Way back in my sailor days I used to work on a lot of gear that had tubes. Open heater circuits are the number one failure mode of tubes. The heaters are always running because the tubes won't work without them, so even if the tube isn't being used the heater is running, so that part fails most commonly. If it ain't glowing, it's bad, lol. That is also why tube radios don;t turn on instantly, till the tubes heat up they won't conduct.
Now granted there are tubes out there that you cannot see the heater filaments, but those aren't used in our stuff.
You did good.