[NostalgiaRadioTime]

Well after all of that hair tugging and beating my head against the wall it turns out it was the (blankety blank) 50A5 tube after all! I had two different used tubes in the socket and both had tested good on my tube tester. I then found an NOS tube in the box I forgot I had and when I popped that one in the voltage came down in line, albeit a few volts low. But all tubes were consistent in the few volts low and most importantly the voltage stayed stable...as did the dial light brightness and the volume / clarity of the radio. I guess both of those used tubes had the same internal short that my tube tester didn't detect.

So my thanks to all for your help. Bill and 45Phicodon, you pointed my grey matter in the right direction!