This exhibit is unmarked and has probably lost the paper identification that it once carried. It looks to be a GEC E4205/B/7 and without the paper covering the internal construction is observable.
As with the Mullard ECR30, these are small electrostatically deflected cathode ray tubes with a medium persistence green phosphor. Both were designed for oscilloscope use.
As with most of the CRT's from this period the heater-cathode insulation is not good and the maximum voltage differential between them is limited to 50 Volts. In practice with the cathode at about -800 Volts, the heater would be fed from its own transformer winding.
The a1,a3 voltage is given as 800 volts and the focus anode a2 would be held at about 135 Volts.
In the centre are the two hollow cylinders that form the focus anode. The two parts are coupled together as can be seen. In the right the cathode screen can be seen with a central hole for the emission of electrons. Passing into the bell on the left are the flared X deflection plates. To their right are the Y plates seen here only as a bright rectangle.
Here with the tube rotated through 90° the smaller Y plates can be seen. There are simple folded pieces of metal that are held on the longitudinal support rods. All along the supports are mica discs to keep the electrodes properly aligned.
The end window is 67 mm in diameter and domed for strength. The glass technology required to make a strong flat face-plate was not available in the 1940s.
The pinch is on the left. The electron lens components can be seen fixed to the mica discs.
The end window envelope is 40 mm in diameter, and excluding the B12B base pins is 180 mm long.
