12FA6 — Heptode Miniature Pentagrid Converter

An indirectly heated, low-voltage heptode designed for combined mixer and oscillator service in automotive superheterodyne receivers.

Overview

The 12FA6 is a miniature heptode, designed as a pentagrid converter for use in superheterodyne radio receivers. Its primary application is to serve as a combined local oscillator and mixer stage. The tube is specifically engineered for mobile and automotive environments where all operating potentials—including the heater, plate, and screen grids (Grids No. 2 and No. 4)—are supplied directly from a nominal 12-volt vehicle battery system. This design eliminates the need for a high-voltage vibrator power supply or step-up DC-to-DC converter for the receiver's front-end stages.

While some early manufacturer documentation contains a typographical reference to the designation "12AF6" within the introductory text, the device is officially designated as the 12FA6. It must not be confused with other tube types, and direct substitution should only be performed with verified equivalent designations.

Construction, Physical Specifications, and Heater

The 12FA6 is constructed in a compact 7-pin miniature glass envelope. It features a coated unipotential cathode and can be operated in any mounting position.

Physical Dimensions

• Bulb Type: T-5 1/2 glass bulb
• Base Type: Miniature button 7-pin base (7CH basing configuration)
• Maximum Diameter: 3/4 inch (0.75 in)
• Maximum Seated Height: 1 7/8 inches
• Maximum Overall Length: 2 1/8 inches

Heater Characteristics

Because the tube is intended for automotive service, its heater is designed to withstand the wide voltage fluctuations typical of vehicle charging systems. The heater is rated to operate reliably over a voltage range spanning from 10.0 to 15.9 Volts, providing an adequate safety margin against battery overcharging or voltage drops.

Pinout and Electrode Connections

The 12FA6 utilizes the 7CH basing arrangement. When viewed from the bottom of the tube, the pin connections are assigned as follows:

Direct Interelectrode Capacitances

The capacitances of the 12FA6 are measured with an external shield (specifically shield #316) connected to Pin 2 (Cathode & Grid No. 5). The direct interelectrode capacitances are:

Maximum Ratings

The maximum electrical ratings for the 12FA6 are established under the Design Center System. Operating the tube beyond these limits may impair performance or shorten its service life.

Typical Operating Conditions and Characteristics

The 12FA6 is versatile in its application as a frequency converter, supporting both self-excited and externally excited circuit topologies. Additionally, the characteristics of its oscillator section can be evaluated independently under non-oscillating conditions.

Converter Service (Self-Excitation)

In self-excited converter service, screen feedback is utilized. The feedback is configured such that the $G_{2\&4}$ to cathode voltage is approximately 13% of the $G_1$ to cathode voltage. Typical operating parameters under these conditions are:

• Heater Voltage: 12.6 V
• Plate Voltage: 12.6 V
• Grid No. 3 Voltage: 0.50 V (Average contact potential developed across a 2.2 MΩ grid resistor)
• Grids No. 2 & No. 4 Voltage: 12.6 V
• Grid No. 1 Voltage (Oscillator Grid) RMS: 2.5 V
• Grid No. 1 Resistance (Oscillator Grid): 33,000 Ω
• Plate Resistance (Approximate): 0.8 MΩ
• Grid No. 1 Current (Oscillator Grid): 60 µA
• Conversion Transconductance ($G_c$): 320 µmhos
• Plate Current: 450 µA
• Grids No. 2 & No. 4 Current: 1,000 µA
• Cathode Current: 1,500 µA
• Grid No. 3 Voltage for $G_c = 5$ µmhos (Approx.): -3.5 V
• Grid No. 3 Voltage for $G_c = 20$ µmhos (Approx.): -3.0 V

Cathode Current Verification (Calculation)

Using the individual electrode currents under self-excited operation, the total cathode current ($I_k$) can be calculated as: $I_k = I_b + I_{[g2](/tubes/G2)\&4} + I_{g1}$ Inputting the typical values: $I_k = 450\ \mu\text{A} + 1,000\ \mu\text{A} + 60\ \mu\text{A} = 1,510\ \mu\text{A}$ This calculated total of 1,510 µA closely aligns with the published typical cathode current of 1,500 µA.

Converter Service (External Excitation)

When the local oscillator signal is injected from an external source, and Grids No. 2 and No. 4 are connected directly to the plate, the typical operating parameters are:

• Heater Voltage: 12.6 V
• Grid No. 3 Voltage: 0 V
• Grid No. 1 Voltage (Oscillator Grid) RMS: 0.85 V
• Grids No. 2 & No. 4 Connected to Plate Voltage: 12.6 V
• Conversion Transconductance ($G_c$): 300 µmhos
• Plate Current: 670 µA
• Grids No. 2 & No. 4 Current: 1,200 µA
• Cathode Current: 1,900 µA
• Grid No. 1 Current: 45 µA
• Grid No. 3 Voltage for $G_c = 10$ µmhos: -3.2 V

Cathode Current Verification (Calculation)

Using the individual electrode currents under externally excited operation, the total cathode current ($I_k$) is calculated as: $I_k = I_b + I_{[g2](/tubes/G2)\&4} + I_{g1}$ Inputting the typical values: $I_k = 670\ \mu\text{A} + 1,200\ \mu\text{A} + 45\ \mu\text{A} = 1,915\ \mu\text{A}$ This calculated total of 1,915 µA is in close agreement with the published typical cathode current of 1,900 µA.

Oscillator Characteristics (Non-Oscillating)

To evaluate the performance of the oscillator section independently, the tube can be tested under non-oscillating conditions with Grids No. 2 and No. 4 connected directly to the plate:

• Grid No. 3 Voltage: 0 V
• Grid No. 1 Voltage (Oscillator Grid): 0 V
• Grid No. 2 & No. 4 Connected to Plate Voltage: 12.6 V
• Transconductance (between Grid No. 1 and Grids No. 2 & No. 4): 4,000 µmhos
• Amplification Factor (between Grid No. 1 and Grids No. 2 & No. 4): 9
• Cathode Current: 4.5 mA
• Grid No. 1 Voltage (Approximate) for $I_b = 10$ µA: -3.5 V

Technical Summary

The 12FA6 is a specialized, low-voltage heptode pentagrid converter designed for automotive superheterodyne radio receivers. By operating directly from a nominal 12-volt battery supply (with a ruggedized heater designed for the 10.0 to 15.9 V range), it eliminates the need for high-voltage power generation in mobile receiver designs.

The tube features a 7-pin miniature construction, a coated unipotential cathode, and a 7CH basing configuration. Under typical operating conditions at 12.6 V, it provides a conversion transconductance of 300 to 320 µmhos, making it an efficient and reliable choice for combined oscillator and mixer service in low-voltage mobile applications.