12BK6 — High-Mu Double-Diode Triode

The 12BK6 is a miniature, multi-unit vacuum tube containing two high-perveance diodes and a single high-mu triode in one envelope. It is designed for use in radio receivers where it can perform the combined functions of AM detection, automatic volume control (AVC), and audio voltage amplification. The tube features a 7-pin miniature construction and is designed to operate with a 12.6 V heater supply.

Mechanical Specifications and Pinout

The 12BK6 is constructed in a T-5 1/2 glass bulb with an E7-1 miniature button 7-pin base. The physical dimensions of the tube are as follows:

The tube can be operated in any mounting position. The basing arrangement (designated as 7BT or 7BT-0-2) provides the following pin connections, viewed from the bottom of the tube:

• Pin 1: Grid (Triode)
• Pin 2: Cathode and Diode Shield
• Pin 3: Heater
• Pin 4: Heater
• Pin 5: Diode Plate #2
• Pin 6: Diode Plate #1
• Pin 7: Plate (Triode)

The internal diode shielding is connected directly to Pin 2 (along with the cathode), which reduces undesirable audio coupling between the diode units and the triode section.

Heater and Cathode Characteristics

The 12BK6 utilizes a coated unipotential cathode. The heater is designed for operation on either AC or DC power.

The calculated heater power is derived from the nominal values: Power = 12.6 V * 0.150 A = 1.89 W. The maximum rating for the voltage between the heater and the cathode is 90 V.

Triode Electrical Characteristics and Class A1 Operation

The triode section of the 12BK6 is a high-mu voltage amplifier. Typical operating conditions and electrical characteristics under Class A1 amplifier service are specified at two distinct plate voltage points:

The fundamental triode relationship where amplification factor is approximately equal to transconductance multiplied by plate resistance is verified by the typical characteristics:

• At 100 V Plate Voltage: 0.00125 S * 80,000 ΓΑ = 100
• At 250 V Plate Voltage: 0.00160 S * 62,500 ΓΑ = 100

The calculated plate power dissipation at these typical operating points is:

• At 100 V Plate Voltage: 100 V * 0.0005 A = 0.05 W (50 mW)
• At 250 V Plate Voltage: 250 V * 0.0012 A = 0.30 W (300 mW)

Diode Unit Characteristics

The two diode units are independent of each other, except for their common cathode connection and the internal shielding. They are designed with high perveance to ensure high rectification efficiency even when handling low-level signals.

• Maximum Continuous Diode Current: 1.0 mA per plate.
• Diode Current at 10 V DC: When a DC potential of 10 V is applied to an individual diode plate, the resulting current is specified as:
  • Minimum: 1.5 mA
  • Average: 4.0 mA

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 12BK6 are specified both with and without an external shield. When an external shield is used, it is standard RMA shield #316 connected to Pin 2 (cathode).

(Note: h represents heater, k represents cathode, and i.s. represents internal shield).

Maximum Ratings

The following maximum ratings are established under the Design Center system (interpreted according to RMA Standard M8-210):

Analysis of Characteristic Curves

The electrical behavior of the 12BK6 is further illustrated by its characteristic curves:

• Triode Plate Characteristics: The curves of plate current versus plate voltage show that at a grid bias of 0 V, the plate current rises rapidly with plate voltage, reaching approximately 3.5 mA at 150 V. As the grid bias is made more negative (down to -5.0 V), the curves shift to the right, demonstrating typical high-mu triode cutoff behavior where higher plate voltages are required to initiate plate current.
• Diode Characteristics: The diode curves plot the DC voltage developed by the diode against the rectified microamperes for various load resistance values (ranging from 0.1 MΓΑ to 2 MΓΑ) at an RMS signal input of 30 V. These curves show that higher load resistances yield a higher developed DC voltage for a given rectified current, illustrating the rectification efficiency of the high-perveance diode units.
• Triode Parameter Curves: Curves plotting transconductance, plate resistance, and amplification factor against grid voltage show that the amplification factor remains relatively stable near 100 over a wide range of grid bias voltages, while the transconductance decreases and plate resistance increases as the grid bias becomes more negative.

Related Designations

The 12BK6 is part of a family of double-diode triodes with similar internal structures but different heater ratings. The designations 6BK6 and 26BK6 represent tubes with identical electrical characteristics and pinouts, differing only in their heater requirements.

Technical Summary

The 12BK6 is a 7-pin miniature double-diode triode featuring a high-mu triode (amplification factor of 100) and two high-perveance diodes sharing a common unipotential cathode. Operating with a 12.6 V, 150 mA heater, the tube is optimized for detection and audio amplification in compact receiver designs. Its internal shielding, connected to the cathode on Pin 2, minimizes capacitive coupling between the diode and triode sections, ensuring clean signal processing. With a maximum plate voltage rating of 300 V and a maximum continuous diode current of 1.0 mA per plate, the 12BK6 provides a reliable, space-saving solution for multi-function stage designs.