Good Isolation transformer/Variac...

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Good Isolation transformer/Variac...

Post by Carterofmars on Wed Feb 08, 2012 11:45 pm

Can I get recommendations for a good Isolation Transformer/Variac that won't break the bank?

I imagine these are super important when working with old radios.

Thanks.

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Re: Good Isolation transformer/Variac...

Post by Bill Cahill on Thu Feb 09, 2012 3:44 am

E bay often has a number of variacs for sale. Many high priced, but, others reasonable.
Depending on how far you plan on going, my reccommendation is to get a good one rated at 10A maximum, 130 ac maximum.
I paid 85.00 for my last one two years ago.
Bill Cahill

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Re: Good Isolation transformer/Variac...

Post by sprman on Wed Mar 07, 2012 7:54 pm

Hi 10 am should be good ..Unless you use it on a 15 tube plus set then you need a bigger amp rating. Some have a small meter in the top so you can see what voltage you have it set at.I got a 20 amp Variac at a swap fest for 45$ works great to.Sprman

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Re: Good Isolation transformer/Variac...

Post by westcoastjohn on Thu Mar 08, 2012 12:19 pm

I bought 2 big 500/1000 va isolation transformers at an army surplus flea market, $10 each. I traded vacuum tubes for my variac, which is not isolated, so it sits on top of the big isolation transformer. Traded the other off.

A home made isolation transformer/autotransformer is easy to build. Find two large as possible power transformers with identical secondaries, that is, they each have a 25 volt sec for example. Salvage them from old stereo tuners, or TV's. If the tuner is heavy, it's got a big tranny in it. If you hook those secondary winding together, you have isolated 120 vac out of the primary windings of the opposite end. A mismatch produces a slightly higher or lower output.
The other windings can provide lower voltages, but they may be too low to be useful for powering up a radio. You can step down the output by connecting a secondary from tranny 1 to one with a longer winding (higher sec voltage rating). If you have a 35 volt sec on T1 and a 40 volt sec on T2, you've got a stepdown isolation transformer.
You might need to add a third transformer to connect to the first to put out 80 or 90 volts isolated. That's enough to light up the tubes, but only start to melt the wax on the bad caps. :>)

I'm a little rusty on this stuff, so feel free to correct what I just posted if I got my terminals crossed.

Do you know the razor-only outlets for older bathrooms? Those are 20 va isolation transformers. I've installed them in plastic table radios for safety, when I want to plug other devices in. One of those on the bench will power a standard AA5. NFG for the bigger sets, but the big sets have power transformers anyway, so isolation is not so necessary.

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Re: Good Isolation transformer/Variac...

Post by sprman on Fri Mar 09, 2012 12:19 pm

Hi cool tips there.Sprman

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