1959 Predicta Tandem - spot suppression?

[DaveWM]

I had an old 27" short neck Maggie that used to do that. IIRC I concocted some RC combo on the grid or cathode that would charge up and then after shut down would discharge slowly but keep the tube fully cut off. Pretty simple from what I recall. Pretty sure the reason for the spot is a tube that is way past its prime from reading about it.