Actually, you seem to me to have diagnosed your problem. Sounds like one of your tubes died. A tube checker would tell you something more; if you don't have access to one, doing your Voltage readings may tell you more.
Bottom line, though, is that if your problem follows the tube, that's a pretty clear indication that you've got a bad tube. A certain amount of "infant mortality" in tubes does exist, I'm afraid. To be doubly sure, running through your Voltage checks again would help you to make sure that nothing caused the tube to fail, beyond some bad luck.
