12SP7B — Radar Indicator Cathode Ray Tube

The 12SP7B is a 12-inch, direct-viewed, round glass cathode ray tube designed primarily for radar indicator applications. It features magnetic deflection and magnetic focus, utilizing a spherical faceplate made of gray filter glass. The tube is equipped with an aluminized screen and a long-persistence P7 phosphor, which provides high-contrast displays suitable for radar systems.

Screen and Optical Characteristics

The 12SP7B is characterized by its specialized screen and faceplate design, optimized for radar indicator service:

• Phosphor: The tube uses a P7 phosphor, which exhibits blue-white fluorescence and yellow phosphorescence. It is a long-persistence phosphor designed to retain images for extended observation.
• Screen Type: Unlike the non-aluminized 12SP7, the 12SP7B features an aluminized screen, which enhances brightness and display efficiency.
• Faceplate: The spherical faceplate is constructed of gray filter glass with an approximate light transmittance of 74 percent, improving contrast under ambient light conditions.
• Useful Screen Area: The tube provides a minimum useful screen diameter of 11 inches.

Mechanical Specifications and Basing

The physical structure and electrical connections of the 12SP7B conform to standard industrial specifications for cathode ray tubes of its class:

• Bulb: J99 1/2 A or equivalent.
• Bulb Contact: A recessed small cavity cap (J1-21) is located on the side of the bulb for the high-voltage anode connection.
• Base: Small shell Dudecal 5-pin base (B5-57).
• Basing Configuration: Labeled as 12D, which provides the following pinout connections (viewed from the bottom of the base):
  • Pin 1: Heater
  • Pin 2: Grid No. 1
  • Pin 10: Grid No. 2
  • Pin 11: Cathode
  • Pin 12: Heater
  • Recessed Cavity Cap (A): Anode
• Physical Dimensions:
  • Overall Length: 18 3/4 ± 3/8 inches
  • Greatest Bulb Diameter: 12 7/16 ± 1/8 inches
  • Neck Diameter: 1 7/16 ± 1/16 inches
• Deflection Angle: Approximately 54 degrees.
• Mechanical Alignment: Base pin position No. 3 is aligned with the anode contact (J1-21) within a tolerance of 10 degrees.

Electrical Ratings and Capacitances

The 12SP7B operates with a standard heater supply and has specific absolute maximum ratings that must be observed to prevent damage.

Heater and Capacitance Data

• Heater Voltage: 6.3 Volts
• Heater Current: 0.6 ± 10% Ampere
• Direct Interelectrode Capacitances:
  • Cathode to all other electrodes: Approximately 5 µµf
  • Grid No. 1 to all other electrodes: Approximately 6 µµf

Absolute Maximum Ratings

The absolute maximum ratings for the 12SP7B are defined as follows:

• Anode Voltage: 11,000 Volts dc
• Grid No. 2 Voltage: 550 Volts dc
• Grid No. 1 Voltage:
  • Negative bias value: 140 Volts dc
  • Positive bias value: 0 Volts dc
  • Positive peak value: 2 Volts
• Peak Heater-Cathode Voltage:
  • Heater negative with respect to cathode: 200 Volts
  • Heater positive with respect to cathode: 200 Volts

These ratings differ from related variants; for example, the 12SP7 has a lower Grid No. 2 maximum rating of 450 Volts and lower heater-cathode voltage limits of 150 Volts, while the 12SP7D has higher maximum ratings across several parameters.

Typical Operating Conditions and Circuit Values

For typical operation, the 12SP7B is designed to run under the following electrical conditions:

• Anode Voltage: 9,000 Volts dc (Note: Brightness and focus quality decrease with decreasing anode voltage; operation should generally not fall below 4,000 Volts).
• Grid No. 2 Voltage: 250 Volts dc
• Grid No. 1 Voltage for Cutoff: -27 to -63 Volts dc (defined as the voltage required for visual extinction of an undeflected focused spot).
• Focusing Coil Current: Approximately 110 mA dc. This value is specified using a RETMA focusing coil No. 106 positioned at a distance of 3 1/4 inches from the yoke reference line to the center of the air gap.
• Spot Position: The center of the undeflected, unfocused spot will fall within a circle of 20 mm radius concentric with the tube face.
• Maximum Grid No. 1 Circuit Resistance: 1.5 Megohms.

Safety and Operational Considerations

When operating the 12SP7B, appropriate safety precautions must be taken regarding high voltage and radiation:

• X-Ray Radiation: X-ray radiation shielding may be necessary to protect against potential personal injury from prolonged exposure at close range if the tube is operated at voltages higher than its maximum rated anode voltage of 11,000 Volts (or 16,000 Volts, whichever is less). Since the maximum rating of the 12SP7B is 11,000 Volts, shielding considerations apply if this limit is exceeded.

Technical Summary

The 12SP7B is a magnetic-deflection, magnetic-focus radar indicator cathode ray tube featuring a 12-inch round glass envelope, a spherical gray filter glass faceplate, and an aluminized screen. Utilizing the long-persistence P7 phosphor, it is optimized for radar display applications requiring high contrast and image retention. Operating with a 6.3 V, 0.6 A heater, the tube supports an absolute maximum anode voltage of 11,000 V dc and a maximum Grid No. 2 voltage of 550 V dc. Its physical design includes a small shell Dudecal 5-pin base and a recessed cavity cap for high-voltage anode connection, ensuring reliable integration into specialized indicator circuits.