The cathode is on both pins 4 and 11 on the 6HE5. You count pins clockwise from the wide indexing space between the pins looking into the bottom end of the tube. Look for an electrolytic cap from here to chassis ground. Positive end will go towards the cathode of the tube. It could be one of the sections in a combo can capacitor. I am at a complete loss as I don't have a schematic of your set. The circuit might not even be like I'm thinking. I'm only an amateur with extremely limited experience with tube color TV circuits, mostly RCA at that. My biggest claim to fame is that I got one of my vintage sets working well enough to make my avatar.
Let us know what kind of cap it is and then I can suggest where to get one.
Rob