Solas are very useful devices, a bit heavy, with a required resonating capacitor. Some comments:
1. They are/were available either as approximately square wave outputs (great for rectifiers) or "harmonic neutralized" with reasonably sinusoidal outputs.
2. They are good at rejecting changes in input voltage and load current, but the output varies directly with frequency, so they must be purchased for 50 or 60 Hz use.
3. Back before 1970, the electronics shop at Argonne National Laboratories built most of their vacuum-tube circuits using Sola transformers that combined plate supply and heater supply windings to simplify voltage regulation requirements.