[sdiy] diy autotansformer?

[JH.]

As you might know, I have replaced a lot of transformers in old synths and
FX to reduce
the mechanical hum, caused by undersized transformers and increased mains
voltage:
When vintage synths were new, we had 220V, now we have 230V.

I still have some synths where replacement would be very difficult (many
secondary
windings etc.).

I think I should try to reduce the mains voltage for these - build a box
with 230V
input and 220V output. As no isolation is needed, an autotransformer (one
winding
with a tap at the right place) would be fine. Actually, there are even
variable
toroidal transformers that work like this.

But I want the voltage fixed (no accidential change when a knob is touched),
and I want it
as unexpensive as possible. If I were to wind it myself (or have it custom
wound), an
autotransformer would certainly be the best solution. But buying a stock
product,
the easiest way might be this:

Use a 230V primary and 12 V secondary transformer, connect pri and sec in
series,
connect the mains to both windings and connect to synth to the primary only.
This configuration could stand 242V at its input (good to have some
"headroom"),
and the output would be 230V * 230/242 = 219V.

Will this work as intended, and be foolproof, or is there some catch?

JH.