Hi Damien.
My 16-inch Zenith porthole and 1948 teleking is running with direct input. When it works it works great. To make this work properly you need a video source of good quality. It is easy to get disturbance on the video if your media device has a cheap power supply and so on. My 16 Zenith using a media device with SD card and USB that just running in loop with old movies and commercials. To get a good result I had to add some ferrite core filters on media power supply and a galvanic isolator on the video AV connection. I leave you a link to a link to Antiqueradios for directions how to connect to the video amplifier tube and the audio connection. See also the link to my sets on youtube if you want to see my sets in use. Good luck
Best regards Kristian
https://antiqueradio.org/A-V_AdapterForVintageTVs.htm
https://www.youtube.com/channel/UCZm...yZtbykPkhUzCdg