1AF4 — Sharp-Cutoff Filamentary Pentode

The 1AF4 is a miniature, sharp-cutoff filamentary pentode designed primarily for use as an audio-frequency (AF) or radio-frequency (RF) voltage amplifier in portable receivers. It features a coated filament with extremely low power consumption, requiring only 1.4 V at 25 mA. This represents a 50% reduction in filament power compared to older vacuum tubes designed for similar portable services.

Physical and Mechanical Specifications

The 1AF4 is constructed in a small T-5 1/2 glass bulb with an Outline 5-2 designation. Its physical dimensions are as follows:

• Maximum Diameter: 3/4 inch
• Maximum Overall Length: 2 1/8 inches
• Maximum Seated Height: 1 7/8 inches
• Base: Miniature Button 7-Pin (E7-1)
• Mounting Position: Any

Pinout Configurations

The 1AF4 uses a 7-pin miniature base, designated under the 6AR (or 6AR-0-1&5) basing standard. However, three distinct pinout configurations are documented for this designation. Designers must verify the specific wiring of the physical device:

• Configuration A:

  • Pin 1: Filament Negative (F-), Grid No. 3 (G3), Internal Shield (IS)
  • Pin 2: Grid No. 1 (G1)
  • Pin 3: Grid No. 2 (G2)
  • Pin 4: No Connection (NC)
  • Pin 5: Filament Negative (F-), Grid No. 3 (G3), Internal Shield (IS)
  • Pin 6: Plate (P)
  • Pin 7: Filament Positive (F+)

• Configuration B:

  • Pin 1: Filament Negative (F-), Grid No. 3 (G3), Internal Shield (IS)
  • Pin 2: Plate (P)
  • Pin 3: Grid No. 2 (G2)
  • Pin 4: No Connection (NC)
  • Pin 5: Filament Negative (F-), Grid No. 3 (G3), Internal Shield (IS)
  • Pin 6: Grid No. 1 (G1)
  • Pin 7: Filament Positive (F+)

• Configuration C:

  • Pin 1: Filament Negative (F-)
  • Pin 2: Grid No. 3 (G3)
  • Pin 3: Filament Positive (F+)
  • Pin 4: Grid No. 1 (G1)
  • Pin 5: Grid No. 2 (G2)
  • Pin 6: Plate (P)
  • Pin 7: Filament Negative (F-)

Filament Ratings

The 1AF4 utilizes a directly heated, coated filament designed for battery or power-line operation. For power-line operation, the filament voltage is centered at 1.4 V for a normal 117 V line.

• Filament Voltage: 1.4 V (AC or DC)
• Filament Current: 25 mA (0.025 A)
• Filament Power (Calculated): 1.4 V % 0.025 A = 0.035 W (35 mW)

Maximum Ratings

The maximum ratings for the 1AF4 are established under the Design Center / Design-Maximum systems (RMA Standard M8-210):

• Maximum Plate Voltage (DC): 110 V
• Maximum Grid No. 2 (Screen) Supply Voltage (DC): 110 V
• Maximum Grid No. 2 (Screen) Voltage (DC): 90 V
• Maximum Positive Grid No. 1 (Control Grid) Voltage (DC): 0 V
• Maximum Cathode Current: 2.5 mA

Electrical Characteristics and Typical Operation

The 1AF4 operates as a Class A (or Class A1) amplifier. The typical operating characteristics are documented under two primary voltage conditions. Depending on the manufacturer's specifications, there are slight variations in the typical currents, plate resistance, and transconductance values.

Typical Operating Conditions

Interelectrode Capacitances

The direct interelectrode capacitances of the 1AF4 are extremely low, making it highly suitable for RF amplification. These values are documented both with and without an external shield (specifically Shield #316, which is connected to Pins 1 and 5 when used):

• Grid No. 1 to Plate (max):
  • Without Shield: 0.008 pF to 0.009 pF
  • With Shield: 0.008 pF
• Input (G1 to [F + G2 + G3 + IS]):
  • Without Shield: 3.8 pF
  • With Shield: 3.8 pF
• Output (P to [F + G2 + G3 + IS]):
  • Without Shield: 7.5 pF to 7.6 pF
  • With Shield: 7.6 pF

Technical Summary

The 1AF4 is a highly efficient, sharp-cutoff filamentary pentode designed for portable receiver applications. By drawing only 25 mA at 1.4 V, it minimizes battery drain in portable equipment. It provides high plate resistance (up to 2.2 MΩ) and moderate transconductance (up to 1050 %mhos) under typical Class A operating conditions. With its low interelectrode capacitances and compact T-5 1/2 miniature envelope, the 1AF4 serves as an effective voltage amplifier for both RF and AF stages.