12QP4 — Cathode Ray Picture Tube

A glass-envelope, magnetic-deflection and magnetic-focus direct-view picture tube designed for television receivers, featuring a 12-inch screen and an external single-magnet ion-trap.

Overview

The 12QP4 is a specialized cathode ray tube (CRT) designed for direct-view television display applications. It provides a rectangular raster size of 7 1/2" by 10" on a screen with a nominal diameter of 12 7/16 inches. The tube utilizes magnetic deflection and magnetic focusing, with a nominal deflection angle of approximately 55 degrees. To prevent the formation of ion-spot blemishes on the phosphor screen, the electron gun is designed to operate with an external, single-field magnetic ion-trap.

Construction and Physical Specifications

The 12QP4 is constructed with a glass bulb envelope and a small shell duodecal 7-pin base. The screen is coated with a phosphor that exhibits white fluorescence and phosphorescence with medium persistence, providing a clear image suitable for television broadcast reproduction.

Mechanical Dimensions

The physical layout of the tube requires specific clearance for mounting and the installation of external magnetic components:

• Overall Length: 17 1/2" ± 3/8"
• Screen Diameter: 12 7/16" ± 1/8"
• Minimum Useful Screen Diameter: 11"
• Neck Diameter: 1 7/16" ± 1/16"
• Neck Length: A minimum of 4 1/4" of straight neck space must be kept clear to accommodate the external single-field ion-trap magnet.
• Anode Terminal Position: The recessed small cavity cap for the high-voltage anode connection is located on the side of the glass cone, positioned 7 1/2" ± 1/4" from the faceplate.
• Cone Height: 10 1/2" ± 3/16" from the faceplate to the neck transition.

The tube is designed to operate in any mounting position, allowing flexibility in receiver chassis design.

Base and Pin Connections

The electrical connections to the electron gun are made through a small shell duodecal 7-pin base at the end of the neck, while the high-voltage anode connection is made via a recessed small cavity cap on the bulb. The pinout configuration, viewed from the bottom of the base, is as follows:

Heater and Interelectrode Capacitances

The 12QP4 incorporates an indirectly heated, coated unipotential cathode. The heater is designed for operation on either alternating current (AC) or direct current (DC).

The direct interelectrode capacitances of the tube, measured approximately without external shielding, are:

• Grid No. 1 to all other electrodes: 7 µµF
• Cathode to all other electrodes: 5 µµF

Maximum Ratings

The maximum ratings for the 12QP4 are established under the RMA Standard M8-210. These ratings represent the absolute limits within which the tube must be operated to prevent damage and ensure reliable service.

Heater-Cathode Voltage Limits

The insulation between the heater and the cathode is rated to withstand specific peak voltages depending on the operating state and polarity:

• Heater negative with respect to cathode:
  • During equipment warm-up period (not exceeding 15 seconds): 410 V maximum
  • After equipment warm-up period: 140 V maximum
• Heater positive with respect to cathode: 140 V maximum

These limits prevent breakdown of the heater-cathode insulation during the initial power-on phase and during steady-state operation.

Typical Operating Conditions and Characteristics

In typical television receiver applications, the 12QP4 is operated under the following conditions:

• Anode Voltage: 10,000 V
• Grid No. 2 Voltage: 250 V
• Grid No. 1 Voltage (for visual extinction of undeflected focused spot): -27 V to -63 V
• Focusing Coil Current (approximate): 135 mA
• Ion-Trap Current (approximate): 80 mA

The focusing coil current of approximately 135 mA is required to focus the electron beam into a sharp spot on the screen. The ion-trap current of approximately 80 mA applies to a standard single-field ion-trap magnet, which must be physically adjusted on the neck of the tube to align the electron beam and eliminate ion-spot blemishes.

Technical Summary

The 12QP4 is a 12-inch magnetic-deflection and magnetic-focus cathode ray picture tube featuring a 6.3 V, 0.6 A heater, a coated unipotential cathode, and a small shell duodecal 7-pin base. It is designed to provide a 7 1/2" x 10" picture with a white, medium-persistence phosphor screen. The tube operates at a typical anode voltage of 10,000 V (with a maximum rating of 12,000 V) and requires an external single-field magnetic ion-trap drawing approximately 80 mA to protect the screen from ion damage. With its flexible mounting position and standard electrical characteristics, the 12QP4 serves as a robust display solution for vintage television receiver designs.