As Sandy said, 20 watt 24" tubes should use the same ballasts you have.
Originally Posted by shinn
Rubylith can be found at better art supply stores or probably online, like at Daniel Smith. I tape my neg to the light table and tape a sheet of reubylith over it, trace the areas I want to cut out (for burning) or leave intact (for dodging) and then cut the rubylith (NEVER over the neg in case you cut through the rubylith. You only need to score the rubylith the cut away the red part and a swivelling x-acto knife works great because you can make precise tight turns with it. good luck