The one-trimmer control board above adjusted voltage for both 2A3 and 300B at the same time. No way to adjust voltage for one without effecting the other.
I assumed that it should be possible to set voltages for one tube and have the other unaffected. But as my brain isn’t as fast as the two comparators in LM2903 I made a test board with seven trimmers.
After some trimming back and forth I found one trimmer that adjusted voltage for both tubes and an another that just affected 300B,
So, next is a two-trimmer control board, same size as the one-trimmer. If you don’t understand how a circuit works, the trial-and-error method might help you.