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 Post subject: leads from Variac ?
PostPosted: Nov Sun 17, 2019 7:47 pm 
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Joined: Nov Sat 02, 2019 2:22 pm
Posts: 6
This from a guy who is in the learning process. To test my old tube radio I plug my new variac into a wall outlet and the variable variac output is fed to the radio plug pins (correct?). Is there a particular minimum gauge of banana plug/alligator clip test leads that should be used? Like maybe a too small gauge might heat the leads up too much. Is this a dumb question?


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Sun 17, 2019 8:29 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
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Location: St. Louis, MO
Uhh, most variacs have a receptacle on them that you plug the power cord of your equipment directly into. If yours doesn't have one, I'd look into adding a single receptacle to it along with a box to protect the terminals.


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Sun 17, 2019 9:19 pm 
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Location: Ohio 45177
Is that variac in an enclosure or what? ON mine I had a regular 2 pin old style receptacle, but punched a round hole in the back and installed a modern receptacle for the use with grounded or polarized plugs, too. Unless you have a really high current rated variac, 18 ga. wire should be sufficient for radios. Or even 16 ga. Don't want to get ridiculous with big stiff wiring.

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 Post subject: Re: leads from Variac ?
PostPosted: Nov Sun 17, 2019 9:58 pm 
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Location: Rochester NY USA
18 gauge is good for 10 amps but don't use anything smaller - if there's a short somewhere, you want your 15 or 20A circuit breaker to blow, not your wiring...

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 Post subject: Re: leads from Variac ?
PostPosted: Nov Sun 17, 2019 10:07 pm 
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Joined: Jul Sun 21, 2019 3:32 pm
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Location: Upstate NY
Please consider getting a isolation transformer as well, Many old radios did not have power transformers, and could have a live chassis. Meaning you could get a nasty shock or worse.
Mike


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Sun 17, 2019 11:55 pm 
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Thanks, for the good advice. First, I apologize for not giving enough information. Yes, it has a modern output receptacle. Just bought it. What I want to do is put an ammeter between the variac and one of the radio plug prongs in order to observe current rise or fluctuation when I slowly turn up the voltage on the variac.

What would be an appropriate isolation transformer for these old tube radios? I appreciate the need for these.


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Sun 17, 2019 11:59 pm 
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This is the one I used for years, good for table top radios like AA5.
viewtopic.php?f=15&t=367750

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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 18, 2019 12:13 am 
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Location: Upstate NY
I was fortunate enough to find a retiring repair shop, and they were more than happy to have a buyer for their old equipment that they haven't used in years.

Ebay has some bargains, but it's a roll of the dice. You might buy a paperweight..
Mike


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 18, 2019 1:38 am 
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Joined: Jan Tue 10, 2012 8:39 am
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I did a search for isolation transformers and was apalled at the high cost.

I own a couple of Triad isolation transformers I don't use much but frankly I want to keep them for now.

However I do have an interesting item. It's a power transformer scavenged from a high end audio receiver. Its main output is about 90 VCT so if reversed (input to 90V winding from variac) should make for a nice isolation transformer. Its size and weight imply a power capability of around 500 Watts. Of course, care must be exercised not to put full power line voltage into the 90 V winding, else saturation and high current spikes may occur. A stop added to the variac to prevent full output could be added.

From its look, it's a toroid. It's fully encapsulated in a drawn cylindrical metal can and there are a couple of low power windings as well.

I see a nice project here - put it in a box with a suitable variac (5A), and a voltmeter and ammeter and fuse/ckt brkr and switch and pilot lamp and outlet. I already have many of the parts in the junk box.

Any comments?


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 18, 2019 1:43 am 
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Joined: Jan Tue 10, 2012 8:39 am
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I just ran a test by exciting the 96 V (sorry error, not 90 V as posted above) from a variac and observing the input current.

As long as the source remains below about 105 Volts the waveform looks good. For short term use, I think 110 V input wouldn't be too much.

Since I already have two transformers I don't use, building this project isn't something I care about. But maybe it will inspire some ideas. Certainly for any ordinary use of an isolation transformer it would suffice.


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 18, 2019 3:08 am 
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Joined: Feb Sat 22, 2014 1:40 am
Posts: 204
You might be able to save some money using two step down transformers wired back to back.
Junk box or bought for this purpose.
120V to 12V connect 12V to 120V. You need to make sure the Primary can handle the current
of the radio. This will have a little extra loss because of the extra core and wire, but shouldn't be a problem.
Especially with a variac, which usually go higher than the line voltage.
It doesn't have to be a 12v secondary either, it can be any reasonable voltage.

Mikek


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 18, 2019 5:11 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
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Location: St. Louis, MO
Darwin1943 wrote:
What I want to do is put an ammeter between the variac and one of the radio plug prongs in order to observe current rise or fluctuation when I slowly turn up the voltage on the variac.


