Sorry I forget that not everyone has boxes of these sort of devices laying around from scrapping work. Cheap for me.
However that said a good used 100 VA variac can usually be had for around $25. Given that it would be running in an autotransformer type application he wouldn't need a very big one to drive his 150 VA transformer.
All done and cost nowt.
A variac is something I dont have, not scrapped anyhting that old.
My mig welder uses a variac to control the wirefeed motor, but thats staying put.