12BF6 — Twin Diode-Medium-Mu Triode

A multi-unit miniature vacuum tube containing two diodes and a medium-mu triode in a single envelope, designed for detector, amplifier, and automatic volume control (AVC) applications.

Overview

The 12BF6 is a miniature multi-unit tube of the heater-cathode type. It integrates two independent diode units and a single medium-mu triode within a compact 7-pin envelope. The tube is designed primarily for use as a combined detector, amplifier, and automatic volume control (AVC) stage in automobile radio receivers operating from a 12-volt storage battery system.

The electrical characteristics of the triode section allow it to be either impedance-coupled or transformer-coupled to the receiver's power output stage. The triode unit is capable of supplying a high output signal with low distortion, sufficient to drive a pair of 12V6-GT power tubes operating at maximum plate voltage. In overall performance, the 12BF6 is equivalent to the larger metal-envelope type 12SR7.

Construction, Heater, and Base Connections

The 12BF6 features a coated unipotential cathode shared by the triode and both diode units. The tube is housed in a T-5 1/2 glass bulb with a small-button miniature 7-pin base (JETEC No. E7-1). It can be mounted in any physical position.

Physical Dimensions

• Maximum Overall Length: 2-1/8 inches (2.125 inches)
• Maximum Seated Length: 1-7/8 inches (1.875 inches)
• Maximum Diameter: 3/4 inch (0.750 inch)

Heater Characteristics and Ratings

The heater is designed for operation under standard design-maximum limits:

• Average Heater Voltage: 12.6 V (AC or DC)
• Average Heater Current: 0.150 A (150 mA)
• Heater Voltage Supply Limits: 12.6 ± 1.3 V
• Heater Current Supply Limits: 150 ± 15 mA
• Maximum Peak Heater-Cathode Voltage: 90 V (with heater either negative or positive with respect to the cathode)

Pinout Configuration

The 7-pin miniature base (basing diagram JEDEC 7BT) utilizes the following pin assignments, viewed from the bottom of the tube:

• Pin 1: Triode Grid
• Pin 2: Cathode (common to triode and diode units)
• Pin 3: Heater
• Pin 4: Heater
• Pin 5: Diode Plate No. 2
• Pin 6: Diode Plate No. 1
• Pin 7: Triode Plate

Electrical Characteristics and Class A1 Operation

The triode section of the 12BF6 operates as a Class A1 amplifier. Under typical operating conditions, it exhibits a low amplification factor which facilitates large output voltage swings with minimal distortion.

Typical Triode Operating Characteristics (Transformer-Coupled)

• Plate Voltage: 250 V
• Grid Voltage: -9 V
• Amplification Factor: 16
• Plate Resistance (Approximate): 8,500 Ω
• Transconductance: 1,900 µmhos
• Plate Current: 9.5 mA
• Load Resistance: 10,000 Ω
• Total Harmonic Distortion: 6.5%
• Power Output: 300 mW

Maximum Ratings (Design-Center / Design-Maximum Values)

• Maximum Plate Voltage: 300 V
• Maximum Plate Dissipation: 2.5 W
• Maximum Average Diode Current (Each Unit, Continuous): 1.0 mA

Resistance-Coupled Amplifier Operation

The triode unit can be operated as a resistance-coupled voltage amplifier. The following operating conditions outline performance across different plate-supply voltages:

Note: The cathode bypass and blocking capacitors are selected to provide an output voltage at 100 Hz ($f_1$) that is equal to 0.8 of the mid-frequency value. The peak output voltage is obtained across the grid resistor of the following stage at any frequency within the flat region of the output-versus-frequency curve, up to the point where the grid begins to draw current.

Diode Units and Application Considerations

The 12BF6 contains two diode units alongside the triode unit, all sharing a common cathode (Pin 2).

• Diode Current: Each diode plate must deliver a minimum of 0.8 mA when 10 Volts DC is applied.
• Biasing: Diode biasing of the triode unit of the 12BF6 is explicitly not recommended.
• Continuous Operation: The average diode current for each unit must not exceed the maximum rating of 1.0 mA.

Storage-Battery Operating Considerations

When the 12BF6 is used in storage-battery-operated equipment, designers must adhere to specific design-center maximum guidelines to ensure reliable performance:

• Without Charger: The circuit must be designed so that the published maximum values of plate voltages, grid-No.2 supply voltages, dissipations, and rectified output currents are never exceeded for a terminal potential of 2.0 volts per cell at the battery source (equivalent to 12.0 V for a 6-cell battery).
• With Charger: The circuit must be designed so that 90 percent of the published maximum values are never exceeded for a terminal potential of 2.2 volts per cell at the battery source (equivalent to 13.2 V for a 6-cell battery).

Direct Interelectrode Capacitances

The interelectrode capacitances of the 12BF6 vary depending on whether an external shield is employed. Measurements are typically taken either without a shield or with an external shield connected to the cathode (or Pin 2).

Triode Section Capacitances

• Grid to Plate: 1.9 pF (without shield) / 1.9 pF (with shield) [Alternative measurement: 2.0 pF / 2.0 pF]
• Input (Grid to Cathode + Heater): 1.8 pF (without shield) / 1.9 pF (with shield) [Alternative measurement: 1.8 pF / 1.8 pF]
• Output (Plate to Cathode + Heater): 0.7 pF (without shield) / 1.2 pF (with shield) [Alternative measurement: 0.8 pF without shield / 1.1 pF with shield]

Diode Section Capacitances

• Diode Plate No. 1 to Grid (Max): 0.07 pF (without shield) / 0.06 pF (with shield)
• Diode Plate No. 2 to Grid (Max): 0.06 pF (without shield) / 0.05 pF (with shield)
• Diode Plate No. 2 to Heater and Cathode: 0.95 pF (without shield)
• Diode Plate No. 1 to Heater and Cathode: 0.66 pF (without shield)
• Diode Plate No. 1 to Cathode: 0.7 pF (without shield) / 1.4 pF (with shield)
• Diode Plate No. 2 to Cathode: 0.1 pF (without shield) / 1.5 pF (with shield)
• Diode Plate No. 1 to Triode Grid (Max): 0.07 pF (without shield) / 0.06 pF (with shield)
• Diode Plate No. 2 to Triode Grid (Max): 0.06 pF (without shield) / 0.05 pF (with shield)

Related Designations

The 12BF6 is identical to the 6BF6 except for its heater characteristics. The 12BF6 requires a heater voltage of 12.6 V and a heater current of 0.150 A, whereas the 6BF6 operates on a different heater configuration. Additionally, the 12BF6 serves as a direct performance equivalent to the larger metal-type 12SR7 tube.

Technical Summary

The 12BF6 is a 7-pin miniature double-diode triode designed for multi-functional service as a detector, amplifier, and AVC tube, particularly in 12-volt automotive radio systems. It features a shared unipotential cathode, a 12.6 V / 150 mA heater, and a low-mu triode unit with an amplification factor of 16. The triode's low amplification factor allows for high-amplitude output signals with minimal distortion, making it highly effective for driving subsequent power amplifier stages. With design-center maximum ratings of 300 V plate voltage and 2.5 W plate dissipation, the 12BF6 offers robust performance in a compact, versatile miniature glass envelope.