18A5 — Beam-Power Pentode

The 18A5 is a beam-power pentode designed primarily for use as a horizontal-deflection amplifier in compact, 300-milliampere series-string television receivers. It is characterized by its small physical size, low power requirements, high performance when operating at low d-c supply voltages, and a controlled heater warm-up characteristic.

Mechanical Specifications and Pinout

The 18A5 is constructed with a T-9 glass envelope and can be mounted in any position. It utilizes either a B6-8 (Intermediate Shell Octal 6-Pin) or a B6-60 (Short Intermediate Shell Octal 6-Pin) base.

The physical dimensions of the tube conform to RETMA 9-15 or 9-43 standards, with a maximum overall length of 3-7/16 inches and a maximum diameter of 1-9/32 inches.

The electrical connections are arranged according to the RETMA 6CK basing diagram, which provides the following pinout configuration:

Heater Ratings

The heater of the 18A5 is designed for series-string operation and features a controlled warm-up characteristic to ensure reliable performance in series-connected heater chains.

Direct Interelectrode Capacitances

Measured approximately without an external shield, the direct interelectrode capacitances of the 18A5 are:

• Grid-Number 1 to Plate: 0.7 μμf
• Input: 13 μμf
• Output: 7.0 μμf

Electrical Characteristics and Operating Points

The 18A5 can be operated either in its standard pentode configuration or as a triode by connecting the screen grid directly to the plate.

Pentode Operation

The tube's characteristics under pentode operation are documented for two distinct conditions. Condition A represents a short-interval measurement designed to evaluate peak performance without damaging the tube, while Condition B represents a typical operating point.

*Note: Condition A parameters must only be applied for a short interval of two seconds maximum to prevent thermal damage to the tube.

Triode-Connected Operation

When the screen grid (Grid-Number 2) is tied directly to the plate, the 18A5 exhibits the following characteristics:

• Plate Voltage ($E_b$): 125 V
• Screen Voltage ($E_{c2}$): 125 V
• Grid-Number 1 Voltage ($E_{[c1](/tubes/C1)}$): -17 V
• Triode Amplification Factor (μ): 4.6

Maximum Ratings

The maximum ratings of the 18A5 are established under the Design-Maximum system for horizontal-deflection amplifier service in 525-line, 30-frame television systems. The ratings assume a pulse duty cycle of 15% or less of one scanning cycle.

• DC Plate-Supply Voltage (Boost DC Power Supply): 350 V
• Peak Positive Pulse Plate Voltage: 3,000 V
• Peak Negative Pulse Plate Voltage: 600 V
• Screen Voltage (Grid-Number 2): 160 V
• Peak Negative Grid-Number 1 Voltage: 250 V
• Plate Dissipation: 9.0 W
• Screen Dissipation: 2.5 W
• DC Cathode Current: 90 mA
• Peak Cathode Current: 310 mA
• Grid-Number 1 Circuit Resistance: 1.0 MΩ
• Bulb Temperature (at hottest point): 190 °C

Heater-Cathode Voltage Limits

The potential difference between the heater and the cathode must be kept within the following limits:

• Heater Positive with Respect to Cathode:
  • DC Component: 100 V
  • Total DC and Peak: 200 V
• Heater Negative with Respect to Cathode:
  • Total DC and Peak: 200 V

Qualitative Curve Observations

The operating behavior of the 18A5 is illustrated by its characteristic curves, which reveal several key performance traits:

• Average Plate Characteristics at Zero Bias ($E_{[c1](/tubes/C1)} = 0\text{ V}$): When plate current ($I_b$) and screen current ($I_{c2}$) are plotted against plate voltage (from 0 to 500 V) across screen voltages ($E_{c2}$) of 50, 75, 100, 125, and 150 V, the plate current curves exhibit a sharp knee. This knee occurs consistently between 30 V and 50 V, indicating efficient operation and low knee voltage, which is highly desirable for deflection amplifier service.
• Average Plate Characteristics at Nominal Screen Voltage ($E_{c2} = 125\text{ V}$): The family of plate current curves plotted for grid-1 voltages ranging from 0 V to -35 V shows the control grid's influence over plate current. A boundary line representing the 9.0-Watt plate dissipation limit is typically marked on these curves to define the safe operating envelope.
• Average Transfer Characteristics ($E_b = 200\text{ V}$): Plots of plate current ($I_b$) and screen current ($I_{c2}$) against grid-1 voltage (from 0 to -50 V) for screen voltages ($E_{c2}$) of 50, 75, 100, 125, and 150 V demonstrate the transconductance behavior and cutoff characteristics of the tube.

Technical Summary

The 18A5 is a specialized, octal-based beam-power pentode designed for horizontal-deflection amplifier service in series-string television receivers. Operating with an 18.5 V, 0.3 A heater with an 11-second warm-up time, it is optimized for high-performance operation at low DC supply voltages. It features a maximum plate dissipation of 9.0 Watts, a peak positive pulse plate voltage rating of 3,000 Volts, and a sharp plate current knee between 30 V and 50 V. These characteristics, combined with its compact T-9 envelope, make it highly suited for its intended television receiver applications.