1631 — Beam Power Amplifier Vacuum Tube

Overview

The 1631 is a metal-envelope beam power amplifier vacuum tube designed for applications requiring high performance and strict uniformity of characteristics. It is electrically similar to the classic 6L6 beam power tube but features a 12.6 V heater and modified dissipation ratings. The 1631 is particularly suited for push-pull Class AB1 audio frequency power amplification where matched performance and reliability are critical.

Construction, Heater, and Base Pinout

The 1631 is constructed with a rugged metal envelope, which provides self-shielding and mechanical durability. It utilizes an indirectly heated, coated unipotential cathode.

Heater Characteristics

The heater is designed for operation from a 12.6 V supply, making it suitable for mobile, aircraft, or industrial equipment utilizing 12 V electrical systems.

• Heater Voltage: 12.6 V (AC or DC)
• Heater Current: 0.45 A
• Calculated Heater Power: 5.67 W

Physical Dimensions

• Maximum Overall Length: 4-5/16 inches
• Maximum Diameter: 1-5/8 inches
• Envelope Type: Metal

Base and Wiring Connections

The tube uses a standard octal base with a 7AC pinout configuration. The pin connections, viewed from the bottom of the base, are as follows:

• Pin 1: Metal Shell (S) / Shield
• Pin 2: Heater (H)
• Pin 3: Plate (P)
• Pin 4: Grid No. 2 (Screen Grid, G2)
• Pin 5: Grid No. 1 (Control Grid, G1)
• Pin 7: Heater (H)
• Pin 8: Cathode (K)
• Note: Pin 6 is omitted from the active base pins.

Maximum Ratings

To ensure reliable operation and prevent premature tube failure, the 1631 must be operated within its maximum design limits. These absolute maximum ratings are:

• Maximum Plate Voltage: 360 V
• Maximum Grid-No. 2 (Screen) Voltage: 270 V
• Maximum Plate Dissipation: 16 W
• Maximum Grid-No. 2 Input: 2.5 W

Push-Pull Class AB1 Operating Conditions

The primary application of the 1631 is in push-pull Class AB1 amplifier circuits. The published operating data are specified for a pair of tubes operating together.

Common Operating Parameters (Two Tubes)

• Plate Supply Voltage: 360 V
• Grid-No. 2 Supply Voltage: 270 V
• Grid-No. 1 Bias Voltage: -22.5 V
• Total Zero-Signal Plate Current: 88 mA (44 mA per tube)
• Total Zero-Signal Grid-No. 2 Current: 5.0 mA (2.5 mA per tube)

Operating Configurations and Output Power

Depending on the plate-to-plate load impedance selected, the push-pull stage can be optimized for maximum power output or alternative performance characteristics:

Quiescent Dissipation Calculations

Using the common operating parameters, the quiescent (zero-signal) power dissipation for each tube can be calculated:

• Quiescent Plate Dissipation per Tube: 360 V * 0.044 A = 15.84 W. This quiescent dissipation represents approximately 99% of the maximum rated plate dissipation (16 W), indicating that the Class AB1 bias point is configured near the thermal limit of the plate structure for maximum efficiency and linearity.
• Quiescent Grid-No. 2 Dissipation per Tube: 270 V * 0.0025 A = 0.675 W. This is well within the maximum grid-no. 2 input rating of 2.5 W.

Comparison with Related Designations

The 1631 belongs to a family of specialized industrial and military-grade beam power tubes.

• Comparison with 6L6: The 1631 is described as similar to the 6L6 but differs in its heater voltage (12.6 V / 0.45 A versus the 6L6's 6.3 V / 0.9 A) and its maximum plate dissipation rating (16 W). Additionally, the 1631 is manufactured to tighter tolerances for applications critical as to the uniformity of characteristics.
• Comparison with 1632: While both are beam power tubes with 12.6 V heaters, the 1632 is similar to the 25L6 (except for its 12.6 V heater and dissipation ratings). The 1632 operates at much lower voltages (maximum plate and screen voltage of 117 V) and is rated for a lower power output of 2.1 W in Class A service, compared to the high-power capabilities of the 1631.

Technical Summary

The 1631 is a rugged, metal-envelope beam power amplifier featuring a 12.6 V, 0.45 A heater and a 7-pin octal base (7AC). Designed for applications requiring high uniformity of characteristics, it is similar to the 6L6 but optimized with a 16 W plate dissipation rating and a 12.6 V heater. In push-pull Class AB1 service, a pair of 1631 tubes can deliver up to 26.5 W of power into a 6,600 ̩ plate-to-plate load, or 18 W into a 3,800 ̩ load, under a 360 V plate supply and 270 V screen supply.