Dang, that's a puzzler for sure
. In the vid, there was definitely a 60hz component mimicking B+ ripple (hum) from a bad filter cap. But it wasn't that, obviously.
Lacking a cap tester, have you checked the old cap for leakage with an ohmeter? A good cap will show infinite resistance on the highest R range (after a brief charge-up blip).
Any leakage might be a clue to what was going on. If the old cap's good, maybe the problem was a tin whisker.
(This is not a good test for electrolytic caps though, as good ones will always show a small amount of leakage.)
Edit. .. Since one end of C169 goes to the cathode of the 6AC7, if something was allowing 6V AC heater voltage to get onto the cathode, it could certainly cause the problem