Antenna connection on DuMont TV

I'm finally starting to look at my DuMont closely and I noticed that the antenna connected through an RCA type plug instead of twin screw-on terminals. Is this still a 300ohm connection? And in any case, how would I connect a 75ohm lead? Center to Center? or does it need a conversion first?

Also, I have in hand a variac from ebay and I'm wondering if I should try bringing it up slowly before I start recapping so I can see if I get anything on the picture tube. If I have some bad components do I run the risk of shorting something else out if I do this?