[Mondial]

I really doubt there should be an 81 tube installed because it is a half wave rectifier and it was usually used in pairs. Most likely there should be an 80 used instead. 

How many terminals are connected on the rectifier socket? For an 81 there should be only three terminals connected with wires from the transformer, with one vacant. If the socket was wired for an 80 all four terminals will be used.

Are you using an 81 or 83 rectifier? It looks like an 81 in the photo. The 83 never would have been originally installed as it first was manufactured in 1933.