1204HM — Flat-Faced Radar Display Tube

A 12-inch diameter, flat-faced electromagnetic radar display tube designed primarily for Plan Position Indicator (P.P.I.) applications.

Overview

The 12/04 HM (also designated as 1204HM and formerly known as the A12/04) is a specialized cathode-ray tube developed by Ferranti for radar display systems. It features a 12-inch flat faceplate, magnetic focusing, and magnetic deflection. The tube is designed to provide high-resolution visual outputs for Plan Position Indicator (P.P.I.) systems, utilizing a metal-backed screen to optimize light output and prevent ion burn.

A closely related variant, the 12/04 JM, shares the same physical dimensions, electrical ratings, and pinout configuration but differs in its screen phosphor chemistry and persistence characteristics.

Screen and Phosphor Characteristics

The 12/04HM is equipped with a Type 'H' magnesium fluoride screen. This phosphor configuration is characterized by:

• Fluorescence: Orange
• Afterglow: Orange
• Persistence: Long
• Screen Backing: Metal-backed

Because of the specific properties of the magnesium fluoride Type 'H' phosphor, the screen is highly sensitive to thermal and electrical stress. It is liable to burn even at low values of beam current if operated with a stationary or slow-moving electron spot. Consequently, scanning failure protection and beam-current gating are critical design considerations when integrating this tube.

In contrast, the sibling 12/04JM variant utilizes a Type 'J' phosphor which exhibits blue fluorescence, yellow afterglow, and long persistence.

Physical Dimensions and Geometry

The tube features a flat-faced circular envelope with a nominal screen diameter of 12 inches (305 ± 2 mm). The faceplate has a spherical radius of 1000 mm.

The physical dimensions and mechanical tolerances are detailed below:

• Maximum Overall Length: 520 mm (with the mechanical drawing specifying 510 ± 10 mm)
• Nominal Neck Diameter: 35 mm (35 ± 0.9 mm)
• Anode Cap: CT.2 type, with a 9.25 mm diameter. The side-mounted anode cap can lie within 15° on either side of the tube's vertical center line.
• Distance from Screen Centre to Anode Cap: 180 ± 5 mm
• Distance from Screen Centre to Neck Joint (36 Ring Gauge Position): 272 ± 3.5 mm

Ring Gauge Dimensions

The outer cone profile of the envelope is defined by specific ring gauge reference positions measured as distances from the center of the screen:

Base and Electrical Connections

The 12/04HM utilizes a B12A (Duodecal) base. The electrical connections, viewed from the underside of the base, are assigned as follows:

Heater and Electrostatic Capacitances

The tube incorporates an indirectly heated cathode. The heater is designed for AC or DC operation with the following nominal characteristics:

• Heater Voltage ($V_h$): 6.3 V
• Heater Current ($I_h$): 0.3 A

The interelectrode capacitances are kept low to maintain signal integrity at the grid and cathode:

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

Absolute Maximum and Minimum Ratings

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

Typical Operating Conditions

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

• 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 Modulation Drive (for 50 µA beam current): 20 V
• Focus Coil Requirements: Approximately 640 ampere-turns. The focus coil must be positioned such that the center of its gap is approximately 120 mm from the reference line (at the 36 mm Ring Gauge Position).

Operating Characteristics and Graphical Analysis

The relationship between the grid voltage, first anode voltage, and final anode current determines the modulation performance of the tube.

Final Anode Current vs. Grid Voltage

The final anode current ($I_{a2}$) is controlled by the negative bias applied to the grid ($V_g$). This relationship is highly dependent on the first anode voltage ($V_{a1}$):

• At a maximum rated $V_{a1}$ of 600 V, the grid voltage required to cut off the beam current completely is approximately -95 V.
• At $V_{a1} = 450\text{ V}$, the cut-off point shifts to approximately -70 V.
• At $V_{a1} = 300\text{ V}$, the cut-off point is approximately -48 V.
• At the minimum rated $V_{a1}$ of 250 V, the cut-off point is approximately -40 V.

For any given first anode voltage, increasing the grid voltage (making it less negative) results in a steep, non-linear increase in final anode current up to the maximum rated limits.

First Anode Voltage vs. Grid Voltage Limits

The permissible operating region for the grid bias relative to the first anode voltage is bounded by a specific design envelope. For first anode voltages ranging from 250 V to 600 V, the grid cut-off voltage must fall within a defined shaded zone:

• At $V_{a1} = 250\text{ V}$, the grid cut-off voltage must lie between -25 V and -75 V.
• At $V_{a1} = 600\text{ V}$, the grid cut-off voltage must lie between -55 V and -165 V.

Operating outside of this boundary may result in poor focus quality, inadequate beam modulation, or excessive grid current.

Technical Summary

The 12/04 HM is a high-performance 12-inch electromagnetic radar display tube featuring a flat faceplate, magnetic focus, and magnetic deflection. Utilizing a B12A Duodecal base and a 6.3 V, 0.3 A heater, it operates with a first anode voltage of 250 V to 600 V and a final anode voltage of up to 15 kV. Its Type 'H' magnesium fluoride screen provides long-persistence orange fluorescence and afterglow, making it highly suited for P.P.I. radar displays, though it requires careful beam management to prevent phosphor burns.