1A3 — High-Frequency Diode Vacuum Tube

An indirectly heated miniature diode designed for high-frequency detection, FM discriminator circuits, and portable measuring equipment.

Overview

The 1A3 is an indirectly heated miniature diode designed primarily for high-frequency (HF) and radio-frequency (RF) applications. It is commonly employed as a discriminator tube in portable FM receivers and in portable high-frequency measuring equipment. The tube features a coated unipotential cathode, a 7-pin miniature construction, and is capable of operating at frequencies up to approximately 1000 MHz (1000 Mc/s), which is its approximate resonant frequency.

Physical and Mechanical Specifications

The 1A3 is constructed with a T-5 1/2 glass bulb (JEDEC 5-2 outline, also designated as A19-2) and a miniature button 7-pin base (E7-1, Pico 7, or 7C10).

Dimensions and Weight

• Maximum Diameter: 0.750 inches (approximately 19 mm)
• Maximum Overall Length: 2.125 inches (2-1/8 inches or 47.6 mm envelope height with a maximum pin length of 7 to 7.14 mm)
• Maximum Seated Length: 1.875 inches (1-7/8 inches)
• Weight: Approximately 7 grams

Installation and Mounting

The tube can be mounted in any position. When installing the 1A3, certain mechanical considerations apply:

• Sockets (such as RCA Stock No. 9914) must not have material obstructing the center hole. This accommodates potential manufacturing variations where the exhaust-tube tip is located at the base end of the tube.
• Special precautions should be taken to prevent the tube from becoming dislodged in portable or high-vibration environments.
• The tube is rated for operation at altitudes up to 3 km without pressurization.

Pinout and Basing Configurations

The 1A3 utilizes a 7-pin base layout (JEDEC 5AP or 7C10). Depending on the specific manufacturing standard, the pin assignments are generally configured as follows:

Note on Pins 4 and 5: Pins designated as "No Connection" or "Internal Connection" should not be used as external support tie points or connected to any external circuitry. In some alternative configurations, Pin 5 is designated as a heater connection.

Heater and Cathode Characteristics

The 1A3 features an indirectly heated, coated unipotential cathode designed for parallel AC or DC operation.

Heater-to-Cathode Voltage Ratings

The insulation between the heater and the cathode is rated to withstand significant potential differences:

• Maximum Heater-Cathode Voltage (DC, Design Center): 140 V
• Maximum Peak Heater-Cathode Voltage (Absolute Maximum): 100 V (or ±100 V)

Electrical Ratings and Operating Conditions

The electrical limits of the 1A3 are defined under two rating systems: Design-Center Values (EIA Standard RS-239) and Absolute Maximum Ratings.

Maximum Ratings (Rectifier Service)

Typical Half-Wave Rectifier Operation (Condenser-Input Filter)

When operated as a half-wave rectifier with a condenser-input filter, the typical operating parameters are:

• AC Plate Supply Voltage (RMS): 117 V (also specified as the maximum RMS plate voltage)
• Filter Input Condenser: 2 µF
• Minimum Total Effective Plate-Supply Impedance: 0 Ω

Interelectrode Capacitances

The direct interelectrode capacitances of the 1A3, measured without an external shield, are approximately:

• Plate to Cathode ($C_{p/k}$ or $C_{a/k}$): 0.4 pF
• Plate to Heater ($C_{p/h}$ or $C_{a/f}$): 0.6 pF to 0.8 pF
• Heater to Cathode ($C_{h/k}$ or $C_{k/f}$): 0.6 pF

Electrical Characteristics and Curves

Under test conditions with a diode voltage ($U_d$) of 4 V, the diode current ($I_d$) is typically 0.6 mA when measured over a 30-second interval.

Average Plate Characteristics

The relationship between plate voltage and plate current exhibits distinct regions:

• Low Voltage Region (0 to 5 V DC): The characteristic is non-linear and exponential. The current rises from 0 mA at 0 V to approximately 0.7–0.75 mA at 4 V or 5 V.
• Linear Region (5 to 20 V DC): Above 5 V, the plate current becomes highly linear. Typical current values along this curve include:
  • At 8 V DC: Approximately 1.45 mA
  • At 10 V DC: Approximately 1.8 to 2.0 mA (or up to 2.4 mA / 2400 µA depending on the specific curve)
  • At 15 V DC: Approximately 3.0 to 3.3 mA
  • At 20 V DC: Approximately 4.2 to 4.6 mA

Technical Summary

The 1A3 is a high-frequency miniature diode featuring an indirectly heated, coated unipotential cathode. Operating with a nominal 1.4 V, 150 mA heater supply, it is housed in a 7-pin miniature glass envelope and can function at frequencies up to 1000 MHz. Its low interelectrode capacitances (0.4 pF plate-to-cathode) and linear plate characteristics above 5 V make it highly suitable for RF detection and FM discriminator circuits in portable equipment. The tube's ratings support a maximum peak inverse voltage of 330 V (Design Center) or 365 V (Absolute Maximum), with a maximum DC output current of 0.5 mA to 0.55 mA.