Well, there are plug in ammeters like the Kill A Watt. The problem is that I doubt that their internal electronics would be happy getting less than 120V in. You may have to add an ammeter to your variac yourself.


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 18, 2019 6:40 pm 
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Location: Ohio 45177
I was looking at the Chinese cheap digital AC volt and ammeter combos for mine but then you drop the variac down too much and you go below the minimum operating voltage of the unit. As it samples the voltage it is connected to. What would be needed is one that has separate connections for power and the AC voltage monitor so it can operate from the input to the variac regardless of the variac setting, or just get an analog AC ammeter somewhere and tap it into the circuit. In series with the high side no doubt?

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 Post subject: Re: leads from Variac ?
PostPosted: Nov Tue 19, 2019 2:29 am 
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Joined: Aug Tue 04, 2009 2:15 pm
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Location: Elmira, NY
Variac is a trade name similar to Powerstat.. What you have is a variable transformer. Look for the current rating (amperes) of your Variac, and or VA rating(AC Wattage). 5A is typically good for most tube radios and some TV. 10A and up is better. Wire size is by gauge. NEC is the minimum standard. Check the code book and size your wire per code for the power source.

If you find a variable transformer without a case you might consider building a combination variable source with an isolation transformer (a must for safety to you and your equipment). I built a variable supply/isolation/dim bulb tester in the same enclosure. I wired a switch activated by the wiper of the variable transformer which energizes a relay andVariac only when the wiper is set to zero. This way you don’t forget and energize your Device Under Test At full power when you turn it on. My box has an AC ammeter and an old screw in type fuse at the output before the DUT. I can unscrew the fuse and replace with a light bulb for current limiting.

Search dim-bulb tester in this forum. There are many good designs and discussion.


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Tue 19, 2019 2:43 am 
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Joined: Jul Mon 01, 2019 4:42 pm
Posts: 220
Location: St. Louis, MO
Y2KEDDIE wrote:
Search dim-bulb tester in this forum. There are many good designs and discussion.


I've always wondered about the future of those, considering that incandescent bulbs just became obsolete.


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Wed 20, 2019 12:05 am 
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Joined: Aug Tue 04, 2009 2:15 pm
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Location: Elmira, NY
ZombieElvis wrote:
Y2KEDDIE wrote:
Search dim-bulb tester in this forum. There are many good designs and discussion.


I've always wondered about the future of those, considering that incandescent bulbs just became obsolete.

Seems to be no shortage where I live, Loews, Home Depot, Walmart, etc. I probably have 100 various wattages from. When I replaced house lamps. With LED’s


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 25, 2019 4:09 am 
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Joined: Jul Mon 01, 2019 4:42 pm
Posts: 220
Location: St. Louis, MO
Wait a minute... We still got vacuum tubes out the wazoo and most of those stopped being made decades ago once they became obsolete... Never mind! I'm no longer concerned.


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 25, 2019 9:32 pm 
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Joined: Oct Thu 04, 2018 2:11 pm
Posts: 190
Location: Suburban Chicago
I got a $14 panel meter off the bay for my variac clone. It is rated at 60-300 Vac. The contrast gets poor but it worked down to 30 Vac before I could no longer read it and the value compared well with my Fluke. The voltage did anyway, I did not measure the current but the current and wattage numbers seemed reasonable. I opened it up at work this morning to take a look at the circuit, hoping to find an easy way to separate the power feed for the meter from the sense lines. It should be possible to do that but it was not obvious how to do that. The LDC is a loose piece connected to the PCB with zebra strip connectors. I hope I got all that back together right, I'll find out tonight!


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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 25, 2019 11:16 pm 
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Location: Liberty, Missouri
There probably isn't going to be any "action" in anything below about 70VAC or so, and probably closer to 90. Last time I checked Watts are Watts, assuming I&V are in phase, whether you measure before, or after the Variac. I simply use a Watt meter and watch Watts What for the first couple minutes VERY CAREFULLY.

My FLOATING A.C. source is configured as follows.

Image

I never have used a Dim-Wit tester, although some folks swear by them, and I seldom use a Variac.

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 Post subject: Re: leads from Variac ?
PostPosted: Nov Mon 25, 2019 11:58 pm 
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The problem with installing an ammeter is that you're restricted to a certain range, unless you use a cheap digital one, which I found to be inaccurate measuring small currents. Instead, I use my digital VOM, which can handle micro-amps up to 10 amps AC or DC. I took a cheap power cord and split out the hot side to insulated connectors that I can plug my meter into. I plug the modified power cord into either my variac or isolation transformer, depending on what I'm doing.

BTW, I have tried a Kill-A-Watt and had to get my variac up to 90 volts before it would show anything. By then your radio could be smoking.

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