I did some tests with my two ADAs.

Both work stable at 190V.
The older one starts humming at 185V, the new at 180V.
At 190V the temperature at the voltage regulators for +/-15V is about 15 or 20...

@cgrafx
Thanks for the idea.

The transformer is used as an autotransformer (http://en.wikipedia.org/wiki/Autotransformer) or in german "Spartransformator"...