6SN7GT — Medium-Mu Twin Triode Vacuum Tube

An indirectly heated, dual-section receiving valve for voltage amplification, phase inversion, oscillation, multivibrator circuits, resistance-coupled amplifiers, and television deflection applications.

Overview

The 6SN7GT is a general-purpose medium-mu double triode containing two electrically independent triode systems within a single glass envelope. Each section has its own cathode, control grid, and plate (anode); the two sections share a common heater. The tube uses an eight-pin international octal base and a nominal 6.3 V, 0.6 A heater supply.

Brimar, Ferranti, General Electric, Sylvania, and Tung-Sol variously describe the device as a low-mu double triode, medium-mu twin triode, low-impedance double triode, or twin-triode amplifier. The design combines two substantially similar triodes, each with an amplification factor near 20, in one envelope. The independent electrode connections permit separate amplification, cascaded gain stages, phase-inverter operation, and combinations of oscillator and amplifier functions.

The documented applications include resistance-coupled and low-frequency amplification, audio amplification, phase inversion, oscillators, converters, multivibrators, and television vertical-oscillator and vertical-deflection amplifier circuits. Sylvania additionally publishes ratings for horizontal-deflection oscillator service. This does not make the 6SN7GT a horizontal-output power tube.

Construction, Heater, and Independent Sections

The cathodes are indirectly heated, coated unipotential cathodes. Both triodes share the heater, but their grids, plates, and cathodes have separate base connections. The cathodes are not internally common, so the two sections can be biased at different potentials, provided heater-to-cathode limits are observed.

Brimar and Ferranti state that the electrical characteristics of each 6SN7GT triode are equivalent to those of the 6J5GT single triode. This is a characteristic comparison, not a statement of pin-for-pin interchangeability.

Tung-Sol recommends minimizing the voltage difference between heater and cathode. Ferranti lists a 100 V heater-to-cathode limit; Sylvania distinguishes heater-positive and heater-negative conditions, including a 100 V heater-positive DC limit and 200 V limits under certain stated conditions. These figures are manufacturer- and condition-specific, not interchangeable universal ratings.

Octal Base and Physical Dimensions

The 8BD basing arrangement provides the following connections, viewed from the underside of the base:

Sylvania describes a T-9 glass bulb with intermediate-shell or short-intermediate-shell octal base configurations. Ferranti specifies a maximum overall length of 84 mm, maximum seated height of 70 mm, and maximum base diameter of 33 mm. The manufacturers permit mounting in any position.

Electrical Characteristics and Class A Operation

The 6SN7GT combines moderate intrinsic amplification with relatively low plate resistance. Its principal characteristics, at the manufacturers' stated operating points, are:

• Amplification factor (μ): approximately 20.
• Plate resistance (rₚ): approximately 6,700–8,000 Ω.
• Transconductance (gₘ): approximately 2.5–3.0 mA/V (2,500–3,000 µmhos).
• Typical plate current: approximately 9–10.6 mA.

General Electric, Sylvania, and Tung-Sol document two closely corresponding Class A operating points:

Brimar and Ferranti also give a point near 100 V plate voltage, 0 V grid bias, 10.5–10.6 mA plate current, 8,000 Ω plate resistance, and 2.5 mA/V transconductance.

The familiar triode relationship μ ≈ gₘ × rₚ is reflected in the published figures: 0.0026 S × 7,700 Ω ≈ 20. The tabulated characteristics change with operating point; the amplification factor remains approximately 20 in these examples.

Ratings and Their Context

Published maximum ratings differ among manufacturers and service categories. They must be applied according to the particular rating convention and circuit service rather than merged into a single permissive set.

Sylvania's 5 W combined limit is important: the two plates must not simply be assumed to dissipate 3.5 W each simultaneously. Likewise, specialized pulse ratings are not continuous DC ratings.

Resistance-Coupled Voltage Amplification

Brimar and Ferranti supply detailed resistance-coupled amplifier examples for each section:

The examples illustrate how greater supply voltage and appropriately chosen resistors allow a larger output-voltage swing while loaded stage gain remains in the neighborhood of 13–14. Stage gain is lower than the tube's intrinsic amplification factor because of the external circuit.

Tung-Sol publishes an alternative set of resistance-coupled conditions with a 100-kΩ plate resistor and supply voltages of approximately 90, 180, and 300 V. The associated cathode resistors are 2,940, 2,830, and 2,440 Ω; the published peak output voltages are 17, 34, and 56 V, and the corresponding voltage gains approximately 13, 14, and 14. Tung-Sol also specifies cathode-bypass and coupling components for these example circuits.

Both Brimar and Ferranti explicitly permit connecting the two triodes in cascade to obtain higher overall voltage gain. As a mathematical illustration, two idealized loaded stages each with gain 14 would have a combined gain of 14 × 14 = 196, before allowing for real loading and coupling effects. This is an illustration calculated from the stage-gain figures, not a separately guaranteed manufacturer specification.

Phase Inversion, Oscillation, and Multivibrators

Brimar and Ferranti identify phase-inverter service as an intended application. The two independently accessible triodes can be arranged in circuits that develop related signals of opposite polarity. Actual output balance and gain depend on the circuit topology and chosen external components; the sheets do not specify one universal phase-inverter circuit.

