1208HM — Flat-Faced Radar Display Tube

The 12/08 HM (also designated as 1208HM) is a 12-inch diameter, flat-faced cathode-ray tube designed primarily for Plan Position Indicator (P.P.I.) radar display applications. It is engineered as an improved, higher-resolution successor to the CV.429 industrial standard. The tube utilizes magnetic focusing and magnetic deflection to control its electron beam, delivering high-precision line widths suitable for detailed radar imaging.

Screen and Phosphor Characteristics

The 12/08HM features a metal-backed screen utilizing a Type 'H' phosphor. This phosphor is characterized by:

• Fluorescence: Orange
• Afterglow: Orange
• Persistence: Long

The Type 'H' screen is composed of magnesium fluoride. Because of the chemical and physical properties of this material, the screen is highly susceptible to burning. Screen burn can occur even at very low values of beam current if the electron beam spot remains stationary or is allowed to move slowly across the face. Consequently, scanning failure protection and beam-blanking circuits are critical design requirements when operating this tube.

The 12/08HM is closely related to the 12/08 JM, which is identical in its mechanical and electrical specifications but employs a Type 'J' phosphor. The Type 'J' screen exhibits blue fluorescence, a yellow afterglow, and long persistence, but does not share the same extreme vulnerability to low-current stationary spot burns associated with the magnesium fluoride Type 'H' screen.

Physical Dimensions and Mechanical Construction

The tube is constructed with a large-diameter bulb, a flat faceplate, and a narrow neck designed to accommodate external magnetic deflection and focusing coils.

The key physical dimensions of the envelope are:

• Maximum Overall Length: 520 mm (with a nominal dimension of 513 ± 7 mm)
• Nominal Neck Diameter: 35 mm (specifically 35 mm +0.5/-1.0 mm)
• Nominal Face Diameter (D): 305 ± 2 mm (12.000 ± 0.079 inches)
• Minimum Useful Screen Diameter (E): 250 mm (9.842 inches)
• Faceplate Spherical Radius (R1): 1000 mm (39.370 inches)
• Anode Cap: Recessed cavity type, conforming to the CT8 standard.
• Base: B12A Duodecal base.

The reference line is established at a diameter of 36 mm (1.417 inches), which corresponds to the 36 mm Ring Gauge Position.

Base Connections and Pinout

The B12A Duodecal base provides electrical connections to the internal gun structure. The pin configuration, viewed from the underside of the base, is detailed below:

Heater and Electrical Ratings

The 12/08HM contains an indirectly heated cathode. The heater is designed for AC or DC operation.

Heater Specifications

• Heater Voltage: 6.3 V
• Heater Current: 0.3 A
• Calculated Heater Power: 1.89 W

Absolute Maximum and Minimum Ratings

To ensure reliable operation and prevent premature degradation of the cathode and screen, the operating parameters must be maintained within the following limits:

Typical Operating Conditions

Under typical operating conditions, the tube is configured to balance brightness, focus quality, and resolution. The standard operating parameters are:

• 1st Anode Voltage ($V_{a1}$): 300 V
• 2nd Anode Voltage ($V_{a2}$): 15 kV
• Grid Voltage ($V_g$) for Visual Cut-off: -60 V
• Average Grid Drive for 50 µA Beam Current: 20 V
• Line Width (at $I_b = 50$ µA): 0.25 mm

Focusing Coil Requirements

Focusing is achieved via an external magnetic focus coil. Under typical operating conditions, the focus coil must provide approximately 640 ampere-turns.

For optimum focus quality, the focus coil must be positioned precisely along the neck. The center of the focus coil gap should be located approximately 120 mm from the reference line (which is defined by the 36 mm Ring Gauge Position).

Interelectrode Capacitances

The internal electrode capacitances are kept low to prevent signal distortion and ensure responsive grid modulation:

• Cathode to all other electrodes ($C_{k-\text{all}}$): < 8.0 pF
• Grid to all other electrodes ($C_{g-\text{all}}$): < 8.0 pF

Technical Summary

The 12/08 HM is a specialized 12-inch flat-faced radar display tube featuring magnetic focus, magnetic deflection, and a B12A Duodecal base. It operates with a 6.3 V, 0.3 A heater and is optimized for high-resolution P.P.I. applications. The tube features a Type 'H' magnesium fluoride screen with orange fluorescence and long-persistence orange afterglow, which requires careful circuit design to prevent screen burn. With a typical operating second anode voltage of 15 kV and a first anode voltage of 300 V, it achieves a narrow line width of 0.25 mm at a beam current of 50 µA, utilizing an external focus coil of approximately 640 ampere-turns.