The DF703 is an electrometer pentode that would be used where an extremely high input impedance is required such as in pH meters. Operation is at low voltage with very low current.
The beam plates are split with each side connected to one side of the filament.
The bright anode with gettering into the top seal.
The split beam plates can be seen to go to either side of the filament.
The screen grid seen under the microscope.
Higher magnification, the screen grid wires are on the left and the control grid support is the lower horizontal rod.
The screen grid wires are the blur to the left and the fine wire secured to the support below is the control grid. Naturally the thickness of the glass has an effect on image quality at this magnification.
The thin glass tube envelope measures 9.74 x 6.8 mm and, excluding the base wires, is 36 mm tall.
Reference: Data-sheet. Type DF703 was first introduced in 1963.