12SF7 — Diode Remote-Cutoff Pentode Vacuum Tube

Overview

The 12SF7 and 12SF7-GT are receiving tubes containing a diode and a remote-cutoff pentode within a single envelope. Designed for operation with a 12.6-volt heater supply, these tubes are typically utilized in Class A amplifier service where the remote-cutoff characteristic of the pentode section is advantageous, alongside the diode section for detection or auxiliary functions.

Physical and Construction Specifications

The 12SF7 and 12SF7-GT feature an indirectly heated cathode design. The physical packaging differs between the two designations, as indicated by their respective outline drawings:

• 12SF7: Utilizes outline drawing 8-1.
• 12SF7-GT: Utilizes outline drawing 9-18.

Both variants share the 7AZ base connection configuration.

Heater Characteristics

The heater is designed for series or parallel operation in receiving equipment. The electrical requirements for the heater are:

Note: The heater power of 1.89 W is a calculated value derived from the nominal voltage of 12.6 V and nominal current of 0.15 A (12.6 V * 0.15 A = 1.89 W).

Direct Interelectrode Capacitances

The capacitance values are identical for both the metal 12SF7 and the glass-octal 12SF7-GT. These values are specified without an external shield:

Maximum Ratings

To ensure reliable operation, the electrical potentials applied to the pentode electrodes must be maintained within the specified maximum limits:

• Maximum Plate Voltage: 300 V
• Maximum Screen (Grid No. 2) Voltage: 150 V

Class A Amplifier Operating Points

The pentode section of the 12SF7 and 12SF7-GT is characterized for Class A amplifier service. Two distinct typical operating points are established, demonstrating the performance of the tube under different plate voltage conditions while maintaining a constant screen voltage of 100 V and a control grid bias of -1.0 V.

At the higher plate voltage of 250 V (Operating Point I), the plate resistance is approximately 700,000 \u0026Omega; with a transconductance of 2,050 \u0026micro;mhos. When the plate voltage is reduced to 100 V (Operating Point II) with the screen voltage held at 100 V, the plate current decreases slightly from 12.4 mA to 12.0 mA, while the screen current increases slightly from 3.3 mA to 3.4 mA. Under these lower-voltage conditions, the approximate plate resistance drops significantly to 200,000 \u0026Omega;, and the transconductance is slightly reduced to 1,975 \u0026micro;mhos.

Technical Summary

The 12SF7 and 12SF7-GT are multi-unit tubes combining a diode and a remote-cutoff pentode. Operating with a 12.6 V, 0.15 A heater, they provide compact amplification and detection capabilities. The pentode section supports plate voltages up to 300 V and screen voltages up to 150 V. In typical Class A amplifier service, the tube delivers a transconductance of 1,975 to 2,050 \u0026micro;mhos depending on the plate voltage configuration, with an extremely low grid-to-plate capacitance of 0.004 pF when operated without an external shield.