change, so whatever that current rating is, is the maximum current that can safely be drawn at any output voltage. In other words, if it says 5 amps output, then you can draw 5 amps from it at 10 volts, 60 volts, 120 volts, etc.

What is more complicated is when they are rated in VA or watts. This number only obtains at the rated voltage, input and output. For example, if a transformer is rated for 480 VA at 120 volts, it means that with 120 volts in and 120 volts out, it's good for four amps, not any more. So if you set it to a different voltage, the VA or wattage it can handle will be lower. If you set it to 24 volts output, then the VA is 24 x 4 = 48 VA. Set it to a voltage higher than 120 volts, and you are still limited to 480 VA max. This means the current has to be de-rated accordingly. As an example, if it is a 480 VA, 120 volt transformer and you set it to 140 volts output (using an over-voltage tap) then the current has to be limited to 480 / 140 = 3.43 amps.

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