1203HB — Electrostatic Focus Radar Display Tube

The 1203HB (also designated as 12/03 HB) is a 12-inch diameter cathode-ray tube manufactured by Ferranti for radar display applications. It features a metal-backed screen, magnetic deflection, and low-voltage electrostatic focusing. The tube is characterized by its Type 'H' phosphor screen, which provides orange fluorescence and an orange afterglow with very long persistence, making it highly suitable for radar PPI (Plan Position Indicator) displays where target tracks must remain visible between antenna sweeps.

A closely related variant, the 12/03 JB (or 1203JB), shares the same physical dimensions, electrical ratings, and base connections but utilizes a Type 'J' phosphor screen with blue fluorescence, yellow afterglow, and long persistence.

Physical Construction and Screen Characteristics

The 1203HB is constructed with a glass envelope having a maximum overall length of 485 mm (nominally 478 ± 7 mm) and a maximum bulb diameter of 305 ± 2 mm. The useful screen diameter is 265 mm, and the faceplate has a spherical radius of 1000 mm. The neck of the tube has a diameter of 35 +1/-0 mm, with a reference line diameter of 36 mm. The deflection angle is 50 degrees.

Screen and Phosphor

The 1203HB features a metal-backed screen with a Type 'H' phosphor. This phosphor is optimized for radar display, exhibiting:

• Fluorescence: Orange
• Afterglow: Orange
• Persistence: Very long

Because of the nature of the Type 'H' screen material, it is highly susceptible to burning if operated with a stationary or slow-moving electron beam spot. To prevent permanent damage to the phosphor, the tube must not be operated under stationary or slow-scan conditions, even when the beam current is set to a low level.

External Conductive Coating

The tube is equipped with an external conductive coating. The capacitance between the anode and this external coating is approximately 1200 pF, allowing the coating to be utilized as an integrated capacitor for extra-high tension (E.H.T.) smoothing in the high-voltage power supply circuit.

Heater and Base Connections

The 1203HB uses an indirectly heated cathode. The heater is designed for either parallel or series operation.

Heater Specifications

• Heater Voltage: 6.3 V
• Heater Current: 0.3 A

The nominal heater power can be calculated from the rated voltage and current as follows: $\text{Heater Power} = 6.3\text{ V} \times 0.3\text{ A} = 1.89\text{ W}$

When the heater is configured for series-string operation, the surge heater voltage must not exceed 9.5 V r.m.s. To protect the heater filament from damage during startup, a current-limiting device must be incorporated into the circuit to restrict the initial switching surge.

Base and Pinout

The tube utilizes a B12A (Duodecal) base. High-voltage connections to the second and fourth anodes are made via a side contact of the CT8 Cavity Type (also designated as Cap Type C.T. 2). The mounting position for the tube is unrestricted (any position).

The pin connections, viewed from the underside of the base, are as follows:

Electrical Ratings and Operating Limits

The 1203HB operates with high voltages on its accelerating anodes and requires careful adherence to maximum and minimum ratings to ensure reliable operation and prevent internal arcing.

Absolute Maximum and Minimum Ratings

• First Anode ($A_1$) Voltage: 200 V (Min) to 500 V (Max)
• Second and Fourth Anode ($A_2 + A_4$) Voltage: 8 kV (Min) to 15 kV (Max)
• Third Anode ($A_3$) Voltage: -500 V (Max. Negative) to +500 V (Max. Positive)
• Heater-to-Cathode Voltage ($V_{h-k}$): 200 V (Max)
• Grid-to-Cathode Circuit Resistance ($R_{g-k}$): 1.5 MΩ (Max)
• Heater-to-Cathode Circuit Resistance ($R_{h-k}$): 1.0 MΩ (Max)

Typical Operating Conditions

In typical radar display applications, the tube is operated under the following conditions:

• Heater Voltage: 6.3 V
• First Anode ($A_1$) Voltage: 300 V
• Second and Fourth Anode ($A_2 + A_4$) Voltage: 12 kV
• Third Anode ($A_3$) Voltage for Focus: -300 V to +300 V
• Grid Voltage ($V_g$) for Visual Cut-off: -30 V to -70 V

The third anode ($A_3$) serves as the electrostatic focusing electrode. The optimum focus point is achieved at a voltage within the range of -300 V to +300 V relative to the cathode.

Interelectrode Capacitances

The interelectrode capacitances of the 1203HB are low, minimizing the loading on drive circuitry:

• Cathode to all other electrodes ($C_{k-\text{all}}$): < 8 pF
• Grid to all other electrodes ($C_{g-\text{all}}$): < 8 pF
• Anode to external conductive coating ($C_{a-\text{ext. coating}}$): Approximately 1200 pF

Physical Dimensions and Ring Gauge Data

The physical profile of the bulb is precisely defined to facilitate mounting and magnetic deflection yoke alignment. The distance from the center of the screen to various ring gauge diameters along the bulb body is specified as follows:

These dimensions ensure proper seating of the deflection coils and alignment within the radar console housing.

Technical Summary

The 1203HB is a specialized 12-inch radar display tube featuring low-voltage electrostatic focus and 50-degree magnetic deflection. Its Type 'H' metal-backed screen provides orange fluorescence and very long orange afterglow, making it ideal for displaying slow-scan radar sweeps. Operating with a typical accelerating potential of 12 kV on the combined second and fourth anodes, the tube achieves sharp focus using a third anode adjustable between -300 V and +300 V. Housed in a glass envelope with a B12A Duodecal base and a CT8 cavity-type side contact, the 1203HB incorporates an external conductive coating of approximately 1200 pF that serves as an auxiliary E.H.T. smoothing capacitor.