I investigated around the tube sockets, the filament wiring and the choke no paths to ground found.
I removed the tubes and checked for shorts with an Ohmmeter all 4 tubes tested good, no shorts between any pins or the anode (except the filament pins of course). The negative moving Ig suggested a filament/cathode-grid short (although the initial upscale movement is confusing if that were the case). At this point, the amp was putting out about 100W. The Ip would move up nicely, but I stopped at about 200 mA when I saw the Ig start to drop. Upon increasing the drive power, I noticed that the gird meter would move upscale but very slightly, maybe one or two needle widths and then start moving downscale and below zero. The DBC seems to work fine no idle current with no RF and about 80 mA of plate current with a little bit of RF. The unit is cosmetically fine and the inside was a little dusty, which I brushed and then blew out the fan was not dusty, so I'm assuming it was not operated in a dusty environment, but as suggested, sat on a shelf for a while. As best as I can tell, the unit was manf'd in 2004 as I found a QA technician's (an assumption) initials on the red HV label with a July 2004 date. The ham assisting the SK's family told me the SK bought the amp, fired it up, reported it would not key and set it on a shelf. As such, the history, type of usage, hours, etc. I recently bought an AL-572 from a SK's estate.