12ABP7 — Electrostatic-Focus Magnetic-Deflection Cathode-Ray Tube

A 12-inch, round glass, direct-viewed cathode-ray tube designed primarily for radar indicator applications, featuring a low-voltage electrostatic focusing system and high-resolution electron gun.

Overview

The 12ABP7 is a specialized 12-inch cathode-ray tube (CRT) engineered for radar display systems. It incorporates a low-voltage electrostatic focus lens and a magnetic deflection system. The electrostatic focus lens is designed to operate at or near cathode potential, which provides substantially automatic focus that remains stable and independent of variations in the accelerator (or collector) voltage. The tube is equipped with a high-resolution electron gun and a spherical faceplate made of gray filter glass, which provides a light transmittance of approximately 74 percent to enhance contrast under ambient light.

The 12ABP7A is a direct variant of the 12ABP7. It is identical in all electrical and mechanical characteristics except that it utilizes an aluminized (metallized) screen. This aluminized layer provides greater light output, improved image contrast, and minimizes screen charging effects during operation.

The underlying design of the 12ABP- family also supports other phosphor configurations, such as the 12ABP14 (and its aluminized counterpart 12ABP14A) and the 12ABP19 (and its aluminized counterpart 12ABP19A), to meet different persistence and spectral requirements.

Phosphor and Screen Characteristics

The 12ABP- tube family is available with three distinct phosphor types, each tailored to specific radar and indicator requirements:

• P7 (12ABP7 / 12ABP7A): Features a blue fluorescence and a yellow phosphorescence. It provides a long persistence, making it highly suitable for slow-scan radar applications where target trails must remain visible between antenna sweeps.
• P14 (12ABP14 / 12ABP14A): Features a blue (or purple) fluorescence and an orange (or yellow) phosphorescence. It provides a medium-long persistence.
• P19 (12ABP19 / 12ABP19A): Features an orange fluorescence and an orange phosphorescence. It provides a long persistence.

The "A" suffix designates the presence of a metallized backing on the phosphor screen. This aluminization reflects backward-emitted light toward the viewer, increasing the useful light output and preventing the accumulation of negative electrostatic charges on the screen surface.

Mechanical Specifications and Physical Dimensions

The physical envelope of the 12ABP7 is a round glass bulb with a spherical faceplate. The mechanical dimensions and installation parameters are standardized as follows:

Bulb Contact and Alignment

The J1-21 recessed small cavity cap serves as the electrical connection for the high-voltage collector (anode). To facilitate proper orientation of the deflection yoke and external components, the bulb contact is aligned such that the J1-21 contact aligns with the vacant pin position No. 3 on the base within a tolerance of 10 degrees.

Reference Line and Handling

The reference line of the tube's neck is mechanically defined by the position where a standardized ring gauge with a 1.500 (+0.003 / -0.000) inch internal diameter and a length of 2 inches will stop when slid down the bulb cone.

An anti-corona coating is applied to the neck and cone area, extending to a minimum radius of 1-1/2 inches concentric with the high-voltage contact. Because of the delicate nature of this conductive coating, the tube must never be handled by the portion of the bulb carrying the anti-corona coating.

Pinout and Terminal Connections

The 12ABP7 utilizes the 12M basing arrangement. The terminal connections, viewed from the bottom of the base, are assigned as follows:

For proper circuit operation, the cathode must be returned to either one side or the center-tap of the heater transformer secondary winding.

Electrical Characteristics and Operating Points

The 12ABP7 operates with a standard 6.3 V heater supply and utilizes magnetic deflection with a nominal deflection angle of 55 degrees.

Direct Interelectrode Capacitances

The approximate capacitances between the internal electrodes are:

• Cathode to all other electrodes: 5 f (pF)
• Grid No. 1 to all other electrodes: 6 f (pF)

Typical Operating Conditions

Under typical operating conditions, the tube is set up with the following electrical parameters:

• Collector (Anode) Voltage: 10,000 Volts D-C
• Grid No. 4 (Focusing Electrode) Voltage: 0 to 300 Volts D-C
• Focusing Electrode Current: -15 to +15 A D-C
• Grid No. 2 Voltage: 300 Volts D-C
• Grid No. 1 Voltage (for Cutoff): -28 to -72 Volts D-C
• Alignment Magnet Field Strength: 0 to 4 Gausses
• Undeflected Spot Position: Within a circle of 5/8-inch radius concentric with the center of the tube face (measured with the tube properly shielded from external magnetic fields).

