11AR11 — Semiremote-Cutoff Twin Pentode

Overview

The 11AR11 is a duodecar-type, semiremote-cutoff twin pentode designed primarily for use as an intermediate-frequency (IF) amplifier in television receivers. It houses two electrically independent pentode units within a single Compactron T-9 glass envelope. This dual-unit configuration allows a single tube to perform multiple amplification stages, saving space and reducing component count in television chassis designs.

The 11AR11 belongs to a family of identical twin pentodes that includes the 6AR11 and 8AR11. These three types share identical physical dimensions, internal construction, electrode structures, and electrical characteristics, differing solely in their heater ratings and warm-up characteristics to accommodate different power supply configurations.

Construction, Physical Dimensions, and Base Connection

The 11AR11 features a Compactron T-9 glass envelope construction with a button 12-pin base (designation E12-70). The tube requires a standard duodecar 12-contact socket and can be mounted in any position.

The physical dimensions of the device are as follows:

• Maximum Diameter: 1.188 inches
• Maximum Seated Height: 2.000 inches
• Maximum Overall Height: 2.375 inches

The internal layout utilizes the 12DM basing arrangement, which provides separate pin connections for the plates, screen grids (Grid No. 2), suppressor grids (Grid No. 3), control grids (Grid No. 1), and cathodes of both pentode units, alongside a shared heater and an internal shield. The pinout configuration, viewed from the bottom of the base, is assigned as follows:

The inclusion of the internal shield (connected to Pin 7) is critical for minimizing capacitive coupling and crosstalk between the two high-gain pentode sections.

Heater Characteristics and Ratings

The 11AR11 is designed for series-string heater circuits or specific parallel arrangements. Its heater is designed to operate under the following parameters:

For comparison, the related 6AR11 operates at a nominal heater voltage of 6.3 V with a current of 0.8 A (800 mA), while the 8AR11 operates at 8.4 V with a current of 0.6 A (600 mA) and shares the 11-second average warm-up time.

To ensure reliable operation and prevent insulation breakdown, the heater-to-cathode voltage must be maintained within strict limits:

• Heater Negative with Respect to Cathode: 200 V maximum (total DC and peak)
• Heater Positive with Respect to Cathode: 100 V maximum DC, 200 V maximum (total DC and peak)

Electrical Characteristics and Typical Operation

Under typical Class A1 amplifier operating conditions, each pentode unit of the 11AR11 exhibits high transconductance and high plate resistance. The typical operating parameters for each individual section, with the suppressor grid connected directly to the cathode at the socket, are outlined below:

The semiremote-cutoff characteristic is demonstrated by the control grid (Grid No. 1) voltage required to reduce the transconductance to a low value. A bias voltage of approximately -15 V reduces the transconductance to 50 µmhos, allowing effective automatic gain control (AGC) action in television IF stages without sudden cutoff or severe cross-modulation.

Maximum Ratings (Design-Maximum System)

The maximum ratings are established under the Design-Maximum Rating System. These limits must not be exceeded under the worst-case operating conditions of supply voltage, component tolerances, and environmental variations. The ratings apply to each individual pentode unit:

• Plate Voltage: 330 V maximum
• Grid No. 3 (Suppressor Grid) Voltage, Positive Value: 0 V maximum
• Grid No. 2 (Screen Grid) Supply Voltage: 330 V maximum
• Grid No. 2 (Screen Grid) Voltage: Dependent on operating conditions (subject to specific rating curves)
• Grid No. 1 (Control Grid) Voltage, Positive-Bias Value: 0 V maximum
• Plate Dissipation: 3.1 W maximum
• Grid No. 2 (Screen Grid) Input:
  • For Grid No. 2 voltages up to 165 V: 0.65 W maximum
  • For Grid No. 2 voltages between 165 V and 330 V: Subject to specific rating curves

Interelectrode Capacitances

Because the 11AR11 is designed for high-frequency IF amplification, minimizing internal capacitive feedback is essential to prevent instability and oscillation. The direct interelectrode capacitances are highly controlled.

When measured with an external shield (such as EIA 309) connected to the cathode of the section under test, the typical capacitances are:

Cross-unit capacitances are also minimized to ensure excellent isolation between the two pentode sections:

• Plate of Unit No. 1 to Plate of Unit No. 2: 0.016 pF to 0.02 pF maximum
• Grid No. 1 of Unit 1 to Plate of Unit 2: 0.002 pF maximum
• Grid No. 1 of Unit 2 to Plate of Unit 1: 0.004 pF maximum

These extremely low feedback capacitances (such as the 0.026 pF grid-to-plate capacitance) allow high-gain, stable IF amplification at television operating frequencies without requiring complex neutralization circuits.

Technical Summary

The 11AR11 is a high-performance, duodecar-type semiremote-cutoff twin pentode designed for television IF amplifier applications. By integrating two independent, high-transconductance (10,500 µmhos) pentode units into a single Compactron T-9 envelope, it provides a compact and efficient solution for multi-stage bandpass amplification. Operating with an 11.2 V, 0.450 A heater, the 11AR11 features robust design-maximum ratings (330 V plate voltage, 3.1 W plate dissipation per unit) and exceptionally low interelectrode and cross-unit capacitances, ensuring stable and reliable operation in high-frequency circuits.