The 6KD6 is described as a Compactron beam pentode but we would call it a beam tetrode. The design purpose was as a line output valve in colour televisions and thus it is a substantial valve. The anode dissipation is 33 Watts and the maximum positive peak anode voltage is 7,000 Volts. The cathode current is 400 mA with a peak of 1.4 Amps.
The GE logo.
The aligned grids passing flat across the cathode can be seen through the holes in the side of the anode.
The wide glass tube envelope is 38 mm in diameter, and excluding the B12C base pins is 98 mm tall.
Reference: Datasheet. Type 6KD6 was first introduced in 1967.