12LP4A — Magnetic Deflection and Focus Cathode-Ray Tube

The 12LP4A is a 12-inch round, glass, direct-view cathode-ray tube (CRT) designed primarily for television receiver applications. It features magnetic deflection and magnetic focusing, utilizing an electron gun designed for use with an external double-field ion-trap magnet. A defining characteristic of the 12LP4A is its gray, neutral-density (filter glass) faceplate, which distinguishes it from the clear-glass 12LP4 and provides enhanced picture contrast and detail under high ambient light conditions.

Optical and Screen Characteristics

The 12LP4A utilizes a P4 sulfide-type phosphor screen. This phosphor produces a white fluorescence and a white phosphorescence. The persistence of the screen is characterized differently across manufacturer specifications, being designated as "medium" in some documentation and "short" in others.

The faceplate is spherical and constructed of gray filter glass. This gray glass has an approximate light transmission of 66 percent at the center of the screen. By absorbing a portion of both the ambient light entering the tube and the reflected light exiting it, the gray faceplate significantly improves contrast and image detail when operated in brightly lit environments.

The tube provides a picture size of approximately 11 1/4 by 8 1/2 inches with rounded sides. The minimum useful screen diameter is specified as either 11 inches or 11 1/4 inches depending on the manufacturer.

Mechanical Construction and Basing

The 12LP4A is an all-glass tube with a nominal bulb diameter of 12 inches. Its physical dimensions and mechanical features include:

Bulb Contact and Alignment

The high-voltage anode connection is made via a recessed small cavity cap located on the cone of the bulb. The anode contact is aligned with the vacant pin position No. 3 (or Pin No. 3 position) on the base, with an alignment tolerance of ±10 degrees or ±30 degrees depending on the manufacturer's specification.

Base and Pin Connections

The tube utilizes a small-shell duodecal base. Depending on the manufacturer, this base is supplied as either a 5-pin configuration (JETEC No. B5-57) or a 7-pin configuration (12D). In both configurations, the active electrical connections remain identical, with the 7-pin version including pins 6 and 7 as "no connection" (NC) pins.

The pinout connections, viewed from the bottom of the base, are as follows:

• Pin 1: Heater
• Pin 2: Grid No. 1
• Pin 6: No Connection (omitted on 5-pin bases)
• Pin 7: No Connection (omitted on 5-pin bases)
• Pin 10: Grid No. 2
• Pin 11: Cathode
• Pin 12: Heater
• Cap: Anode (Anode and Grid No. 3 are connected together internally and terminated at the recessed small cavity cap)

Electrical Ratings and Design-Center Values

The maximum ratings for the 12LP4A are established under the Design-Center system (such as RMA Standard MB-210). Under this system, the ratings provide a ten-percent safety factor; the tube is designed to withstand the combined effects of variations in line voltage and associated components, provided the design-center values are not exceeded by more than ten percent.

Heater and Cathode Ratings

The heater is designed for operation from either an AC or DC supply. The nominal heater voltage is 6.3 Volts, and the nominal heater current is 0.6 Amperes (with a ±10% tolerance specified by some manufacturers).

The peak heater-cathode voltage ratings vary between manufacturer specifications:

• Heater Negative with Respect to Cathode (Warm-up): During an equipment warm-up period not exceeding 15 seconds, the maximum peak voltage is rated at 410 Volts DC by some manufacturers, while others specify a limit of 125 Volts.
• Heater Negative with Respect to Cathode (After Warm-up): The maximum peak voltage is rated at 140 Volts DC.
• Heater Positive with Respect to Cathode: The maximum peak voltage is rated at 140 Volts DC by some manufacturers, and 125 Volts by others.

To ensure proper operation and longevity, the cathode should be returned to one side or to the center tap of the heater transformer winding.

Typical Operating Conditions

The 12LP4A is typically operated with an anode voltage of 11,000 Volts DC. While the tube can operate at lower anode potentials, both brightness and focusing quality decrease with decreasing anode voltage; operation below 8,000 Volts is generally not recommended.

The typical operating parameters vary slightly depending on the design of the receiver and the manufacturer's recommended operating point:

• Anode Voltage: 11,000 Volts DC
• Grid No. 2 Voltage: 250 Volts DC or 300 Volts DC
• Grid No. 1 Voltage (for visual extinction of undeflected focused spot or raster): -27 to -63 Volts DC, or -28 to -72 Volts DC
• Grid No. 1 Circuit Resistance: 1.5 Megohms (Maximum)

Focusing and Deflection Requirements

The 12LP4A requires external magnetic focusing and deflection components. The deflection angle of the tube is approximately 54 degrees.

The focusing coil current depends on the specific coil used and its physical placement along the neck of the tube:

• When using a standard focusing coil (such as JETEC No. 106 or equivalent) positioned at a distance of 3 1/4 inches from the reference line to the center of the focus coil air gap, the approximate focusing current required is 110 mA DC. This is typically measured with the bias adjusted to produce a highlight brightness of 20 foot-lamberts on a 7 1/2 by 10 inch picture area, sharply focused at the center of the screen.
• When the focusing coil is positioned at a distance of 4 1/2 inches from the yoke-reference-line to the center of the air gap, the approximate focusing current required is 96 mA DC.

Ion-Trap Magnet

The electron gun of the 12LP4A must be used with an external double-field ion-trap magnet. The ion-trap magnet must be adjusted to its optimum position on the neck of the tube (approximately 6 1/4 inches from the base). The required magnetic field strength is approximately 35 Gausses, which is equivalent to a current of 120 milliamperes through a standard JETEC ion-trap magnet No. 108.

Capacitances and Circuit Function

The direct interelectrode capacitances of the tube are approximately:

• Cathode to all other electrodes: 5 µµF
• Grid No. 1 to all other electrodes: 6 µµF

The tube also features an external conductive coating. When this coating is properly grounded, it forms a capacitor in conjunction with the internal anode coating. The capacitance of this structure is rated as:

• Maximum External Coating to Anode: 3,000 µµF
• Minimum External Coating to Anode: 750 µµF

In practical receiver circuits, this capacitance acts as a high-voltage filter capacitor for the anode power supply, helping to smooth the rectified high-voltage output.

Safety and X-Ray Radiation Considerations

When operating the 12LP4A at high voltages, safety precautions regarding X-ray radiation must be observed. Prolonged exposure to X-rays at close range can present a danger of personal injury. If the tube is operated at voltages exceeding the manufacturer's maximum rated anode voltage (12,000 Volts DC) or 16,000 Volts (whichever is less), appropriate X-ray radiation shielding may be necessary to protect users and service technicians.

Technical Summary

The 12LP4A is a 12-inch, all-glass, round television picture tube utilizing magnetic deflection (54-degree angle) and magnetic focusing. It is characterized by its gray neutral-density faceplate, which provides a 66% light transmission rate to enhance picture contrast under high ambient light. The tube operates with a nominal 6.3 V, 0.6 A heater and is designed for a maximum anode voltage of 12,000 Volts DC, with typical operation at 11,000 Volts DC. It features a duodecal base (5-pin or 7-pin) and requires an external double-field ion-trap magnet (35 Gausses) and a focusing coil (typically requiring 96 to 110 mA DC depending on positioning). Grounding the external conductive coating provides an integrated high-voltage filter capacitance of 750 to 3,000 µµF.