17DM4 — Half-Wave Vacuum Rectifier

The 17DM4 is an indirectly heated, half-wave vacuum rectifier designed specifically for service as a damper diode in the horizontal-deflection circuits of black-and-white television receivers. It features a coated unipotential cathode and is housed in a glass envelope with an octal base. The tube is electrically identical to the 6DM4 and 12DM4 except for its heater characteristics and heater warm-up time.

Mechanical Construction and Pinout

The 17DM4 is constructed using a T-9 glass bulb with a JEDEC 9-44 outline. It utilizes a short intermediate-shell, 5-pin octal base with external barriers, designated as the JEDEC B5-85 (or 85-85) base. The tube can be mounted and operated in any physical position.

Physical Dimensions

• Maximum Diameter: 1.188 inches
• Maximum Bulb Height: 3.250 inches
• Maximum Overall Height: 3.812 inches

Pin Connections and Socket Considerations

The 17DM4 uses the JEDEC 4CG bottom-view basing diagram. The pin assignments are as follows:

Pins are physically omitted from the base in positions 1, 4, and 6. However, positions 1 and 6 contain internal connections that terminate in stem leads inside the envelope. Position 2 has a physical base pin that may also connect to an internal stem lead. To prevent high-voltage breakdown between adjacent pins in television damper service, the socket clips corresponding to positions 1, 2, 4, and 6 must be removed from the tube socket.

Heater Characteristics and Ratings

The heater of the 17DM4 is designed for series-string current-operated heater chains. It features a controlled warm-up time to ensure reliable operation in series configurations.

The average heater warm-up time of 11 seconds is specifically defined under standardized test conditions: the time required for the heater voltage to reach 80% of its rated value when four times the rated heater voltage (67.2 V) is applied in series with an external resistance equal to three times the nominal heater operating resistance.

Heater-Cathode Voltage Limits

Because the cathode of a damper diode operates at high pulse potentials relative to ground, the heater-to-cathode insulation must withstand severe electrical stress. The design maximum ratings (under EIA Standard RS-239) specify:

• Heater Negative with Respect to Cathode: 5000 V peak maximum, with the DC component not exceeding 900 V.
• Heater Positive with Respect to Cathode: 300 V peak maximum, with the DC component not exceeding 100 V.

Direct Interelectrode Capacitances

Measured without an external shield, the approximate direct interelectrode capacitances of the 17DM4 are:

• Plate to Heater and Cathode: 8.5 pF
• Cathode to Heater and Plate: 11.5 pF
• Heater to Cathode: 4.0 pF

Electrical Characteristics and Plate Curves

The 17DM4 exhibits a low internal voltage drop, which is critical for maintaining efficiency in horizontal-deflection damping applications. Under typical operating conditions, the tube voltage drop is approximately 35 V at a forward plate current of 400 mA.

The average plate characteristics under rated heater conditions demonstrate the following relationship between plate voltage and plate current:

• At 10 V Plate Voltage: ~75 mA
• At 20 V Plate Voltage: ~200 mA
• At 35 V Plate Voltage: ~400 mA
• At 50 V Plate Voltage: ~660 mA
• At 70 V Plate Voltage: ~1040 mA

The plate characteristic curve is nearly linear for plate voltages above 20 V, indicating stable internal resistance during the conduction cycle.

Maximum Ratings for Damper Service

The maximum ratings of the 17DM4 are defined under the Design Maximum System (EIA Standard RS-239) for television damper service:

• Peak Inverse Plate Voltage: 5000 V
• Peak Plate Current: 1100 mA
• DC Plate Current: 175 mA
• Plate Dissipation: 6.5 W

Pulse Duty Cycle Constraints

The peak inverse plate voltage rating of 5000 V is specified for operation in a standard 525-line, 30-frame television system (FCC standards). Under these operating conditions, the duty cycle of the horizontal voltage pulse must not exceed 15% of one scanning cycle. For a standard horizontal scanning line, this 15% duty cycle corresponds to a maximum pulse duration of 10 microseconds.

Technical Summary

The 17DM4 is a specialized half-wave vacuum rectifier designed for television horizontal damper service. It features a 16.8 V, 450 mA heater with a controlled 11-second warm-up time for series-string applications. Utilizing a T-9 glass envelope and a 5-pin octal base (JEDEC 4CG), it requires the removal of unused socket clips to prevent high-voltage breakdown. With a peak inverse voltage rating of 5000 V, a peak current capability of 1100 mA, and a low typical voltage drop of 35 V at 400 mA, the 17DM4 provides robust and efficient damping performance in black-and-white television receivers.