Thanks for that !
Good to know the procedure isn't accepted on a fully ubiquitous level .
FYI, I searched ( Variac ) briefly over at AudioKarma & DIY ( solidstate ) and from my quick perusals it seems that a Variac is typically used "hand-in-glove" with the ( current limiting ) light-bulb .
Some manufacturers ( such as Adcom ) specifically mention ( in their service manuals ) not to Variac .