Hi, Vacuumlanders.
I've finally sorted out my workshop bench AC supply. In an earlier
post I described a fixed voltage-bucking transformer intended to reduce
the typical 122 VAC where I live to a more amenable 110-115 VAC for
vintage radios and tube gear. Well, it dropped too many volts! The 12
VAC transformer to hand took the nominal voltage down to 108 or so -
not fatal, but I wanted to do a bit better. The solution was to set up
a system wherein a small variac controls the bucking transformer
primary voltage.

I had a 250 VA industrial power transformer to hand with 110, 120, 130
VAC tapped primary and a 14 VAC, 14 amp secondary (plus some smaller LV
windings.) I connected the secondary in series with line-in (from the
mains) and the line-out to the bench via two Hammond 1:1 isolating
transformers, each to its own power bar. The primary (at the 120 VAC
tap presently) goes to a small 0 - 120 VAC variac.

Now, with 120 VAC in, I can adjust the bucking voltage from zero (a
nominal 120 VAC out) to 14 VAC (nominal 106 VAC out) by setting the
variac from 0 % to 100%. The load (before the Hammond transformers,
only!) could be quite high, up to around 1.5 KVA since the series
winding is designed to take the high current and the variac would only
have to supply about 1.6 amps maximum.

I've designed the system to plug together with 3-pin cables and
standard 120 VAC sockets so that, if needed, I can re-deploy all the
parts. It works well. The only inconvenience found so far is that I
don't have a fixed AC voltmeter on the output so it needs a quick
check with my DMM so set up each radio test, but just leaving the
variac at 50% is quite close.

The variac is still available for the slow powering-up of vintage gear,
although I generally use a second one with a voltmeter on it for that
purpose.

I have one minor problem to solve. A couple of my vintage radios are
elsewhere in the house and each needs a dedicated 110-115 VAC supply.
I plan to cannibalize a couple of wall-wart transformers (6 to 9 VAC
secondary when I can find them) to use as fixed bucking or
autotransformers in the line.

I trust the above is clear - any questions, email me.

Cheers,
Roger