17D4A — Single Indirectly Heated Diode

The 17D4A is a single, indirectly heated diode featuring a unipotential cathode. It is designed specifically for television horizontal frequency damper service. The tube's construction enables it to withstand high-voltage pulses between the cathode and both the heater and plate elements, making it highly suited for use in "direct drive" damper circuits.

Mechanical Specifications and Pinout

The 17D4A is constructed with a T-9 glass bulb envelope conforming to the JEDEC 9-41 outline. It features an intermediate shell 5-pin octal base (B5-85) and can be mounted in any position.

The physical dimensions of the tube are as follows:

• Maximum Diameter: 1.188 inches
• Maximum Seated Height: 2.750 inches
• Maximum Overall Height: 3.313 inches

The tube utilizes the JEDEC 4CG bottom-view basing diagram. The pin connections are assigned as follows:

Wiring Note: Socket pins 1, 2, 4, and 6 must not be used as tie points for external circuit connections.

Heater Characteristics

The 17D4A is equipped with an indirectly heated filament designed for series or parallel heater strings. The nominal heater specifications and warm-up characteristics include:

• Heater Voltage: 16.8 V
• Heater Current: 0.450 ± 0.030 A (450 mA)
• Heater Warm-up Time: Approximately 11.0 seconds

The heater warm-up time is specifically defined under controlled conditions: it represents the time required for the voltage across the heater to reach 80% of its rated value when 4 times the rated heater voltage is applied to a circuit consisting of the tube heater in series with an external resistance equal to 3 times the nominal heater operating resistance.

Electrical and Capacitive Characteristics

In typical operation, the 17D4A exhibits a low internal voltage drop. When conducting a plate current of 340 mA, the tube voltage drop is 30 V.

The direct interelectrode capacitances of the unshielded tube are documented with minor variations in electrode grouping across different technical records:

• Heater to Cathode (H to K): 3.0 pF
• Cathode to Plate and Heater: 7.0 pF or 9.0 pF
• Plate to Cathode and Heater: 7.0 pF or 9.0 pF

These low capacitances help minimize unwanted coupling in high-frequency television sweep circuits.

Design-Maximum Ratings

The ratings for the 17D4A are established under the Design-Maximum System. Operating the tube beyond these limits may impair its performance or cause permanent damage.

Plate and Current Limits

• Maximum Peak Inverse Plate Voltage: 5000 V
• Maximum DC Plate Current: 185 mA
• Maximum Steady State Peak Plate Current: 900 mA
• Maximum Plate Dissipation: 8.0 W

For peak inverse plate voltage operation in a standard 525-line, 30-frame television system, the duty cycle of the voltage pulse must not exceed 15% of one horizontal scanning cycle (equivalent to a maximum pulse duration of 10 µs).

Heater-Cathode Voltage Limits

Because the cathode in damper service experiences high pulse potentials, the maximum allowable voltage between the heater and cathode is strictly defined based on polarity:

• Heater Negative with Respect to Cathode:
  • DC Limit: 1000 V
  • Total DC and Peak Limit: 5000 V (The pulse duration must not exceed 15% of one horizontal scanning cycle, or 10 µs in a 525-line, 30-frame system).
• Heater Positive with Respect to Cathode:
  • DC Limit: 100 V
  • Total DC and Peak Limit: 300 V

Technical Summary

The 17D4A is a specialized, octal-based, single indirectly heated diode designed for television horizontal frequency damper service. Operating with a 16.8 V, 450 mA heater and featuring a controlled 11-second warm-up time, it is optimized for reliability in series-string television receivers. Its robust internal insulation allows it to withstand peak inverse plate voltages and heater-cathode pulse potentials up to 5000 V in direct-drive horizontal deflection circuits. With a low typical voltage drop of 30 V at 340 mA and a maximum plate dissipation of 8.0 W, the 17D4A provides efficient and stable damping performance under demanding high-voltage pulse conditions.