Maximum Ratings and Safety Constraints

The maximum ratings for the 12ABP7 family are defined under two different rating systems depending on the manufacturer's specifications: the Design Center system and the Absolute Maximum system. Designers must adhere strictly to these limits to prevent premature tube failure or hazard.

Power and Screen Protection

At or near the maximum collector voltage rating, the effective resistance of the high-voltage supply must be high enough to limit the continuous collector input power to a maximum of 6 watts.

The phosphor screens of the long-persistence variants, particularly the 12ABP19 and 12ABP19A, are highly susceptible to permanent thermal damage and burning if the electron beam current density is allowed to rise too high. To prevent screen burning, circuits must be designed to employ the minimum practical beam current densities.

Operating Voltage Limits

The brilliance and definition of the displayed image degrade rapidly as the collector voltage is decreased. To maintain acceptable focus and resolution, the collector voltage should not be operated below a minimum limit of 8,000 volts.

X-Ray Radiation Warning

When the 12ABP7 is operated at voltages near or exceeding its maximum ratings, it can produce soft X-rays. X-ray radiation shielding may be necessary to protect personnel from prolonged exposure at close range if the tube is operated at voltages higher than the manufacturer's maximum rated anode voltage or 16,000 volts, whichever is less.

Performance and Focus Standards

Line Width and Resolution

The resolution of the 12ABP7 is characterized by its line width under standardized test conditions. The nominal line width is 0.019 inch maximum for the P7 and P14 phosphor versions. For the P19A phosphor version, the maximum line width is 0.018 inch when measured with a reduced screen current of no more than 50 A to prevent screen damage.

The measurement is performed using a 525-line interlaced and synchronized raster pattern with interlaced line blanking. The pattern width is adjusted to 90% of the minimum useful screen diameter (11 inches). The test pattern dimensions are 7-1/2 by 10 inches, which represents a calculated aspect ratio of:

$\text{Aspect Ratio} = \frac{10\text{ inches}}{7.5\text{ inches}} = 1.33 \text{ (or } 4:3\text{)}$

The beam current ($I_b$) is set to 100 A (measured before applying blanking). The line width is calculated by dividing the merged raster height by the number of active lines (262.5), measured at the center of the tube face.

Focus and Alignment Procedures

To achieve the best overall focus of the 7-1/2 x 10-inch raster pattern, Grid No. 1 voltage is adjusted to produce an anode current of 100 A, and the Grid No. 4 voltage is adjusted within its 0 to 300 V range. Visual extinction of the focused raster is used as the reference point for cutoff; achieving complete visual extinction of a stationary focused spot requires the Grid No. 1 bias to be approximately 5 volts more negative than the raster extinction value.

For optimum spot quality, the use of an adjustable beam alignment magnet is highly recommended. The magnet should have a field strength adjustable from 0 to 4 Gausses. Depending on the specific manufacturer's mechanical layout, this magnet must be positioned along the neck of the tube at a distance of approximately 5-3/16 inches or 5-7/16 inches from the reference line.

Technical Summary

The 12ABP7 is a 12-inch, electrostatic-focus, magnetic-deflection cathode-ray tube designed for high-resolution radar indicator service. It features a low-voltage electrostatic focus lens that operates near cathode potential to provide automatic focus tracking independent of anode voltage fluctuations. Operating with a 6.3 V, 0.6 A heater, the tube utilizes a 12M duodecal base and a J1-21 high-voltage cap.

Under typical operating conditions, the tube runs at an anode potential of 10,000 V, requiring a focusing voltage of 0 to 300 V and a control grid cutoff bias of -28 to -72 V. The 12ABP7 utilizes the long-persistence P7 phosphor (blue fluorescence, yellow phosphorescence), while the 12ABP7A variant incorporates an aluminized screen for enhanced brightness and contrast. Designers must observe strict limits on anode input power (6 W maximum) and maintain anode voltage above 8,000 V to preserve image definition, while implementing appropriate X-ray shielding if operating near the absolute maximum anode voltage limit of 13,200 V.