The DH3-91 is a one inch oscilloscope tube with a medium persistence green phosphor. They were made in large numbers from the early 1950s and were used for transmitter monitors as well as miniature oscilloscopes.
The final anode voltage of 1,000 is the maximum and good results can be achieved with 500 - 650 volts. This can be achieved with a simple voltage doubler from the main HT transformer winding. With much lower a3 voltages the image becomes dim. The sensitivity of the electrostatic deflection is a function of the final anode voltage. For Y sensitivity the formula is 95/a3 mm/Volt. The X formula is 110/a3 mm/Volt.
Electrostatic deflection systems are precision devices and here the electron gun is on the left and the Y plates on the right. Between are the anodes all held in glass rods.
The end window is rounded. Flat screens developed later when the glass technology allowed.
The small hole in the gun on the left is where the electron beam exits. Each of the following hollow cylinders also have small central holes.
Here the hole in the final anode can be seen, as can the more sensitive Y plates to the right of the anode. The X plates are obscured by the aquadag colloidal graphite coating.
The end window is 29 mm in diameter and excluding the B8B base pins is 99 mm tall.
Reference: Datasheet. Type DH3-91 was first introduced in 1953.