17JK8 — Double Dissimilar Triode

An indirectly heated, double dissimilar triode in a miniature envelope, designed for radio frequency (RF) amplification, oscillation, and mixer applications.

Overview

The 17JK8 is a multi-section vacuum tube containing two distinct, electrically independent triode systems with different characteristics within a single envelope. One section is optimized for oscillator service, while the other is designed for RF amplification. This dissimilar dual-triode configuration allows a single tube to perform multiple stages of a receiver front-end, such as RF amplification and frequency mixing or oscillation, reducing space and component count.

Mechanical Design and Heater Characteristics

The 17JK8 is constructed in a miniature T-6 1/2 glass envelope with a 9-pin button base (designation E9-1). It utilizes the 9AJ basing arrangement and conforms to the 6-2 physical outline. The physical dimensions of the device are as follows:

• Maximum Diameter: 0.875 in
• Maximum Seated Height: 1.937 in
• Maximum Overall Height: 2.187 in

The tube features an indirectly heated cathode system designed for series-string operation with a controlled warm-up time. The heater and heater-to-cathode insulation ratings are:

The maximum heater-cathode voltage limit of 100 V applies with the heater either negative or positive with respect to the cathode.

Interelectrode Capacitances

To prevent unwanted coupling and feedback between the two dissimilar triode sections, the 17JK8 is designed with low internal capacitances. The direct interelectrode capacitances, measured with an external shield, are:

The isolation between the active elements of each section is highly controlled, with the maximum grid-to-grid capacitance rated at 0.003 pF and the maximum plate-to-plate capacitance rated at 0.009 pF.

Design Maximum Ratings

The two triode sections of the 17JK8 have independent maximum ratings tailored to their specific roles. Operating the tube within these limits is necessary to prevent premature degradation.

Typical Operating Characteristics

Under Class A1 amplifier conditions, the two sections exhibit distinct electrical characteristics. Section 1 provides a moderate amplification factor and transconductance suitable for stable oscillation, while Section 2 features high transconductance and lower plate resistance for effective RF amplification.

Section 1 (Oscillator)

At a typical operating point, Section 1 exhibits the following characteristics:

• Plate Voltage: 100 V
• Grid Voltage: -1.0 V
• Plate Current: 5.3 mA
• Transconductance: 6,800 µmhos
• Amplification Factor: 55
• Plate Resistance (Approximate): 8,000 Ω
• Grid Voltage for $I_b = 20$ µA (Approximate): -4.4 V

The relationship between the parameters can be verified by calculation: $\mu \approx g_m \times r_p = 0.0068\text{ S} \times 8,000\ \Omega = 54.4$ This closely matches the nominal amplification factor of 55.

Section 2 (RF Amplifier)

At a typical operating point, Section 2 exhibits the following characteristics:

• Plate Voltage: 135 V
• Grid Voltage: -1.2 V
• Plate Current: 10 mA
• Transconductance: 13,000 µmhos
• Amplification Factor: 70
• Plate Resistance (Approximate): 5,400 Ω
• Grid Voltage for $G_m = 1,500$ µmhos (Approximate): -2.8 V
• Grid Voltage for $G_m = 150$ µmhos (Approximate): -5.5 V

Calculating the relationship for Section 2: $\mu \approx g_m \times r_p = 0.013\text{ S} imes 5,400\ \Omega = 70.2$ This aligns with the nominal amplification factor of 70. The grid voltage values required to reduce transconductance to 1,500 µmhos and 150 µmhos demonstrate the gain-control characteristics of the RF amplifier section.

Technical Summary

The 17JK8 is a specialized double dissimilar triode housed in a T-6 1/2 miniature 9-pin envelope. Featuring a 16.8 V, 150 mA heater with a controlled 17-second warm-up time, it is well-suited for series-string receiver designs. Its internal structure is divided into two specialized sections: Section 1, optimized for oscillator service with an amplification factor of 55 and a transconductance of 6,800 µmhos; and Section 2, optimized for RF amplification with a high transconductance of 13,000 µmhos and an amplification factor of 70. With low inter-section capacitance and robust design maximum ratings, the 17JK8 provides a compact and efficient solution for RF front-end, oscillator, and mixer stages.