The DG7-5 is a small three inch electrostatically focused cathode ray tube. This was probably used for an oscilloscope type instrument. The phosphor is green and this tube came with different colour options for a range of applications.
It is typical of these small oscilloscope tubes that the final anode voltage is of the order of 800 Volts. Normally the final anode is held at + 200 V and the cathode and heater hang 600 V below ground. This arrangement has advantages for driving the deflection plates.
The close-up of the tube neck showing the electron gun on the right and on the left the flaring Y plates are just visible.
The front of the CRT showing the phosphor creeping round the edge of the envelope, together with the internal colloidal graphite coating and the Mullard logo. The screen is 70 mm diameter.
See alsoCV5171 for another in this family of tubes and DG7-32 for the replacement improved low voltage oscilloscope tube.
The end window envelope is 32 mm in diameter, and excluding the B9G base pins is 138 mm tall.
Reference: Data-sheet. Type DG7-5 was first introduced in 1950.