I've tried just using a filament transformer in reverse. I needed about 60Vac so I tried using a 12.6V 4W transformer supplied from the 6.3V filament supply. I don't recall the numbers but I didn't get as much voltage out of the thing as I thought I would. I attribute my problems to the following:
Small transformers have poor voltage regulation. Designers take this into account when they design the parts. So, a 12.6V transformer might actually have a 14 or 15V winding that loads down to 12.6V at the spec'd max current. This eats your lunch and sticks you with the tab when you try to run them in reverse.
I suggest you use a 6.3V 1amp transformer and a full wave bridge on the output side to make your DC. There is a temptation to just raise the impedance in the bias adjust circuit, like using 100K pots, but this could lead to other problems depending on the tubes used.