[Mondial]

Yes, it is perfectly normal for a mercury vapor rectifier like the 82 to have the blue glow between the filament and plate. That is the ionized mercury glowing which results in a constant 15 V voltage drop across the tube, pretty much independent of the DC current draw. The constant drop is important because the Class B output stage draws a varying amount of current with audio signal level.

The RCA AVC circuit is very interesting and is somewhat similar to the amplified AGC of the Apex set you were working on a while back. This one uses a separate plate detector to generate the AVC voltage instead of taking it off the regular audio plate detector of the Apex.