Scott Dorsey ) writes:
Primary 20,000 ohms single plate 10MA DC
Secondary 500 ohms

Ahh, wait, you DID give impedances!

What you want is a 20K to 600 ohm line output transformer. I bet a nickel
Hammond makes one.

There was a time when everyone made them. If you were making a phasing
rig, you'd need those to interface with the B&W phasing network (the
ones that came in a package like a metal tube). The B&W seemed to
be the most common, but I think the other similar networks (Central
Electronics was another manufacturer) used the same sorts of transformers.

So look for a phasing rig, and the transformers will follow.

Michael VE2BVW