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 discolouration of the envelope is a sign of a hard service life. Looking through the holes in the end of the anode the two aligned grids can be seen to pass flat across the cathode.
The anode is made of two nickel sheets that are stitched together.
The lower components showing a rectangular cathode tube and copper supports for the screen grid.
The top components showing a rectangular cathode tube and heat sink fins fitted to the screen grid supports.
The wide glass tube envelope is 38 mm in diameter, and excluding the B12C base pins is 109 mm tall.
Reference: Data-sheet. Type 6KD6 was first introduced in 1967.