12SN7GTA — Medium-Mu Twin Triode Vacuum Tube

The 12SN7GTA is a glass octal, medium-mu twin triode designed for use in television receivers and general-purpose audio and radio equipment. It contains two independent triode units within a single glass envelope, sharing only a common heater. The tube is highly versatile and is commonly employed as a combined vertical oscillator and vertical-deflection amplifier, a horizontal-deflection oscillator, a multivibrator, a phase inverter, or a resistance-coupled amplifier.

Except for its heater ratings, the 12SN7GTA is electrically identical to the 6SN7GTB, and is also identical to the 6SN7GTA except for heater characteristics.

Heater, Basing, and Mechanical Specifications

The 12SN7GTA features an indirectly heated, coated unipotential cathode. It is designed to operate with either an AC or DC heater supply.

The tube utilizes an intermediate shell 8-pin octal base with an 8BD basing designation. The pin connections, viewed from the bottom of the base, are as follows:

• Pin 1: Grid, Triode Unit 2
• Pin 2: Plate, Triode Unit 2
• Pin 3: Cathode, Triode Unit 2
• Pin 4: Grid, Triode Unit 1
• Pin 5: Plate, Triode Unit 1
• Pin 6: Cathode, Triode Unit 1
• Pin 7: Heater
• Pin 8: Heater

Physical Dimensions and Mounting

• Bulb Type: T-9 glass
• Maximum Diameter: 1.188 inches
• Maximum Seated Height: 2.750 inches
• Maximum Overall Height: 3.312 inches
• Mounting Position: Any

Heater-Cathode Voltage Ratings (Design-Center Values)

To prevent breakdown between the heater and cathode, specific voltage limits must be observed:

• Heater Negative with Respect to Cathode: 200 V maximum (Total DC and Peak)
• Heater Positive with Respect to Cathode: 100 V maximum (DC component), 200 V maximum (Total DC and Peak)

Direct Interelectrode Capacitances

The capacitances of the two triode units are measured without an external shield. The values for each independent section are:

Class A1 Amplifier Characteristics and Ratings

When operated as a Class A1 amplifier, each triode section exhibits the following typical characteristics and operating conditions:

Maximum Ratings (Design-Center Values, Each Unit)

• Plate Voltage: 450 V
• Cathode Current: 20 mA
• Plate Dissipation (Either Plate): 5.0 W
• Plate Dissipation (Both Plates, both units operating): 7.5 W
• Maximum Grid-Circuit Resistance (Fixed Bias): 1.0 MΩ
• Maximum Grid-Circuit Resistance (Cathode Bias): 1.0 MΩ

Television Deflection and Oscillator Service

The 12SN7GTA is highly optimized for television sweep circuits. Its ratings are categorized by specific circuit functions under a 525-line, 30-frame system.

Vertical-Deflection Amplifier (Each Unit)

In vertical-deflection amplifier service, the tube must withstand high-voltage flyback pulses.

• DC Plate Voltage: 450 V maximum
• Peak Positive-Pulse Plate Voltage (Absolute Maximum): 1,500 V (The pulse duration must not exceed 15% of a vertical scanning cycle, equivalent to 2.5 milliseconds).
• Peak Negative-Pulse Grid Voltage: 250 V maximum
• Peak Cathode Current: 70 mA maximum
• Average Cathode Current: 20 mA maximum
• Plate Dissipation (Either Plate): 5.0 W maximum
• Plate Dissipation (Both Plates): 7.5 W maximum
• Maximum Grid-Circuit Resistance (Cathode Bias): 2.2 MΩ

Oscillator Service (Each Unit)

The tube can be configured as a vertical or horizontal deflection oscillator.

Note: In stages operating with grid-leak bias, an adequate cathode-bias resistor or other suitable protective means is required to safeguard the tube in the event that excitation is lost.

Resistance-Coupled Amplifier Operation

For general-purpose voltage amplification, each section of the 12SN7GTA can be configured as a resistance-coupled amplifier. The following table outlines typical operating conditions across various plate supply voltages ($E_{bb}$), load resistances ($R_p$), grid resistances ($R_{g1}$), and grid-leak or cathode-bias configurations:

• $E_o$ represents the maximum RMS voltage output for 5% total harmonic distortion.
• Gain is measured at an output level of 2.0 Volts RMS.
• For zero-bias configurations ($R_k = 0$ with a 10 MΩ grid resistor), the generator impedance is negligible.
• Coupling capacitors should be selected to achieve the desired low-frequency response, and the cathode resistor ($R_k$) must be adequately bypassed.

Technical Summary

The 12SN7GTA is a versatile, medium-mu twin triode featuring an 8-pin octal base and a T-9 glass envelope. It is characterized by its 12.6 V, 0.3 A heater, which distinguishes it from its 6.3 V counterparts like the 6SN7GTB. Each independent triode section provides an amplification factor of 20, a transconductance of 2,600 to 3,000 µmhos, and a plate resistance of 6,700 to 7,700 Ω under typical Class A1 operating conditions.

With robust design-center ratings including a maximum plate voltage of 450 V and a combined plate dissipation of 7.5 W, the 12SN7GTA is highly reliable for demanding television sweep applications (such as vertical deflection and horizontal/vertical oscillation) as well as general-purpose audio amplification, multivibrator, and phase-inverter circuits.