18F5P — Triode-Pentode Vacuum Tube

An indirectly heated, miniature glass triode-pentode designed for television receiver frame scanning and vertical deflection applications.

Overview

The 18F5P (Cyrillic: 18Ф5П) is a specialized multi-unit vacuum tube containing an independent triode and a power pentode within a single miniature glass envelope. The tube is designed primarily for television receivers with a 110° deflection angle, where the triode section serves as a master oscillator and the pentode section operates as the vertical deflection output stage.

Engineered for series-connected heater chains, the 18F5P features a controlled heater current. The European equivalent designation for this tube is the PCL85. The design combines a high-mu triode and a high-slope power pentode, allowing a single envelope to perform the complete vertical deflection function in television chassis.

Physical Construction and Base Configuration

The 18F5P is housed in a miniature glass envelope (type 21П) with a standard 9-pin Noval base. The physical dimensions and mechanical limits are as follows:

• Maximum Diameter: 22.5 mm
• Maximum Seated Height: 71.5 mm
• Maximum Total Height: 78.5 mm
• Mass: Approximately 20 g
• Mounting Position: Unrestricted

Pinout Connection

The 9-pin Noval base configuration provides independent connections for the triode and pentode sections, with the pentode's cathode internally tied to its suppressor grid (grid 3) and an internal shield.

Heater and Environmental Ratings

The heater is designed for series-string operation in television receivers. The nominal heater current is tightly controlled, while the heater voltage is allowed to vary within specified tolerances.

• Nominal Heater Current: 300 mA
• Heater Current Limits: 285 mA to 315 mA
• Nominal Heater Voltage: 18 V
• Heater Voltage Limits: 16.2 V to 19.8 V (18 V ± 1.8 V)

Environmental and Mechanical Limits

The tube is rated to operate under the following environmental conditions:

• Ambient Temperature Range: -45 °C to +70 °C (with some specifications permitting down to -60 °C)
• Relative Humidity: Up to 98% at temperatures up to 25 °C
• Vibration Resistance: Up to 50 Hz with acceleration up to 2.5 g
• Maximum Bulb Temperature: 240 °C

Electrical Characteristics and Typical Operation

The electrical characteristics of the 18F5P are measured independently for the triode and pentode sections under specified DC operating points.

Triode Section

The triode section is characterized by a high amplification factor and moderate transconductance, suitable for oscillator service.

Pentode Section

The pentode section is designed to deliver high peak currents during the flyback and scan intervals of vertical deflection.

Peak and Pulse Characteristics (Pentode)

Under pulsed conditions representing the peak demand of frame scanning (measured at $U_a = 50$ V, $U_{[g2](/tubes/G2)} = 170$ V, and $U_{g1} = -1$ V):

• Peak Anode Current: 165 mA to 200 mA (PCL85: 200 mA nominal; 18F5P minimum limit of 165 mA)
• Peak Grid 2 Current: 50 mA maximum (PCL85: 35 mA maximum)

Interelectrode Capacitances

The internal shielding and physical layout minimize capacitive coupling between the high-voltage pentode anode and the sensitive triode grid.

• Pentode Feedback Capacitance ($C_{g1-a}$): $\le$ 0.6 pF
• Triode Anode to Pentode Grid 1 Capacitance: $\le$ 0.08 pF
• Pentode Anode to Triode Grid Capacitance: $\le$ 0.03 pF
• Triode Grid to Heater Capacitance: $\le$ 0.15 pF (up to 0.2 pF maximum)
• Pentode Grid 1 to Heater Capacitance: $\le$ 0.2 pF

Absolute Maximum Ratings

The absolute maximum ratings define the operating boundaries beyond which tube reliability and service life are severely compromised.

