[Tubeglow20]

I have just finished restoring an RCA 1X591 Gladwynn model radio. New caps and resistors and all tubes checked and replaced as needed. Radio plays and sounds great with it's big 8 inch speaker. Rewired the dial lights so that they are in parallel instead of series. The radio comes with 2 Mazda 1490 bulbs wired in series and when the radio is powered up, the inrush current ususally blows one or both 1490's and can take out the 35Z5 tube. The radio might work for a while,  but eventually the bulbs and tube will blow.  Anyway, here's my problem....the 50L6 tube was changed out with a tested 50L6 that checked good with no grid leakage or shorts, (tested on a B&K 606 tester). When the radio is powered up, it hums very loudly so I tried another 50L6, ( tested good also). This one works with no hum. I knew the first one tested good, so I cleaned the tube socket to see if that would help....nope. Now, I carefully turned the chassis sideways the get underneath to probe connections to check for loose connections; used a plastic probe. I VERY CAREFULLY touched pin 5 of the 50L6 tube with my meter probe, the hum completely goes away and the radio plays very loud and clear. Remove the probe tip and the hum is VERY loud.  Found that if I attach a test lead to pin 5 and let it hang out from under the radio, it plays normal. Just don't connect it to anything, don't want to get zorched!!!! Now, if I swap a different 50L6 tube, some will play just fine without the wire hanging from pin 5. Any suggestions about what might be happening?