The 6D6 is a variable μ pentode for AGC controlled amplifiers at RF and IF in superhet radio receivers.
Electrically the 6D6 (UX6 base) is identical to the Type 58 (UX6 base) and the 6U7G.
The construction is as a classic pentode. The three wire grids are helical and of different pitches. The suppressor is more open than the screen grid. The control grid will be wound with a variable pitch in order to generate the variable μ characteristic. The oxide coated cathode tube shows as a white tube at the centre of the grids.
The grids seen at right angles to the support wires.
The classic envelope is 38 mm in diameter, and excluding the UX6 base pins is 105 mm tall.
References: Datasheet, 1040 & 1043. Type 6D6 was first introduced in 1936.