A Rotary Transformer is really an electric motor and a dynamo on the same rotating shaft. The motor is powered by a low DC voltage of say 12-24Volts but at quite a large current, often many tens of amps.
The dynamo windings are designed to produce the high voltage needed by old valve equipment and a typical output voltage would be 600 volts at 200mA. Although the output is DC it does need filtering with capacitors and chokes to remove spikes and transients produced by the commutator brushes.