12K7GT — Remote-Cutoff RF Pentode

The 12K7GT (also designated as J5-12K7GT) is a glass-envelope, remote-cutoff (variable-mu) RF pentode designed for use as a radio-frequency or intermediate-frequency amplifier. It is particularly suited for series-heater AC/DC radio receivers, where its low current requirement optimizes overall power efficiency.

Construction, Heater, and Base Connections

The 12K7GT features a T-9 glass bulb envelope with a small wafer 7-pin octal base with a metal sleeve (base designation B7-27 or 7R). The tube incorporates a coated unipotential cathode and a skirted miniature top cap (C1-3) for the control grid connection. It can be mounted and operated in any position.

The heater is designed for series operation, operating at a nominal voltage of 12.6 V (with some early historical specifications listing 12.5 V) and a current of 0.15 A (150 mA).

• Heater Power Calculation: Operating at 12.6 V and 0.15 A, the heater power dissipation is calculated as: $12.6 \text{ V} \times 0.15 \text{ A} = 1.89 \text{ W}$

This 150 mA heater design reduces the total heater power requirement for a receiver by half compared to designs utilizing 0.3 A tubes. Additionally, it minimizes or eliminates the need for large plug-in resistors or specialized line cords by allowing the use of a small series resistance.

The physical dimensions of the 12K7GT are:

• Maximum Overall Length: 3-5/16 inches
• Maximum Seated Height: 2-3/4 inches
• Maximum Overall Height: 2-7/8 inches
• Maximum Diameter: 1-5/16 inches

Pinout Configuration

The bottom-view 7R basing arrangement provides the following connections:

• Pin 1: Base sleeve / connected to metal shell
• Pin 2: Heater (h)
• Pin 3: Anode / Plate (a)
• Pin 4: Grid 2 / Screen Grid (g2)
• Pin 5: Grid 3 / Suppressor Grid (g3) [typically connected to Pin 8 at the socket]
• Pin 6: No Pin (NP)
• Pin 7: Heater (h)
• Pin 8: Cathode and internal shield (k, I.S.)
• Top Cap: Grid 1 / Control Grid (g1)

Maximum Ratings

The maximum electrical limits for the 12K7GT under the JETEC and RMA Standard M8-210 rating systems are:

Typical Operating Conditions and Characteristics

The 12K7GT operates as a Class A1 amplifier. Depending on the plate and screen voltages, the typical operating points and characteristics vary as follows:

Interelectrode Capacitances

The direct interelectrode capacitances of the 12K7GT depend on whether an external shield is used.

Application and Performance Curves

The 12K7GT is designed primarily for RF and IF amplifier stages featuring Automatic Volume Control (AVC). Its remote-cutoff characteristic is specifically tailored to minimize cross-modulation in the presence of strong signals.

The performance of the tube is characterized by several key relationships:

• Plate Characteristics: Curves of plate voltage versus plate current demonstrate the pentode behavior across control grid voltages ($E_{[c1](/tubes/C1)}$) from 0 V to -20 V, under conditions of a 12.6 V heater, 100 V screen grid, and grounded suppressor grid.
• Grid #2 Characteristics: Curves of screen grid voltage versus plate current, screen current, and transconductance illustrate the tube's behavior at a plate voltage of 250 V, control grid voltage of -3 V, and grounded suppressor grid.
• Grid #1 Characteristics: Curves of control grid voltage versus plate current, screen current, and transconductance (plotted on a semi-logarithmic scale) show the remote-cutoff behavior at a plate voltage of 250 V, screen voltage of 100 V, and grounded suppressor grid.

Related Designations

The 12K7GT is electrically identical to the 6K7GT in its ratings, characteristics, and physical dimensions, differing only in its heater requirements (the 6K7GT utilizes a 6.3 V heater).

Technical Summary

The 12K7GT is an octal-based, remote-cutoff RF pentode featuring a 12.6 V, 150 mA heater and a unipotential cathode. It is designed for RF and IF amplification in AC/DC series-string receivers, offering excellent cross-modulation performance and AVC compatibility. With a maximum plate voltage rating of 300 V and typical transconductance values ranging from 1,325 to 1,650 µmhos, it provides a highly efficient amplification solution identical in performance to the 6K7GT but optimized for 12.6 V heater circuits.