12FM6 — Duplex-Diode Triode Vacuum Tube

A miniature, duplex-diode triode designed for combined detector, automatic volume control (AVC) rectifier, and transistor driver or audio-frequency (AF) voltage amplifier applications in automobile receivers.

Overview

The 12FM6 is a multi-unit miniature vacuum tube containing a double detector diode and a medium-mu triode within a single T-5 1/2 glass envelope. The tube is specifically designed for operation in mobile environments, such as automotive receivers, where the heater and plate potentials are obtained directly from a nominal 12-volt storage battery.

The triode section is intended for use as an audio-frequency voltage amplifier or transistor driver, operating into low-impedance loads. The two diode sections share a common unipotential cathode with the triode, providing detection and AVC rectification functions in a single compact envelope.

Construction, Heater, and Pinout

The 12FM6 features a coated unipotential cathode shared by both the triode and the two diode units. It is housed in a T-5 1/2 glass bulb with a miniature button 7-pin base (EIA 7BT or E7-1). The tube can be mounted and operated in any position.

Heater Characteristics

The heater is designed to operate directly from a 12-volt automotive electrical system. Because automotive battery voltages fluctuate significantly depending on the state of charge and generator/alternator operation, specific operating limits are defined for reliability.

Under no circumstances should the heater voltage fall outside the 10.0 V to 15.9 V range when used in automobile service. Operation at or near these extreme limits involves a sacrifice in performance at low heater voltage and a reduction in life expectancy at high heater voltage. Improved supply-voltage regulation is recommended to maximize equipment reliability.

Terminal Connections and Basing

The 12FM6 uses the EIA 7BT basing arrangement. Viewed from the bottom of the tube, the pin connections are as follows:

Physical Dimensions

• Maximum Overall Length: 2 1/8 inches
• Maximum Seated Height: 1 7/8 inches
• Maximum Diameter: 3/4 inch
• Envelope Type: T-5 1/2 Glass
• Base Type: Miniature Button 7-Pin

Direct Interelectrode Capacitances

The capacitances of the 12FM6 electrodes are measured without an external shield:

Electrical Characteristics and Typical Class A Operation

The triode section of the 12FM6 is optimized for low-voltage operation, specifically with a plate potential of 12.6 V. Two typical Class A operating points are documented, illustrating the performance of the triode under different biasing methods:

Diode Characteristics

The two diode units are designed for detection and AVC rectification. Under typical operating conditions, each diode exhibits the following characteristic:

• Average Diode Current: 2.0 mA with 10 Volts DC applied to the diode plate.

Maximum Ratings

The maximum ratings for the 12FM6 are established under the Design-Center System. These ratings are limiting values of operating and environmental conditions applicable to a bogey tube and should not be exceeded under normal conditions.

Circuit Applications and Performance Curves

The 12FM6 is uniquely tailored for hybrid automotive receiver designs of the late 1950s, where vacuum tubes operated alongside transistors. In these circuits, the 12FM6 performs the functions of AM detection and AVC voltage generation, while its triode section serves as a voltage amplifier or driver stage to feed a power transistor output stage. Because the plate voltage is supplied directly from the 12-volt vehicle battery, the need for a high-voltage vibrator power supply is eliminated.

Average Plate Characteristics

The plate characteristics of the triode section demonstrate its capability at very low plate voltages (0 to 50 V). With a grid resistor of 2.2 Megohms (bypassed) and the heater operated at its rated value, the plate current varies smoothly with plate voltage across different grid bias voltages:

• At a plate voltage of 12.6 V and zero grid bias, the plate current is approximately 1.8 mA.
• As the grid is biased more negatively, the plate current decreases, reaching cutoff near -3.0 V to -4.0 V depending on the plate voltage.

Average Transfer Characteristics

The transfer curves illustrate the relationship between the grid supply voltage and the triode's dynamic parameters at a constant plate voltage of 12.6 V:

• Transconductance: Peaks near 1300 to 1400 umhos as the grid supply voltage approaches 0 V, and drops off as the grid is biased negatively.
• Plate Resistance: Increases from approximately 5600 Ohms at zero bias to over 20000 Ohms as the grid bias approaches -2.0 V.
• Amplification Factor: Remains relatively stable, ranging from approximately 10 to 13.5 across the primary operating region.
• Plate Current: Follows a typical triode transfer curve, tapering to zero near -3.0 V grid bias.