I have the same phenomenon with the 872's or 575's (whichever happen to be installed) in a homebrew rig. 3 light but one is dark, and the phenomenon stays at that socket. No obvious other issues.. how curious.
Make sure the tube filament pins are making good contact. I have seen it fail due to corrosion of the contacts in the sockets, or sometimes they get sprung to the point there is no tension on the pin.
Make sure you have the same filament voltage on each tube (check it with a voltmeter by actually touching the tube pin itself, not the socket terminals, with the meter probe). Also observe the glow of the filaments to estimate if they are getting equal voltage.
If the filaments are getting proper voltage, try swapping the leads to the plate caps of the tubes. Maybe the plate transformer has gone defective and is no longer putting out equal voltage on each half of the winding under load.
Don k4kyv
