Not saying this is the cause
But I suggest you Always tap and giggle vacuum tubes in the tester incase they have intermittent issues
Especially old used ones
Often tubes that had intermittent issues but were still “good” were kept because ya never know when you need one for a tv or home radio
I had two with intermittent issues just the other day
One jumped from good to bad drastically with tapping and the other had a mild filament short that could be persuaded to clear itself.
-s
