12SK7GT — Remote-Cutoff RF Pentode

The 12SK7GT is an indirectly heated, remote-cutoff (variable-mu) pentode designed for use as a radio-frequency (RF) or intermediate-frequency (IF) amplifier in radio receivers. Housed in a T-9 glass envelope with an eight-pin octal base, it is particularly suited for AC/DC receivers where series-string heater operation is employed.

Overview

The 12SK7GT is engineered to operate in stages utilizing automatic volume control (AVC). Its remote-cutoff characteristic allows the tube to handle a wide range of signal voltages with minimal cross-modulation and distortion. The design features a single-ended construction, meaning all electrode connections, including the control grid, are brought out through the base pins, eliminating the need for a top cap.

The electrical characteristics, ratings, and physical dimensions of the 12SK7GT are identical to those of the 6SK7GT, with the sole exception of the heater ratings. It is also the glass-envelope equivalent of the metal-shell 12SK7.

Construction, Heater, and Basing

The 12SK7GT incorporates a coated unipotential cathode. The heater is designed to operate from either AC or DC power supplies.

Physical Dimensions

The tube is housed in a T-9 glass bulb with a small wafer octal 8-pin base (basing designation 8N). It can be operated in any mounting position.

• Maximum Diameter: 1.188 inches
• Maximum Seated Height: 2.750 inches
• Maximum Overall Height: 3.312 inches

Pin Connections

The 8N basing configuration provides the following pin assignments, viewed from the bottom of the tube:

In typical applications, Grid No. 3 (Pin 3) is tied directly to the Cathode (Pin 5) at the tube socket.

Electrical Ratings

The maximum ratings for the 12SK7GT are established under the Design Center Rating System (RMA Standard M8-210). These limits represent the maximum values that the tube can safely withstand under normal operating variations:

• Maximum Plate Voltage: 300 V
• Maximum Grid No. 2 (Screen) Supply Voltage: 300 V
• Maximum Grid No. 2 Voltage: See Rating Chart (typically up to 150 V)
• Maximum Positive DC Grid No. 1 Voltage: 0 V
• Maximum Plate Dissipation: 4.0 W
• Maximum Grid No. 2 Dissipation: 0.4 W
• Maximum Heater-Cathode Voltage: 90 V

Typical Operating Conditions and Characteristics

The 12SK7GT can be operated under different typical Class A1 amplifier conditions depending on the plate supply voltage. The following table outlines two standard operating points:

Calculations and Variable-Mu Behavior

The remote-cutoff characteristic is mathematically demonstrated by the relationship between the control grid bias and the transconductance.

• At a grid bias of -3 V (Operating Point B), the transconductance is 2,000 µmhos.
• When the grid bias is increased to -35 V, the transconductance drops to 10 µmhos.

This represents a transconductance reduction ratio calculation of: $\frac{2000\text{ \mu mhos}}{10\text{ \mu mhos}} = 200$

This wide control range allows the tube to handle strong local signals without overloading, as the AVC circuit can apply a highly negative bias to reduce the stage gain by a factor of 200.

Direct Interelectrode Capacitances

The capacitances of the 12SK7GT vary depending on whether the tube is shielded. When measured with a close-fitting external shield connected to Pin 5, the typical capacitances are:

• Grid No. 1 to Plate (Max): 0.005 pF
• Input (Grid No. 1 to all other electrodes except Plate): 6.5 pF
• Output (Plate to all other electrodes except Grid No. 1): 7.5 pF

For the unshielded metal-shell version (12SK7), the capacitances are:

• Grid No. 1 to Plate (Max): 0.003 pF
• Input: 6.0 pF
• Output: 7.0 pF

The extremely low grid-to-plate capacitance (0.005 pF or less) is essential for preventing Miller-effect feedback, which can cause instability or oscillation in high-gain RF and IF amplifier stages.

Characteristic Curves and Qualitative Interpretation

The relationship between the electrode voltages and currents is illustrated by the tube's characteristic curves:

• Plate Current vs. Plate Voltage: The family of curves shows typical pentode behavior. Above a plate voltage of approximately 50 V, the plate current curves become nearly horizontal, indicating that plate current is largely independent of plate voltage and is primarily controlled by the Grid No. 1 and Grid No. 2 voltages.
• Transconductance vs. Grid No. 1 Voltage: At a constant plate voltage of 250 V and screen voltage of 100 V, the transconductance curve decreases smoothly and gradually as the control grid bias is made more negative. This gradual slope is the defining feature of the remote-cutoff design, preventing the abrupt cutoff of plate current that occurs in sharp-cutoff pentodes.
• Transconductance and Plate Resistance vs. Grid No. 2 Voltage: With a fixed plate voltage of 250 V and a grid bias of -3 V, increasing the screen grid (Grid No. 2) voltage from 25 V to 150 V results in a significant increase in transconductance and plate current, while the plate resistance decreases. This highlights the importance of maintaining a stable screen voltage to preserve the desired gain characteristics.

Related Designations and Comparisons

The 12SK7GT is part of a family of closely related tubes:

• 6SK7GT / 6SK7: These tubes share identical electrical ratings, typical operating points, and physical dimensions with the 12SK7GT, but utilize a 6.3 V, 0.3 A heater.
• 12SK7: This is the metal-shell version of the 12SK7GT. It shares the same electrical characteristics and 12.6 V heater rating but is housed in a metal envelope rather than glass.
• 12SF7 / 12SF7GT: While also remote-cutoff pentodes, these types include an internal diode plate, making them distinct from the pure pentode structure of the 12SK7GT.

Direct substitution between these types must take into account the heater voltage differences (12.6 V vs. 6.3 V) and envelope shielding characteristics.

Technical Summary

The 12SK7GT is an octal-based, remote-cutoff RF pentode featuring a 12.6 V, 0.15 A heater. It is designed for RF and IF amplification in radio receivers, particularly in AC/DC series-string configurations. With a maximum plate voltage rating of 300 V and a plate dissipation limit of 4.0 W, it provides reliable service under typical operating conditions of 100 V to 250 V on the plate. Its variable-mu design allows for smooth automatic volume control with a transconductance range extending from 2,000–2,350 µmhos down to 10 µmhos at a grid bias of -35 V.