The GEC E4205/B/7 and the Mullard ECR30 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.
The GEC name on the wrap-round paper label.
In case it was not obvious - this is a Cathode-Ray Tube.
This reads ID CRT 53. The tube face is domed to resist external air pressure and is 69 mm in diameter.
The deflection plates flare in he direction of the screen. The black internal coating is colloidal graphite and forms the final anode.
The base pins look to be pristine and unused.
The outer box shows clear signs of water damage. The box is 105 mm square and 260 mm tall.
The thick card tube protection is stapled together to keep the tube in the centre of the box volume.
The other side of the inner protective insert.
The thick card as it closes over the tube face doubles as a handle to withdraw the tube from the outer box.
The end window envelope is 41 mm in diameter, and excluding the B12B base pins is 179 mm long.
