Don't have a "cable box" - I'm on Direct TV. No jacks on the Direct box.

[sydandjudy]

Connect Seinhauser? headphones to new (2/5/18) Samsung 49" TV. No headphone jacks in back.

 

[Turb0Yoda]

More info will be needed. What Headphones, what TV, etc.

That said, most TVs don't have a 3.5" in AFAIK. I've never seen that before.

 

[americanaudiophile]

Most likely your TV has an optical audio output. You can use something like this
https://www.amazon.com/Avantree-Digital-Converter-Headphone-Warranty/dp/B071WLVXN5/ref=sr_1_1_sspa?s=electronics&ie=UTF8&qid=1517937993&sr=1-1-spons&keywords=headphone+amp+with+optical+input&psc=1
Check the TV manual on how to use the optical audio output. You may have to turn it on, turn the TV speakers off, and change the audio output type to PCM.