The 6AS7GA is a double triode with a very low AC anode resistance. Electrically the GA variant is identical to the G version. The difference is the tubular envelope with a glass foot base in place of the older pinch construction.
The AC anode resistance is 280 Ohms. The literature gives the original design purpose as a series pass element for stabilised power supplies.
With a pair of U channel anodes for each triode, one per side of the flat cathode, the construction closely mirrors other series pass valves.
The central cathode and grid with anode sections on either side and bridged together at the top.
Looking into the flat based U channel anode. The grid winding extends wider than the anode.
The wide glass tube envelope is 39 mm in diameter and, excluding the IO base pins, is 88 mm tall.
Reference: Datasheet. Type 6AS7GA was first introduced in 1955.