18F24 — RF High-Slope Pentode

An indirectly heated, high-slope pentode designed for radio frequency (RF), intermediate frequency (IF), and wideband amplifier applications.

Overview

The 18F24 is a specialized high-slope pentode engineered primarily for high-frequency amplification. Its design features high mutual conductance (transconductance) and very low feedback capacitance, making it highly suitable for RF, IF, and wideband amplifier stages. The tube incorporates an indirectly heated cathode requiring an 18 V heater supply and is housed in an envelope with a 9-pin base.

Mechanical Design and Pinout

The 18F24 is constructed in a physical size designated as Size L 3 and utilizes a standard 9-pin base. The pin configuration is designed to optimize high-frequency performance by providing multiple connections for critical electrodes, which helps minimize lead inductance. Specifically, both the cathode and the suppressor grid are allocated two pins each.

The pin connections, viewed from the underside of the base, are assigned as follows:

The redundant connections for the cathode (pins 6 and 8) and the suppressor grid (pins 1 and 9) allow for low-impedance grounding paths, which are essential for maintaining stability and preventing unwanted oscillations in high-slope RF and wideband circuits.

Heater and Cathode Characteristics

The 18F24 features an indirectly heated cathode designed for series or parallel heater chains. The electrical specifications for the heater are:

The calculated heater power is derived directly from the nominal operating parameters (18 V * 0.165 A = 2.97 W). The maximum allowable potential difference between the cathode and the heater (U_k/f) is rated at 50 V, which must be respected in circuit designs where the cathode is elevated above ground potential.

Electrical Characteristics and Class A RF Operation

The 18F24 is characterized by its high mutual conductance (slope) of 10 mA/V, which allows for high voltage gain in wideband and RF stages. The static characteristics and typical Class A RF amplifier operating conditions are identical:

Circuit Calculations and Operating Analysis

Under these typical Class A operating conditions, several key circuit parameters can be calculated:

1. Total Cathode Current (I_k): I_k = I_a + I_g2 = 15 mA + 2.1 mA = 17.1 mA This total current flows through the cathode self-bias resistor.

2. Cathode Bias Resistor Verification: Using the calculated cathode current of 17.1 mA and the target control grid bias of -2 V, the required cathode resistor is: R_k = |U_g1| / I_k = 2 V / 17.1 mA ≈ 117 Ω This calculated value closely aligns with the recommended standard value of 120 Ω.

3. Anode Dissipation (P_a): P_a = U_a * I_a = 250 V * 15 mA = 3.75 W This operating dissipation is within the maximum limit of 4 W.

4. Screen Grid Dissipation (P_g2): P_g2 = U_g2 * I_g2 = 200 V * 2.1 mA = 0.42 W This operating dissipation is within the maximum limit of 0.45 W.

Cut-off Characteristics

The tube exhibits a sharp cut-off characteristic. When the control grid voltage (U_g1) is biased down to -7 V, the anode current (I_az) is reduced to less than 0.5 mA, allowing for effective automatic gain control (AGC) or gating functions.

Maximum Ratings

To ensure reliable operation and prevent premature failure, the 18F24 must be operated within its absolute maximum ratings. These limits should not be exceeded under worst-case operating conditions:

• Cold Anode Voltage (U_a0): 400 V (maximum voltage when the tube is cold and not conducting)
• Anode Voltage (U_a): 250 V
• Anode Dissipation (W_a): 4 W
• Screen Grid Voltage (U_g2): 250 V
• Screen Grid Dissipation (W_g2): 0.45 W
• Cathode Current (I_k): 20 mA
• Control Grid Resistor (R_g1): 0.5 MΩ
• Voltage between Cathode and Heater (U_k/f): 50 V

The typical operating cathode current of 17.1 mA operates at approximately 85.5% of the absolute maximum cathode current rating of 20 mA, representing a robust design margin.

Interelectrode Capacitances

For high-frequency and wideband applications, the internal capacitances of the tube are critical parameters that dictate input loading, output loading, and high-frequency stability. The 18F24 features the following interelectrode capacitances:

• Input Capacitance (C_g1): 10.5 pF
• Output Capacitance (C_a): 5.9 pF
• Feedback Capacitance (C_a/g1): < 0.035 pF

The extremely low feedback capacitance (less than 0.035 pF) is a key design feature of this high-slope pentode. It minimizes the Miller effect, preventing the amplification of input capacitance and ensuring excellent stability in high-gain RF and IF amplifier stages without the need for complex neutralization circuits.

Technical Summary

The 18F24 is a high-performance RF high-slope pentode featuring an 18 V, 0.165 A indirectly heated heater and a 9-pin base (Size L 3). It is optimized for RF, IF, and wideband amplification where high gain and stability are paramount.

At its typical operating point (U_a = 250 V, U_g2 = 200 V), the tube delivers a high mutual conductance of 10 mA/V and an internal resistance of 0.3 MΩ, while drawing 15 mA of anode current. Its physical design utilizes dual pin connections for both the cathode and suppressor grid to minimize parasitic inductance, and its internal structure achieves an exceptionally low feedback capacitance of < 0.035 pF. Designers must adhere to the maximum ratings, including a 4 W anode dissipation limit, a 20 mA cathode current limit, and a 50 V heater-to-cathode voltage limit, to ensure stable and reliable operation in high-frequency circuitry.