17DM4A — Half-Wave Vacuum Rectifier

An indirectly heated, half-wave vacuum diode designed for high-voltage damper service in the horizontal-deflection circuits of television receivers.

Overview

The 17DM4A is a half-wave vacuum rectifier engineered specifically to serve as a damper diode in horizontal-deflection circuits. Housed in a glass T-9 envelope with an octal base, the tube features a coated unipotential cathode and is designed to withstand the severe high-voltage pulses characteristic of television scanning systems.

The 17DM4A belongs to a family of damping diodes that includes the 6DM4A and 12DM4A, sharing identical electrical and physical characteristics with these types except for its heater ratings. It is also identical to the 6DA4 except for heater ratings. Compared to the prototype 17DM4, the 17DM4A offers higher plate current ratings, providing improved performance margins in demanding deflection circuits. Additionally, some documentation groups the 17DM4A with the 17AX4GTA, reflecting its role as a color television type damper.

Construction, Physical Specifications, and Socketing

The 17DM4A is constructed with a coated unipotential cathode and a single anode (plate) enclosed in a T-9 glass bulb. The tube is designed to operate in any mounting position, provided there is adequate ventilation to dissipate heat during operation.

The physical dimensions of the tube are as follows:

• Maximum Diameter: 1.188 inches (with a maximum base diameter of 1.281 inches).
• Maximum Overall Length: 3.812 inches (or 3-13/16 inches).
• Maximum Seated Height: 2.750 to 3.250 inches (or 3-1/4 inches).
• Maximum Overall Height: 3.313 inches.

The tube is equipped with a 5-pin octal base, which may be a short intermediate-shell octal base (JEDEC B5-85 or B5-147) or an intermediate-shell octal base (JEDEC B5-82). The physical outline corresponds to JEDEC 9-44 or 9-11 configurations.

Socketing Restrictions

Due to the high peak voltages present in horizontal-deflection damper service, specific socketing precautions must be observed:

• Pins 1, 2, 4, and 6 must not be used as circuit tie points.
• Although pins 1, 4, and 6 are physically absent from the base of the tube, positions 1 and 6 may have internal connections to stem leads. Pin 2 is designated as an Internal Connection (IC) and may also connect to a stem lead.
• To prevent voltage breakdown and reduce the risk of electrical arcover, it is recommended to avoid using these socket terminals for any wiring. Some specifications recommend removing the socket clips for positions 1, 2, 4, and 6 entirely.

Heater Characteristics

The heater of the 17DM4A is specifically designed for operation in series-connected heater strings, incorporating a controlled warm-up characteristic to minimize voltage surges across other tubes in the string during start-up.

The heater warm-up time is formally defined as the time required for the heater voltage to reach 80% of its rated value (13.44 V) when a voltage equal to four times the rated heater voltage (67.2 V) is applied to a circuit consisting of the tube heater in series with a resistance equal to three times the nominal heater operating resistance.

Pinout and Interelectrode Capacitances

The 17DM4A utilizes the JEDEC 4CG basing diagram. Depending on the manufacturer's design, the pin assignments for the active electrodes vary, though the physical socket layout remains compatible:

• Pin 2: Internal Connection (IC) — Do not use.
• Pin 7: Heater (H)
• Pin 8: Heater (H)
• Active Electrodes (Configuration A): Pin 3 is connected to the Cathode (K), and Pin 5 is connected to the Plate (P).
• Active Electrodes (Configuration B): Pin 3 is connected to the Plate (P), and Pin 5 is connected to the Cathode (K).

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 17DM4A vary slightly depending on the measurement conditions and shielding. Typical unshielded values include:

• Heater to Cathode: 4.0 pF
• Plate to Cathode (or Plate to Cathode and Heater): 5.0 pF to 8.5 pF
• Cathode to Plate and Heater: 8.5 pF to 11.5 pF

Electrical Ratings and Damper Service

The maximum ratings of the 17DM4A are defined under the Design-Maximum System (EIA RS-239). These ratings are specified for operation in a standard 525-line, 30-frame television system, where the duty cycle of the horizontal voltage pulse must not exceed 15% of one scanning cycle (equivalent to a maximum pulse duration of 10 microseconds).

Typical Operating Characteristics and Plate Curves

Under typical operating conditions, the 17DM4A exhibits a low internal voltage drop, which is critical for maintaining efficiency in television deflection circuits. The typical tube voltage drop is documented as:

• 35 V at an instantaneous plate current ($I_b$) of 400 mA.
• 32 V at an instantaneous plate current ($I_b$) of 250 mA.

Average Plate Characteristics

The average plate characteristic curve, plotted as plate current in milliamperes versus plate voltage in volts at the rated heater voltage of 16.8 V, reveals the following behavior:

• 0 to 10 V: The plate current is low, remaining under 50 mA (with approximately 75 mA at 10 V).
• 20 V: The plate current rises to approximately 180 mA.
• 25 V: The plate current reaches approximately 250 mA.
• 35 V: The plate current is 400 mA, corresponding to the typical voltage drop point.
• 41 V: The plate current reaches approximately 500 mA.
• 50 V: The plate current is approximately 650 to 680 mA.
• 55 V: The plate current reaches approximately 750 mA.
• 68 V: The plate current reaches approximately 1000 mA.
• 70 V: The plate current is approximately 1050 to 1060 mA.
• 72 V: The plate current reaches approximately 1100 mA.

The curve exhibits a smooth, non-linear upward slope at very low voltages (below 20 V) and transitions into a nearly linear region from 20 V up to 72 V, demonstrating the highly efficient conduction characteristics of the tube's large-area cathode and anode structure.

Technical Summary

The 17DM4A is an octal-based, indirectly heated half-wave vacuum rectifier designed specifically for television damper service. Operating with a 16.8 V, 0.45 A heater featuring a controlled 11-second warm-up time, it is optimized for series-string heater applications. The tube is capable of handling peak inverse voltages up to 5000 V and peak plate currents up to 1200 mA, with a low typical voltage drop of 32 to 35 V under normal conduction. Its robust design, high heater-to-cathode insulation ratings, and strict socketing guidelines ensure reliable operation under the severe pulsed voltage conditions of horizontal deflection systems.