12FX8A — Low-Voltage Triode-Heptode Vacuum Tube

The 12FX8A is a miniature multi-unit vacuum tube containing a triode and a heptode section within a single envelope. It is specifically designed for use in automobile radio receivers where plate, screen, and grid voltages are supplied directly from a 12-volt storage battery. In this application, the triode section functions as a radio-frequency (RF) amplifier, while the heptode section serves as a frequency converter.

The 12FX8A is unilaterally interchangeable with the 12FX8. The primary distinction between the two types is that the 12FX8A incorporates an additional control on the conversion-conductance cutoff characteristic of the heptode section.

Physical and Mechanical Specifications

The 12FX8A is constructed in a miniature T-6 1/2 glass bulb with a small button 9-pin base (E9-1). The tube is designed to operate in any mounting position.

Physical Dimensions

• Maximum Overall Length: 2.438 inches (2 7/16 inches)
• Maximum Seated Height: 2.188 inches (2 3/16 inches)
• Seated Height Range: 1.812 ± 0.094 inches
• Maximum Diameter: 0.875 inches (7/8 inches)

Terminal Connections and Basing

The tube utilizes the JEDEC 9KV basing arrangement. The pin connections, viewed from the bottom of the base, are as follows:

• Pin 1: Heptode Grid Numbers 2 and 4 (Screen Grid)
• Pin 2: Heptode Grid Number 1 (Oscillator Grid)
• Pin 3: Heptode Plate
• Pin 4: Heater
• Pin 5: Heater and Triode Cathode
• Pin 6: Triode Grid
• Pin 7: Heptode Cathode, Grid Number 5, and Internal Shield
• Pin 8: Triode Plate
• Pin 9: Heptode Grid Number 3 (Signal Grid)

Heater Characteristics and Supply Limits

The 12FX8A contains two coated unipotential cathodes. The heater is designed to operate from a nominal 12.6-volt supply, but because it is intended for automotive service, it must withstand the voltage fluctuations of a vehicle's electrical system.

Heater Supply Limits

When operated in automobile service from a nominal 12-volt source, the heater voltage must be maintained within strict limits to ensure reliable operation and acceptable tube life:

• Minimum Heater Voltage: 10.0 V
• Maximum Heater Voltage: 15.9 V

Extreme variations within this range may be tolerated for short periods. However, continuous operation near these absolute limits degrades performance at the low end and significantly reduces life expectancy at the high end. Improving supply-voltage regulation is highly recommended to maximize equipment reliability.

Heater-Cathode Voltage Ratings

• Heater Positive with Respect to Cathode (Maximum): 16 V
• Heater Negative with Respect to Cathode (Maximum): 16 V

Maximum Ratings (Design-Center Values)

The maximum ratings for the 12FX8A are established under the Design-Center system. These values represent limiting operating and environmental conditions for a nominal ("bogey") tube and should not be exceeded under normal operating conditions.

Typical Operating Characteristics

The 12FX8A is optimized for low-voltage operation, with typical operating points established at a nominal plate and screen supply of 12.6 V.

Average Characteristics (Class A Amplifier / Converter Service)

** Voltage developed across a 2.2 MΩ Grid #3 resistor.* *** Voltage developed across a 2.2 MΩ Grid resistor.*

Cutoff Characteristics

• Heptode Grid #3 Voltage (Approximate):
  • For a conversion transconductance ($G_c$) of 10 µmhos: -3.0 V
  • For a conversion transconductance ($G_c$) of 1.0 µmhos: -8.0 V
• Triode Grid Voltage (Approximate):
  • For a plate current ($I_b$) of 10 µA: -3.2 V

Heptode Oscillator Characteristics (Not Oscillating)

The heptode section can be evaluated under non-oscillating conditions with the screen connected directly to the plate:

• Plate Voltage: 12.6 V
• Screen (Connected to Plate) Voltage: 12.6 V
• Grid #3 Voltage: 0 V
• Grid #1 Voltage: 0 V
• Amplification Factor (between Grid #1 and Grids #2 & #4 connected to plate): 9
• Transconductance (between Grid #1 and Grids #2 & #4 connected to plate): 3600 µmhos
• Cathode Current: 4.4 mA
• Grid #1 Voltage (Approximate, for $I_b$ = 10 µA): -4.5 V

Direct Interelectrode Capacitances

These capacitance values are measured directly without an external shield.

Heptode Section Capacitances

• Grid #3 to Plate (max): 0.28 pF
• Grid #3 to Grid #1 (max): 0.12 pF
• RF Input (Grid #3 to Heater, Triode Cathode, Heptode Cathode, Grid #1, Grids #2 & #4, Grid #5, Plate, and Internal Shield): 6.0 pF
• Oscillator Input (Grid #1 to Heater, Triode Cathode, Heptode Cathode, Grids #2 & #4, Grid #3, Grid #5, Plate, and Internal Shield): 5.0 pF
• Mixer Output (Plate to Heater, Triode Cathode, Heptode Cathode, Grid #1, Grids #2 & #4, Grid #3, Grid #5, and Internal Shield): 5.0 pF
• Grid #1 to Cathode (Grid #1 to Heptode Cathode, Grid #5, and Internal Shield): 3.0 pF
• Oscillator Output (Heptode Cathode to Heater, Triode Cathode, Grids #2 & #4, Grid #3, and Plate): 17 pF
• Grid #1 to Plate (max): 0.16 pF

Triode Section Capacitances

• Grid to Plate: 1.3 pF
• Input (Grid to Triode Cathode and Heater): 2.2 pF
• Output (Plate to Triode Cathode and Heater): 0.25 pF

Inter-Section Capacitances

• Triode Grid to Heptode Grid #3 (max): 0.01 pF
• Triode Plate to Heptode Grid #3 (max): 0.18 pF
• Triode Plate to Heptode Plate (max): 0.20 pF