General Limits

• Heater-to-Cathode Voltage (Triode and Pentode): 200 V maximum

Triode Section Limits

• Anode Voltage ($U_a$): 250 V maximum (550 V maximum during cold start)
• Anode Dissipation ($P_a$): 0.7 W maximum (PCL85: 0.5 W maximum)
• Cathode Current ($I_k$): 15 mA maximum
• Peak Cathode Current (Pulsed):
  • For pulse duration $\le$ 0.8 ms (duty cycle $\ge$ 25): 100 mA maximum
  • For pulse duration $\le$ 0.4 ms (duty cycle $\ge$ 50): 200 mA maximum
• Grid Circuit Resistance ($R_g$):
  • With Automatic Bias: 3.3 M$\Omega$ maximum
  • With Fixed Bias: 1.0 M$\Omega$ maximum

Pentode Section Limits

• Anode Voltage ($U_a$): 300 V maximum (PCL85: 250 V maximum; 550 V maximum during cold start)
• Peak Anode Voltage (Pulsed, in frame scanning circuits): 2.0 kV maximum
• Grid 2 Voltage ($U_{[g2](/tubes/G2)}$): 250 V maximum (550 V maximum during cold start)
• Anode Dissipation ($P_a$): 9.0 W maximum (PCL85: 7.0 W maximum)
• Grid 2 Dissipation ($P_{[g2](/tubes/G2)}$): 2.0 W maximum (PCL85: 1.5 W maximum)
• Cathode Current ($I_k$): 75 mA maximum
• Grid 1 Circuit Resistance ($R_{g1}$):
  • With Automatic Bias: 2.2 M$\Omega$ maximum
  • With Fixed Bias: 1.0 M$\Omega$ maximum

Residual Pentode Anode Voltage Limits

To prevent excessive screen grid dissipation and maintain linear deflection, the minimum instantaneous anode voltage (residual voltage) during the scan cycle must be restricted:

• At $U_{[g2](/tubes/G2)} = 150$ V: $\ge$ 40 V
• At $U_{[g2](/tubes/G2)} = 190$ V: $\ge$ 52 V

Lifetime and Reliability

The 18F5P is rated for a minimum service life of 800 hours under nominal operating conditions. The end-of-life criteria are defined by the following parameter degradation limits:

• Peak Pentode Anode Current: $\ge$ 130 mA (reduced from the nominal 165–200 mA range)
• Triode Transconductance: $\ge$ 3.3 mA/V (reduced from the nominal 5.5 mA/V)
• Pentode Reverse Grid 1 Current: $\le$ 2.0 $\mu$A (increased from the initial $\le$ 1.2 $\mu$A limit)
• Triode Reverse Grid Current: $\le$ 1.0 $\mu$A (increased from the initial $\le$ 0.6 $\mu$A limit)

Qualitative Curve Observations

The graphical characteristics of the 18F5P illustrate the performance of both sections across their operating ranges:

• Triode Anode Characteristics ($I_a$ vs. $U_a$): The triode curves display standard, evenly spaced triode behavior plotted up to an anode voltage of 300 V. The curves cover grid bias voltages ranging from a positive bias of +1 V down to a cutoff bias of -4.5 V.
• Triode Transfer Characteristics ($I_a$ vs. $U_g$): Plotted across anode voltages from 75 V to 250 V, demonstrating a smooth transition to cutoff and consistent transconductance in the linear region.
• Pentode Anode Characteristics ($I_a$ vs. $U_a$): Plotted at a screen grid voltage ($U_{[g2](/tubes/G2)}$) of 170 V, these curves show a sharp knee below 50 V, transitioning into flat plateaus up to 300 V. This sharp knee is critical for minimizing residual anode voltage during peak deflection current demands. The curves are plotted for control grid voltages ($U_{g1}$) from +0.8 V down to -20 V.
• Pentode Transfer Characteristics ($I_a$ vs. $U_{g1}$): Plotted at an anode voltage of 170 V for screen grid voltages ($U_{[g2](/tubes/G2)}$) of 150 V, 170 V, and 200 V, illustrating the control of plate current and cutoff behavior under varying screen potentials.

Technical Summary

The 18F5P is a high-performance, miniature 9-pin triode-pentode designed specifically for series-string television vertical deflection systems. Combining a high-mu triode ($\mu \approx 50$) and a robust 9-watt power pentode in a single envelope, it provides a compact solution for frame scanning. With a nominal 18 V, 300 mA heater, low interelectrode coupling, and high peak current capabilities (up to 200 mA in the pentode section), the 18F5P and its equivalent PCL85 represent a highly optimized design for consumer television electronics of its era.