[MrFixr55]

DOPF!!!! I thought of that, but I forgot to suggest that. Zenith did not use metal tubes. Zenith tubes at the time were made by Sylvania, and they had an aquadag coating. They referred them to Metal-Glass. For the 6K7 and 6A8, the RCA Receiving Tube manual shows the tube shell connected to Pin 1 for the metal version of the tube. for the G and GT versions of these tubes, there is no connection for Pin 1. For the 6H6, there is a connection for Pin 1 to a shield on both metal and glass versions.

It may not fix your problem, but make sure that Pin 1 of the 6A8, 6K7 and 6H6 was not used as an interconnect terminal and connect them to chassis ground.

For the heck of it, connect the antenna to the 6A8 grid and let it play. One of your problems is the intermittent nature of the issue. If the problem does not appear, it may be the primary of the antenna transformer intermittently opening.