18FX6 — Dual Control Pentagrid Amplifier

The 18FX6 is a miniature heptode designed for combined mixer and oscillator functions, operating as a pentagrid converter. It is primarily intended for converter applications in line-operated AC/DC superheterodyne radio receivers utilizing series-connected heater strings.

Mechanical Construction and Pinout

The 18FX6 is constructed with a T-5 1/2 glass envelope and a miniature button 7-pin base (EIA designation 7CH or E7-1). It features a coated unipotential cathode and can be mounted in any position.

The physical dimensions of the tube are as follows:

• Maximum Diameter: 3/4 inch
• Maximum Overall Length: 2 1/8 inches
• Maximum Seated Height: 1 7/8 inches

The bottom-view pin connections (EIA 7CH) are assigned as follows:

Heater Ratings and Interelectrode Capacitances

The heater is designed for series-string operation in AC/DC receivers, drawing a low current to match standard line-operated configurations.

• Heater Voltage: 18 Volts (AC or DC) with a ± 10% operating tolerance
• Heater Current: 0.10 Amp (100 mA)

Direct Interelectrode Capacitances

The capacitances of the 18FX6 vary depending on whether an external shield is used. The values below compare measurements taken with Shield EIA 316 connected to Pin 2 (cathode) against measurements taken without a shield:

Design-Maximum Ratings

The ratings of the 18FX6 are established under the Design-Maximum System. Operating the tube beyond these limits may impair its performance and longevity.

• Maximum Plate Voltage: 150 Volts
• Maximum Grid #2 & Grid #4 Supply Voltage: 150 Volts
• Maximum Grid #2 & Grid #4 Voltage: 110 Volts
• Maximum Plate Dissipation: 1.0 Watt
• Maximum Grid #2 & Grid #4 Dissipation: 1.2 Watts
• Maximum Heater-Cathode Voltage: 100 Volts (Heater positive or negative with respect to cathode)

Typical Operating Conditions and Characteristics

The 18FX6 can be operated with separate excitation in converter service, or analyzed in a non-oscillating state to determine the characteristics of its oscillator section.

Converter Service (Separate Excitation)

Under typical operating conditions in an EIA conversion conductance test configuration, the tube exhibits the following characteristics:

• Plate Voltage: 100 Volts
• Grid #2 & Grid #4 (Screen) Voltage: 100 Volts
• Grid #3 (Mixer Grid) Voltage: -1.5 Volts
• Grid #1 (Oscillator Grid) Resistance: 20,000 Ohms
• Grid #1 Current: 0.5 mA
• Plate Current: 2.3 mA
• Grid #2 & Grid #4 Current: 6.2 mA
• Cathode Current: 9.0 mA
• Conversion Transconductance: 480 µmhos
• Plate Resistance (Approx.): 0.4 Megohm
• Grid #3 Voltage for Gc = 10 µmhos (Approx.): -21 Volts

Oscillator Section (Non-Oscillating)

When evaluating the oscillator portion of the structure without active oscillation, the screen grids (Grid #2 and Grid #4) are tied directly to the plate to form a triode-connected configuration:

• Grid #3 Voltage: 0 Volts
• Grid #1 Voltage: 0 Volts
• Grid #2 & Grid #4 Connected to Plate: 100 Volts
• Cathode Current: 24 mA
• Transconductance (between Grid #1 and Grids #2 & #4 connected to plate): 7000 µmhos
• Amplification Factor (between Grid #1 and Grids #2 & #4 connected to plate): 22
• Grid #1 Voltage for Ib = 20 µA (Approx.): -9.2 Volts

Analysis of Characteristic Curves

The behavior of the 18FX6 under varying electrical potentials is characterized by several distinct operational curves:

• Average Plate Characteristics (Triode Connection): When Grid #2 and Grid #4 are connected to the plate and Grid #3 is held at 0 V, the tube behaves like a triode. The relationship of plate and grid current versus plate voltage (from 0 to 500 V) shows classic triode-like curves across a range of Grid #1 bias voltages from +4.0 V down to -26 V.
• Average Transfer Characteristics: The conversion transconductance (Gc) peaks near 500 µmhos when Grid #3 is at 0 V and falls off smoothly to near zero as the Grid #3 bias is swept down to -20 V. Throughout this sweep, the screen current (Ic2+4) remains relatively stable, hovering around 10 mA.
• Average Characteristics vs. Plate Voltage: When Grid #3 is biased at -1.5 V, the conversion transconductance (Gc), plate current (Ib), and screen current (Ic2+4) vary with plate voltage from 0 to 200 V. The conversion transconductance rises sharply as plate voltage increases up to 50 V, after which it levels off near 500 µmhos. Similarly, the plate current remains flat at approximately 2.3 mA for plate potentials above 50 V.

Technical Summary

The 18FX6 is a versatile, 7-pin miniature pentagrid converter designed for line-operated superheterodyne receivers. Featuring an 18 V, 100 mA heater, it is optimized for series-string heater circuits. With a maximum plate rating of 150 V and a conversion transconductance of 480 µmhos under typical 100 V operating conditions, the 18FX6 combines the functions of an oscillator and a mixer into a single, compact glass envelope.