This is a project that has been in the making for a long time. I show how to make a variable AC voltage supply with a Variac and isolation transformer, a metal box plus some switches and a contactor. A variable AC supply is very useful to bring up valve equipment slowly as well as to provide a supply for other projects requiring lower voltage AC. A variac is basically a variable autotransformer which is inherently referenced to mains so cannot be considered as a safety device by itself so the use of an isolation transformer is almost an essential item in the workshop of anyone who is dealing with vintage or just unknown history electrical items. An isolation transformer is a 1:1 transformer that separates the line supply from your domestic service to a completely non referenced to earth mains supply. A contactor is a relay but a more industrial version. You can search the web and YouTube to see the contactor vs relay differences in detail. Big Clive has a video on his channel taking a contactor to bits. https://youtu.be/jqNnWa0CHw0
You are usually limited at home by the physical size of a larger variac and isolation transformer but if you do have the space then the larger the better. The reason for that is another safety one, if you have something rated quite high then you are unlikely to stress your supply gear. You wouldn't buy a scooter to cruise the highways but a big engine car which is just ticking along rather than screaming its heart out so why do the same on your workbench? I think I can safely say that I will be unlikely to stress this box of tricks!
Although I don't do this as a screw A into B type build, there is more than enough information to show you how to make a variable AC voltage supply of your own.
Finally, I must thank Christian, who has the very excellent channel @AllTheGearNoIdea for sourcing the metal enclosure and drilling most of the holes which saved me a lot of work as well as is absolutely amazing 3d cad skills for making sure it would all fit! Why not check him out... https://www.youtube.com/channel/UCvBpiuUUnErJlNBm6DWb3Ww
I will put the original schematic on the net for you to download right here!! https://www.dropbox.com/s/suxdxazvblwjbfa/GEORGE-VARIAC-REV-1.pdf?dl=0