12AL11 — Dissimilar Double Pentode Compactron Tube

The 12AL11 is a multi-unit vacuum tube of the duodecar type, housed in a 12-pin compactron envelope. It contains two highly dissimilar pentode sections: a sharp-cutoff, dual-control pentode (Section 2) designed for use as an FM detector, and a high-power pentode (Section 1) designed for use as an audio-frequency output amplifier. This combination makes the 12AL11 particularly suited for television receiver applications, where it can perform both FM sound detection and audio power amplification within a single envelope.

The 12AL11 belongs to a family of tubes that includes the 6AL11 and 10AL11. These types share identical physical construction, electrode structures, and electrical characteristics, differing only in their heater ratings.

Physical Construction, Pinout, and Heater Ratings

The 12AL11 is constructed in a T-9 glass bulb with a button 12-pin base (E12-70 designation) and conforms to the JEDEC 9-58 outline. The tube features coated unipotential cathodes and is designed to operate in any mounting position.

Physical Dimensions

• Maximum Overall Length: 2.375 inches
• Seated Length: 2.000 inches
• Bulb Diameter: 1.062 to 1.188 inches

Heater Characteristics

The heater is designed for series-string or parallel operation, featuring a controlled warm-up time to ensure reliable performance in series-connected receiver designs.

• Heater Voltage (AC or DC): 12.6 V
• Heater Current: 0.450 A (with allowable limits of 0.450 ± 0.030 A)
• Average Heater Warm-up Time: 11 seconds

Heater-Cathode Voltage Ratings

The maximum ratings for voltage between the heater and the cathodes are identical for both Section 1 and Section 2:

• Heater Positive with Respect to Cathode:
  • DC Component: 100 V max
  • Total DC and Peak: 200 V max
• Heater Negative with Respect to Cathode:
  • Total DC and Peak: 200 V max

Pin Connections (JEDEC 12BU Basing Diagram)

The duodecar 12-contact socket connections, viewed from the bottom of the tube, are assigned as follows:

Section 1: Beam Power Unit (Audio-Frequency Output Amplifier)

Section 1 is a power pentode (beam power unit) optimized for Class A1 audio amplification. It is capable of delivering several watts of power output to drive a television receiver's loudspeaker.

Maximum Ratings (Design-Maximum Values)

• Plate Voltage: 275 V
• Grid No. 2 (Screen-Grid) Voltage: 275 V
• Plate Dissipation: 10 W
• Grid No. 2 Input: 2.0 W
• Grid No. 1 Circuit Resistance:
  • With Fixed Bias: 0.25 MΩ max
  • With Cathode Bias: 0.5 MΩ max

Typical Operation and Characteristics (Class A1 Amplifier)

Under typical operating conditions, Section 1 provides the following performance:

• Plate Voltage: 250 V
• Grid No. 2 Voltage: 250 V
• Grid No. 1 (Control-Grid) Voltage: -8.0 V
• Peak AF Grid No. 1 Voltage: 8.0 V
• Zero-Signal Plate Current: 35 mA
• Maximum-Signal Plate Current: 39 mA
• Zero-Signal Grid No. 2 Current: 2.5 mA
• Maximum-Signal Grid No. 2 Current: 7.0 mA
• Plate Resistance (Approximate): 100,000 Ω (0.1 MΩ)
• Transconductance: 6,500 µmhos
• Load Resistance: 5,000 Ω
• Total Harmonic Distortion (Approximate): 10%
• Maximum-Signal Power Output: 4.2 W

Section 2: Sharp-Cutoff, Dual-Control Pentode (FM Detector)

Section 2 is a sharp-cutoff pentode featuring dual-control grids (Grid No. 1 and Grid No. 3), making it highly suitable for FM quadrature detection and similar circuit configurations.

Maximum Ratings (Design-Maximum Values)

• Plate Voltage: 330 V
• Grid No. 2 Supply Voltage: 330 V
• Grid No. 2 Voltage: Subject to rating chart limits (up to 165 V maximum for full dissipation, derating at higher voltages)
• Grid No. 3 (Suppressor-Grid) Voltage: 28 V
• Positive DC Grid No. 1 Voltage: 0 V
• Plate Dissipation: 1.7 W
• Grid No. 2 Input:
  • For Grid No. 2 voltages up to 165 V: 1.1 W
  • For Grid No. 2 voltages between 165 V and 330 V: Derated as shown in the Grid No. 2 rating chart (decreasing linearly to 0 W at 330 V)

Average Characteristics (Class A1 Amplifier Service)

When evaluated as a standard amplifier, Section 2 exhibits the following characteristics:

• Plate Supply Voltage: 150 V
• Grid No. 3 Voltage: 0 V
• Grid No. 2 Supply Voltage: 100 V
• Cathode-Bias Resistor: 560 Ω
• Plate Current: 1.3 mA
• Grid No. 2 Current: 2.1 mA
• Grid No. 1 Transconductance (Grid No. 1 to Plate): 1,000 µmhos
• Grid No. 3 Transconductance (Grid No. 3 to Plate): 200 to 400 µmhos
• Plate Resistance (Approximate): 0.15 MΩ
• Grid No. 1 Voltage (Approximate for Plate Current of 30 µA): -4.5 V
• Grid No. 3 Voltage (Approximate for Plate Current of 50 µA): -4.5 V

Direct Interelectrode Capacitances

The interelectrode capacitances of the 12AL11 are measured without an external shield. The internal shield of the tube is connected to the appropriate reference points during measurement.

Section 1 (Beam Power Unit)

• Grid No. 1 to Plate: 0.26 pF
• Input (Grid No. 1 to Cathode, Heater, Grid No. 2, Grid No. 3, and Internal Shield): 11 pF
• Output (Plate to Cathode, Heater, Grid No. 2, Grid No. 3, and Internal Shield): 12 pF

Section 2 (Pentode Unit)

• Grid No. 1 to Plate: 0.034 pF
• Grid No. 3 to Plate: 3.2 pF
• Grid No. 1 to All Except Plate: 6.5 pF
• Grid No. 3 to All: 7.5 pF
• Grid No. 1 to Grid No. 3: 0.24 pF

Inter-Section Capacitance

• Pentode Plate to Beam Power Plate (Plate to Plate): 0.12 pF

Technical Summary

The 12AL11 is a versatile, dissimilar double pentode compactron tube designed for television receiver applications. By combining a high-power beam pentode (Section 1) capable of delivering 4.2 W of audio output and a sharp-cutoff, dual-control pentode (Section 2) optimized for FM detection, the tube consolidates the entire sound-demodulation and audio-output stages of a television receiver into a single 12-pin envelope. Operating with a 12.6 V, 0.45 A heater with controlled warm-up characteristics, it is highly compatible with series-string heater designs. The physical separation of the two sections, combined with low inter-section plate-to-plate capacitance (0.12 pF), ensures stable operation without unwanted coupling between the high-gain detector and the power output stage.