Thanks for the suggestions.
I've checked and rechecked the tubes for leakage. I could try swapping them all out.
This is a hot chassis set, not floating. There are a few ceramic caps along the filament string to the chassis but I'd expect they'd pop if they were leaky or shorted.
You can get the Sams for free here:
https://www.earlytelevision.org/pdf/...sams_61-16.pdf