If it is a OneHungLow brand the 50VA would be an optimistic rating, one the best one they made first, and before the "Clost Leduction" came in and used fewer thicker laminates, less turns of copper and thinner wire. I would try, but look at the temperature rise. If it goes above 90C then it is overloaded. I have met some that did that even with no load. Some small sub 5W units would draw 10W at full load, and still the same at near no load.