21YP4 vs 21ZP4
 

I have 3 nearly identical mid-50s Philco's, 2 use the 21YP4, 1 uses the 21ZP4 (B, IIRC) Now, I could use a good ZP4, as could another member here, but how hard would it be to swap in a YP4 in place? Z is magnetic focus, Y is electrostatic. Other specs look the same. I would think this would require a ciruit change, and if its not too complex I might be willing to try it, using the Y and freeing up the Z for someone else.