1629 — Electron-Ray Tuning Indicator Tube

An indirectly heated, octal-based "Magic Eye" visual indicator tube containing a triode unit and a fluorescent target, designed for voltage indication and tuning applications.

Overview

The 1629 is a specialized electron-ray tube, commonly referred to as a "Magic Eye" tuning indicator. It integrates a triode amplifier and a circular fluorescent target within a single T-9 glass envelope. The primary function of the 1629 is to provide a visual representation of voltage changes, making it highly useful as a tuning indicator in radio receivers and as a voltage indicator in specialized equipment, such as aircraft electronics.

The 1629 is closely related to the 6E5 electron-ray tube. Physically and electrically, the 1629 and 6E5 are identical in their indicator behavior, operating curves, and internal structure, with the sole exception of their heater ratings. While the 6E5 operates on a standard 6.3 V, 0.3 A heater, the 1629 is designed for a 12.6 V, 0.15 A heater supply, making it suitable for series-string heater chains or higher-voltage mobile and aviation power systems.

Construction, Mechanical Specifications, and Pinout

The 1629 features a coated unipotential cathode that is indirectly heated. The tube is housed in a T-9 glass bulb and utilizes a small shell octal base. The physical construction is designed to allow mounting in any position. To assist with physical alignment in equipment, the plane of the ray-control electrode is aligned such that it passes directly through the tube axis and the base key.

Mechanical Dimensions

• Overall Length: 3-15/16 inches ± 3/16 inch (up to a maximum of 4-3/16 inches)
• Seated Height: 3-3/8 inches ± 3/16 inch
• Maximum Diameter: 1-3/16 inches (or 1.165 inches maximum)

Heater Characteristics

• Heater Voltage (AC or DC): 12.6 V
• Heater Current: 0.15 A
• Heater Supply: AC or DC

Heater Power Calculation

Using the nominal heater voltage of 12.6 V and the nominal heater current of 0.15 A, the calculated power consumption of the heater is:

12.6 V * 0.15 A = 1.89 W

Base Connections (7AL Bottom View)

The 1629 utilizes a small shell octal 7-pin base. The pin assignments, viewed from the bottom of the tube, are as follows:

Electrical Ratings

The maximum and minimum ratings for the 1629 are established under the design-center rating system for indicator service:

The minimum target voltage limit (specified as 125 V by some manufacturers and 100 V by others) is necessary to ensure adequate brightness and definition of the fluorescent pattern on the target.

Typical Operating Conditions and Indicator Characteristics

The 1629 operates by varying the shadow angle of the circular fluorescent target. This shadow angle is controlled by the voltage applied to the triode grid. When the grid bias is zero, the shadow angle is at its maximum of approximately 90° (open eye). As the grid is biased more negatively, the shadow narrows until it reaches 0° (closed eye).

Typical operating parameters across different supply voltages are outlined below:

Note: The target current is subject to wide variations in practical operation.

Circuit Operation and Curve Interpretation

In a typical indicator circuit, the triode plate of the 1629 is connected to the plate supply voltage through a high-value series resistor (typically 0.5 to 1.0 megohm), often designated as resistor R. The triode plate is also internally connected to the ray-control electrode.

When the triode grid voltage is at 0 V, the triode section conducts maximum current. This current flows through the series plate resistor, causing a significant voltage drop across it. As a result, the potential on the triode plate (and thus the ray-control electrode) falls well below the target voltage. The electrostatic field of the ray-control electrode repels the electrons flowing toward the target, casting a wide shadow (approximately 90°) on the circular fluorescent screen.

When a negative control voltage (such as an automatic volume control voltage) is applied to the triode grid, the triode plate current is reduced. This decreases the voltage drop across the series plate resistor, raising the potential of the ray-control electrode closer to the target potential. The electrostatic deflecting field weakens, allowing electrons to strike the target in the shadow area, thereby narrowing the shadow angle toward 0°.

Because the internal triode and indicator structure of the 1629 is identical to that of the 6E5, the characteristic curves—including plate current versus grid voltage and shadow angle versus grid voltage—are identical to those published for the 6E5.

Technical Summary

The 1629 is an octal-based, indirectly heated electron-ray tuning indicator featuring a 12.6 V, 0.15 A heater and a unipotential cathode. It combines a control triode and a circular fluorescent target in a T-9 glass envelope. Designed for visual voltage indication, it operates with a maximum plate and target supply of 250 V and controls a shadow angle from 90° to 0° using a grid control voltage range of 0 V to -8.0 V (with a 1.0 megohm series plate resistor). Except for its 12.6 V heater, the 1629 is identical to the 6E5 and is particularly suited for aircraft and series-string applications.