17AX4GTA — Half-Wave Vacuum Rectifier

The 17AX4GTA (also designated as the 17DM4A) is a single heater-cathode diode designed for use as a damping diode in the horizontal-deflection circuits of both color and black-and-white television receivers. It is particularly suited for autotransformer deflection systems where high pulse voltages are applied to the cathode.

Overview and Relationships

The 17AX4GTA is a half-wave vacuum rectifier utilizing a coated unipotential cathode. It is electrically identical to the 6AX4GTB, 12AX4GTB, and 25AX4GT vacuum tubes, differing only in its heater ratings and characteristics.

Because of its specialized design for handling high-voltage pulses in television deflection systems, operation of the 17AX4GTA as a standard power rectifier is not recommended.

Heater Ratings and Series Operation

The heater of the 17AX4GTA is designed for series-string television receiver applications and can be operated on either AC or DC.

The heater warm-up time is defined as the time required for the heater voltage to reach 80% of its rated value when four times the rated voltage is applied to a circuit consisting of the heater in series with an external resistance equal to three times the nominal heater operating resistance. For reliable series-string operation, the equipment must be designed so that at normal supply voltage, a bogey tube operates at the nominal current of 0.45 A.

Physical Construction, Base, and Pinout

The 17AX4GTA is constructed in a T-9 glass envelope with an intermediate-shell octal base. It can be mounted in any position, but requires adequate ventilation during operation.

Physical Dimensions

• Maximum Overall Length / Height: 3-5/16 inches (3.313 inches)
• Maximum Seated Height: 2-3/4 inches (2.750 inches)
• Maximum Envelope Diameter: 1-3/16 inches (1.188 inches) to 1-5/16 inches (1.3125 inches)
• Maximum Base Diameter: 1-9/32 inches

Pin Connections and Basing

The tube utilizes a 5-pin octal base requiring a standard octal socket. There are variations in the documented pin connections for the 4CG basing diagram:

• Primary Configuration:
  • Pin 3: Cathode (K)
  • Pin 5: Plate (P)
  • Pin 7: Heater (H)
  • Pin 8: Heater (H)
  • Pins 1, 2, 4, and 6 are designated as No Connection (NC) or Internal Connection (IC). Pin 1 may sometimes be omitted from the base entirely.
• Alternative Configuration:
  • Pin 2: Heater (H)
  • Pin 3: No Connection (NC)
  • Pin 5: Plate (P)
  • Pin 7: Heater (H)
  • Pin 8: Cathode (K)

Critical Installation Note: Regardless of the specific pinout configuration, socket terminals 1, 2, 4, and 6 must not be used as tie points for external circuit connections.

Electrical Capacitances

The direct interelectrode capacitances of the 17AX4GTA, measured without an external shield, are approximately:

• Cathode to Plate and Heater: 8.5 pF
• Plate to Cathode and Heater: 5.0 pF
• Heater to Cathode: 4.0 pF

Design-Maximum Ratings for TV Damper Service

The ratings of the 17AX4GTA are established under the Design-Maximum System for operation in a 525-line, 30-frame television scanning system.

* Note on Peak Inverse Voltage: The maximum peak inverse plate voltage is rated at 4400 V or 5000 V depending on the specific equipment design standards. In all cases, the pulse duration must not exceed 15% of a horizontal scanning cycle, or a maximum of 10 microseconds.

Heater-Cathode Voltage Ratings

Due to the high pulse voltages present in damper service, the heater-to-cathode insulation is rated to withstand significant potential differences:

• Heater Positive with Respect to Cathode:
  • DC Component: 100 V
  • Total DC and Peak: 300 V
• Heater Negative with Respect to Cathode:
  • DC Component: 900 V
  • Total DC and Peak: 5000 V

Operating Characteristics and Conduction Curves

Under typical operating conditions, the tube exhibits a low internal voltage drop. When conducting a direct current of 250 mA, the instantaneous tube voltage drop is 32 V.

The plate characteristic curves of the 17AX4GTA show a smooth, non-linear diode conduction behavior. Depending on the specific production design, the relationship between plate voltage and plate current typically follows these trends:

• At 15 V, plate current is approximately 100 mA.
• At 32 V to 36 V, plate current is approximately 250 mA to 300 mA.
• At 50 V to 51 V, plate current is approximately 500 mA.
• At 64 V to 70 V, plate current reaches approximately 700 mA to 750 mA.

Technical Summary

The 17AX4GTA is an octal-based, half-wave vacuum rectifier designed specifically for television damper service. Featuring a 16.8 V, 0.45 A heater with a controlled 11-second warm-up time, it is optimized for series-string heater circuits. The tube is capable of handling peak inverse voltages up to 5000 V and peak currents up to 1000 mA, with a maximum plate dissipation of 5.3 W. Its low internal voltage drop of 32 V at 250 mA and robust heater-cathode insulation make it highly effective in demanding horizontal-deflection environments.