12Q7G — Twin Diode-Triode Amplifier

Overview

The 12Q7G is an indirectly heated twin diode-triode vacuum tube designed for use as an amplifier and detector. It combines a high-mu triode section with two independent diode units in a single envelope, sharing a common indirectly heated cathode. This tube is typically applied in receiver circuits where it can perform simultaneous functions such as signal detection, automatic volume control (AVC), and audio-frequency voltage amplification.

Mechanical Specifications and Basing

The 12Q7G is constructed in an ST-12 glass bulb and utilizes a Small Octal 7-Pin base. It features a top-mounted Miniature cap, which provides the connection for the triode control grid.

The physical dimensions of the tube are:

• Maximum Overall Length: 4-15/32 inches
• Maximum Seated Height: 3-29/32 inches
• Maximum Diameter: 1-9/16 inches

The electrical connections are arranged according to the 7V basing diagram, viewed from the bottom of the base:

• Pin 1: No Connection
• Pin 2: Heater
• Pin 3: Triode Plate
• Pin 4: Diode Plate
• Pin 5: Diode Plate
• Pin 7: Heater
• Pin 8: Cathode
• Top Cap: Triode Grid

Heater and Electrical Characteristics

The 12Q7G features an indirectly heated cathode designed for series or parallel heater string operation. The nominal heater ratings are:

• Heater Voltage: 12.5 V
• Heater Current: 0.150 A
• Calculated Heater Power: 1.875 W (calculated as 12.5 V multiplied by 0.150 A)

Triode Section Characteristics

Under typical operating conditions as a Class A amplifier, the triode section exhibits the following electrical characteristics:

• Plate Voltage: 250 V (maximum)
• Grid Voltage: −3 V
• Plate Current: 1.1 mA
• Mutual Conductance (Transconductance): 1200 µmhos
• Amplification Factor (μ): 70
• Plate Resistance: 58,000 Ω

Resistance-Coupled Amplifier Operation

The 12Q7G is frequently operated as a resistance-coupled voltage amplifier. A typical operating point for this service is defined with the following circuit parameters:

• Plate Supply Voltage: 250 V (maximum)
• Plate Load Resistor: 0.25 MΩ (megohm)
• Grid Resistor of Following Stage: 0.5 MΩ (megohm)
• Grid Bias: −2 V
• Cathode Biasing Resistor: 4500 Ω
• Approximate Voltage Gain: 45

Diode Units and Capacitances

The 12Q7G contains two diode units intended for detection and automatic volume control. These diode units are characterized by:

• Maximum Peak Plate Current: 8 mA per diode
• Approximate Plate Resistance: 6500 Ω (measured at a diode current of 0.25 mA)

Interelectrode Capacitances

The direct interelectrode capacitances of the triode section are specified as follows:

• Grid-to-Cathode Capacitance: 1.6 µµF
• Grid-to-Plate Capacitance: 2.0 µµF
• Plate-to-Cathode Capacitance: 4 µµF

Technical Summary

The 12Q7G is an ST-12 glass envelope, Small Octal 7-Pin twin diode-triode vacuum tube. Operating with a 12.5 V, 0.150 A heater, it integrates a high-mu triode (amplification factor of 70, transconductance of 1200 µmhos) and two independent diode units sharing a common cathode. It is optimized for combined detection, automatic volume control, and voltage amplification, delivering a practical stage gain of approximately 45 in typical resistance-coupled amplifier configurations.