Tung-Sol explicitly identifies oscillator, converter, multivibrator, and audio-amplifier service. The two-section construction allows oscillator and associated amplifier functions within one envelope, or two interacting triode stages in a multivibrator. The datasheets establish these applications without specifying a universal oscillator frequency range or multivibrator timing formula.

Television Vertical Deflection and Oscillator Service

Sylvania describes the 6SN7GT as suitable for a combined vertical oscillator and vertical-deflection amplifier in television receivers. Its ratings distinguish ordinary Class A amplification from vertical-deflection amplifier, vertical-deflection oscillator, and horizontal-deflection oscillator service.

For specified vertical-deflection amplifier operation, Sylvania gives a 1,200 V absolute maximum peak positive plate voltage. This is a specialized pulsed limit and must not be interpreted as permission for 1,200 V continuous DC plate operation. The published television condition refers to a 525-line, 30-frame scanning system and restricts pulse duty cycle to 15% of one scanning cycle.

Sylvania also specifies different oscillator-service peak limits: vertical-deflection oscillator data include 400 V peak negative grid voltage and 70 mA peak cathode current; horizontal-deflection oscillator data include 600 V and 300 mA, respectively. These are application-specific peak ratings, not normal continuous Class A operating values.

Interelectrode and Inter-Section Capacitance

The manufacturers document capacitance within each triode and between the two assemblies. Brimar and Ferranti give approximate values without an external shield:

They also specify approximately 0.5 pF plate-to-plate, 0.1 pF grid-to-grid, and 0.2 pF from either grid to the opposite triode's plate. These small cross-section capacitances are relevant when the sections are used in separate stages or high-gain cascades: the electrode circuits are galvanically independent, but not capacitively isolated.

Tung-Sol provides a different measurement condition, using a close-fitting shield connected to the cathode. Its approximate values are 3.8/4.0 pF grid-to-plate, 2.8/3.0 pF grid-to-cathode, and 0.8/1.2 pF plate-to-cathode for the two triode units. The different conditions and definitions should be preserved when comparing capacitance tables.

The plate-to-grid capacitance also matters in amplifier design because the effective input capacitance can be affected by voltage gain. The precise circuit effect depends on gain, wiring, and external impedances.

Characteristic Curves and Design Interpretation

Brimar, Ferranti, Sylvania, and Tung-Sol include characteristic curves in their PDFs. The available sheets contain plate-current-versus-plate-voltage curves and transfer-characteristic material for the individual sections. These curves complement the tabulated operating points by showing how plate current varies with grid bias and plate voltage, rather than only at one or two specified conditions.

The tabulated values demonstrate that a 6SN7GT section can deliver moderate voltage gain with comparatively low internal plate resistance, while the curves provide the basis for selecting bias and load lines in a particular circuit. The two sections should be analyzed individually when their operating conditions differ.

Related Designations and Manufacturer Comparisons

The General Electric tables group the 6SN7-GT with the 6SN7-WGT for certain characteristics, while separately listing 6SN7-GTA and 6SN7-GTB with differing published maximum-voltage information. Those later suffixes must not be assumed to share every original 6SN7GT rating.

Sylvania compares the 6SN7GT with the 12SN7GT, stating that they have the same electrical characteristics except for heater requirements. The 12SN7GT uses a 12.6 V, 0.3 A heater rather than the 6SN7GT's 6.3 V, 0.6 A heater.

The RCA industrial documentation compares the 6SN7-GT with the premium 5692 twin triode, including different plate ratings, heater-to-cathode limits, and controlled characteristics. The 5692 is therefore a related industrial type, not evidence that its distinct ratings apply automatically to the 6SN7GT.

Brimar, Ferranti, and Tung-Sol also relate individual 6SN7GT sections to the 6J5 family. Such comparisons are useful for understanding the underlying triode characteristic, but they do not by themselves establish unrestricted whole-tube substitution.

Technical Summary

The 6SN7GT is an octal, indirectly heated, medium-mu double triode with two separate cathodes, a shared 6.3 V / 0.6 A heater, and two approximately matched low-impedance triode systems. Each section offers an amplification factor near 20, transconductance of roughly 2,500–3,000 µmhos, and plate resistance of approximately 6.7–8.0 kΩ at the published Class A operating points.

The manufacturer datasheets document a notably versatile range of service: audio and low-frequency voltage amplification, resistance-coupled amplifier stages, cascaded amplification, phase inversion, oscillators, converters, multivibrators, television vertical oscillators, vertical-deflection amplifiers, and horizontal-deflection oscillators. Its separate triode electrodes and documented interelectrode capacitances make it suitable for analyzing both independent-stage and coupled-stage operation.

Designers must distinguish the manufacturers' general amplifier ratings from specialized television pulse ratings and must respect individual as well as combined plate-dissipation limits. Within those constraints, the 6SN7GT provides two flexible, moderate-gain triode elements in a single eight-pin glass receiving valve.

This technical article is original synthesis based exclusively on the seven manufacturer PDF artifacts supplied through the Tube Tinker MCP. Numerical illustrations identified as calculations are derived from the published data.