I have a Naval electronics training manual that describes Selenium rectifiers as being 65-85% efficient. Not sure, but I thought it might mean that 15-35% of the applied voltage is dropped across them. It would be an easy fix if there were only about a one volt drop per plate: it has five 1x1 inch plates. I could then replace with a modern silicon diode as you suggested.

Do let me know what you measure across the stack in the TransOceanic.

Thanks,

Bob