12AXP2 — Electrostatic Deflection and Focus Cathode Ray Tube

The 12AXP2 is a 12-inch direct-viewed, round glass cathode ray tube (CRT) designed for oscilloscope applications. It features electrostatic deflection, electrostatic focus, and post-deflection acceleration to achieve high-performance trace display. The "12AXP2" designation specifically identifies the variant equipped with a P2 phosphor screen, which is optimized for applications requiring medium persistence and green-colored visual output.

Phosphor and Screen Characteristics

The screen of the 12AXP2 is characterized by its specific phosphor composition and faceplate design:

• Phosphor Type: P2
• Fluorescence (Spontaneous Emission): Green
• Phosphorescence (Afterglow): Green
• Persistence: Medium
• Faceplate: Gray filter glass to enhance contrast.
• Minimum Useful Screen Diameter: 11 inches.
• Useful Scan (Centered on Tube Face): A minimum area of 9 3/4 inches by 9 3/4 inches. Under the operating condition where the post-accelerator voltage equals the second anode voltage ($E_{A3} = E_{A2}$), the entire useful screen area can be scanned.

Mechanical Design and Physical Dimensions

The 12AXP2 is a large-format CRT with a robust mechanical structure:

• Bulb Designation: C99 1/2 Exp. 28, or equivalent.
• Bulb Contact: Recessed small cavity cap (J1-21) for the high-voltage post-accelerator connection.
• Base: Medium Shell Diheptal 12-Pin (B12-37).
• Basing Configuration: 14J.
• Weight: Approximately 12 pounds.
• Overall Length: 22 1/4  3/8 inches.
• Maximum Bulb Diameter: 12 7/16  3/16 inches.
• Neck Diameter: 2  1/32 inches.
• Deflection Section Neck Diameter: 3 3/4  3/16 inches.
• Alignment Tolerances: The J1-21 bulb contact aligns with the trace of deflecting plates D1-D2 within 10 degrees, and also aligns with Pin No. 5 within 10 degrees.
• Deflection Direction: A positive voltage applied to deflecting plate D1 deflects the electron beam approximately toward Pin No. 5. A positive voltage applied to deflecting plate D3 deflects the beam approximately toward Pin No. 1.

Pinout Connections (14J Basing)

The 12-pin diheptal base provides the following electrical connections, viewed from the bottom of the tube:

Electrical and Heater Characteristics

The 12AXP2 utilizes an indirectly heated cathode with the following heater specifications:

• Heater Voltage: 6.3 Volts
• Heater Current: 0.6  10% Amperes
• Heater Supply: AC or DC

Direct Interelectrode Capacitances

The approximate interelectrode capacitances of the tube structure are:

Absolute Maximum Ratings

The following absolute maximum values represent the limits within which the 12AXP2 must be operated to prevent damage or premature degradation:

• Anode Input: 6 Watts (calculated as the product of the Anode No. 2 voltage and the average Anode No. 2 current)
• Anode No. 3 Voltage (Post-Accelerator): 12,000 Volts dc
• Anode No. 2 Voltage: 6,000 Volts dc
• Anode No. 1 Voltage (Focusing Electrode): 2,500 Volts dc
• Grid No. 1 Voltage:
  • Negative Bias Value: 200 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
• Peak Voltage Between Anode No. 2 and Any Deflecting Plate: 750 Volts
• Ratio of Post-Accelerator Voltage to Anode No. 2 Voltage ($E_{A3}/E_{A2}$): 2.3:1

Typical Operating Conditions

Under standard operating conditions, the 12AXP2 is typically configured with the following electrical parameters:

• Anode No. 3 Voltage ($E_{A3}$): 10,000 Volts dc
• Anode No. 2 Voltage ($E_{A2}$): 5,000 Volts dc
• Anode No. 1 Voltage for Focus ($E_{A1}$): 1,300 to 2,200 Volts dc
• Grid No. 1 Voltage Required for Cutoff ($E_{G1}$): -60 to -100 Volts dc (defined as the voltage required for visual extinction of the undeflected focused spot)
• Deflection Factors:
  • Deflecting Plates 1-2 (plates nearer the screen): 105 to 145 Volts dc/Inch
  • Deflecting Plates 3-4: 80 to 115 Volts dc/Inch
• Modulation: 30 Volts maximum (measured in accordance with MIL-E-1 specifications at a post-accelerator current of 25 A)
• Line Width "A": 0.5 mm (measured at a post-accelerator current of 25 A)
• Focus Electrode Current: -15 to +10 A dc (measured at a post-accelerator current of 25 A)
• Spot Position (Undeflected): Within a 30 mm square
• Angle Between $D_1$-$D_2$ Trace and $D_3$-$D_4$ Trace: 90  2 Degrees

Circuit Design Constraints

To maintain proper focus and prevent damage to the tube's internal structures, the external circuitry must comply with the following maximum resistance limits:

• Grid No. 1 Circuit Resistance: 1.5 Megohms maximum
• Resistance in Any Deflection Plate Circuit: 5.0 Megohms maximum