[R-390] Bucking Transformer / Autotransformer Question

[Ken]

Hi Barry and list,

I have a transformer with a 12.6v winding from an old domestic tube receiver. It’s wired  up as an auto-transformer and works beautifully. I use it to feed an R-390 and it runs completely cold so I do believe it is very efficient. 

Regards 
Ken 

> El 16 jul 2020, a las 16:02, Larry H <larry41gm at gmail.com> escribió:
> 
> Barry,  I posted an article on this subject on 10/4/2016 and here is the
> link:  http://mailman.qth.net/pipermail/r-390/2016-October/056213.html .
> You will find it helpful.  Unfortunately, the link in it to the article by
> Mr. Elliot is nolonger valid, but here is a valid link to it:
> https://sound-au.com/articles/buck-xfmr.htm .  He claims that the
> autotransformer configuration is more efficient, and I agree with him.
> 
> Regards, Larry
> 
>> On Wed, Jul 15, 2020 at 12:03 PM Barry <n4buq at knology.net> wrote:
>> 
>> I'd like to employ a bucking transformer for some of my equipment -
>> particularly a Fluke 760A calibrator.  I have a pretty hefty old
>> transformer with two, separate 10V secondaries that can deliver 1.0A and
>> 10.0A.  It also has a tapped primary with taps at 115V, 120V, 126V, and
>> 132V (I think I'm remembering all those correctly).
>> 
>> I was thinking that since my mains voltage is right around 126V, then I
>> could connect that to the mains and use the 115V tap in an autotransformer
>> configuration.  I'm unsure, though, whether that is the best way to do this
>> and whether the primary windings will have to carry more current than it's
>> capable of supplying.
>> 
>> While I could connect one of the secondaries as a standard bucking
>> configuration, I was thinking the autotransformer configuration might be
>> better/simpler.
>> 
>> Any thoughts on this approach?  I'm just unsure of how much current that
>> small section of primary would need to supply.  I think it would be the
>> total current drawn by the load but I might be thinking of that incorrectly
>> so thought I'd ask.
>> 
>> Thanks,
>> Barry - N4BUQ
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