1A4T — Remote-Cutoff Amplifier

A battery-operated, remote-cutoff amplifier tube designed for high-frequency amplifier service in battery-powered radio receivers.

Overview

The 1A4T (also designated as 1A4-T) is a battery-operated, remote-cutoff amplifier tube designed primarily for high-frequency amplifier service in battery-powered radio receivers. Characterized by its low filament power requirements and remote-cutoff transfer characteristic, the tube is variously classified as a tetrode or a remote-cutoff RF pentode. It features a glass bulb envelope, a small four-prong base, and a top cap connection for the control grid.

Construction, Filament, and Physical Dimensions

The 1A4T is constructed with a directly heated filament designed for low-current DC operation.

Filament Characteristics

The filament is optimized for battery-operated equipment, requiring a low current to conserve battery life:

• Filament Voltage: 2.0 V DC
• Filament Current: 0.060 A (60 mA)
• Filament Power (Calculated): 2.0 V * 0.060 A = 0.12 W

Physical Dimensions and Envelope

The tube utilizes a glass bulb, specifically an ST 12-D envelope (with an outline drawing designation of 12-6). The physical dimensions are constrained as follows:

• Maximum Outer Diameter: 1-3/16 inches
• Maximum Seated Height: 3-29/32 inches
• Maximum Overall Height: 4-17/32 inches

Base and Pinout

The 1A4T features a small 4-prong base (base connection 4K). The control grid is brought out to a top cap on the glass envelope to minimize input-to-output capacitance, which is critical for stable high-frequency amplification. The bottom-view pin connections are:

Electrical Characteristics and Typical Class A Operation

The 1A4T operates as a Class A amplifier. At its typical operating point, it provides a high amplification factor and high plate resistance, which are characteristic of screen-grid amplifier tubes.

Typical Operating Conditions

Under typical Class A conditions, the tube exhibits the following electrical parameters:

The remote-cutoff characteristic is demonstrated by the significant reduction in transconductance from 750 µmhos at a -3 V bias down to 15 µmhos when the negative grid bias is increased to -15 V. This allows the tube's gain to be controlled over a wide range by varying the grid bias, making it highly suitable for automatic volume control (AVC) circuits.

Interelectrode Capacitances

The interelectrode capacitances of the 1A4T vary depending on whether an external tube shield is employed. The top-cap control grid design helps maintain extremely low grid-to-plate capacitance, reducing the risk of unwanted feedback and oscillation in high-frequency circuits.

Maximum Ratings

To ensure reliable operation and prevent premature failure, the operating voltages of the 1A4T must be kept within the following maximum limits:

• Maximum Plate Voltage: 180 V
• Maximum Screen Voltage: 67.5 V

Characteristic Curves and Behavior

The electrical behavior of the 1A4T is illustrated by its characteristic curves under specific operating conditions.

Average Plate Characteristics

When plotted at a filament voltage of 2.0 V DC and a screen voltage of 67.5 V, with the control grid bias varied from 0 V to -7 V, the plate current curves reveal a distinct region below approximately 60 V of plate voltage. In this low-voltage region, the values are marked as unstable. This instability is a classic characteristic of tetrode-like structures, where secondary emission from the plate to the screen grid causes negative resistance effects when the plate voltage is lower than the screen voltage.

Average Transfer Characteristics

At a plate voltage of 180 V and a screen voltage of 67.5 V, the transfer curves (plotting plate current and transconductance against control grid bias from 0 V to -15 V) show that transconductance peaks near 1000 µmhos at 0 V bias. As the negative bias increases, the transconductance and plate current decrease smoothly, reaching a low value of 15 µmhos at -15 V. This smooth, gradual cutoff behavior confirms the remote-cutoff design, which prevents cross-modulation and distortion when handling strong radio signals.

Related Designations and Equivalents

The 1A4T is closely related to several other vacuum tube types:

• Type 1D5GT: The electrical characteristics and maximum ratings of the 1A4T are identical to those of the 1D5GT.
• Type 1A4P and 1D5GP: For the pentode variant designated as 1A4P, the corresponding equivalent type is the 1D5GP.

These relationships indicate identical electrical performance, though physical envelopes and base styles may differ between the designations.

Technical Summary

The 1A4T is a battery-operated, remote-cutoff amplifier tube featuring a directly heated 2.0 V, 0.06 A filament. Housed in an ST 12-D glass envelope with a small 4-prong base and a top cap control grid, it is designed for high-frequency amplification in battery-powered receivers. Operating at a maximum plate voltage of 180 V and a screen voltage of 67.5 V, it provides a transconductance of 750 µmhos, a plate resistance of approximately 1 MΩ, and an amplification factor of 720 under typical Class A conditions. Its remote-cutoff characteristic allows for effective gain control, while its low grid-to-plate capacitance ensures stable operation in RF and IF amplifier stages.