1AN5 — Variable-Mu Pentode

The 1AN5 is a variable-mu pentode designed primarily for use in battery-operated AM/FM receivers. It is engineered to perform multiple functions, including intermediate frequency (I.F.) amplification, multiplicative mixing, and additive mixing. Featuring a low-current filamentary cathode, the 1AN5 is optimized for portable applications where power conservation is critical.

Mechanical Design and Pinout

The 1AN5 is housed in a T 5/2 glass bulb with a 5-2 outline and an E7-1 miniature 7-pin base. The tube employs a 7 ES basing configuration and can be mounted in any position.

Physical Dimensions

• Maximum Diameter: 3/4"
• Maximum Overall Height: 2 1/8"
• Body Height: 1 1/2" ± 3/32"
• Maximum Height to Top of Bulb: 1 7/8"

Pin Connections

The pinout configuration, as viewed from the bottom of the base, is as follows:

Filament and Interelectrode Capacitances

The 1AN5 features a coated filamentary cathode designed for low-power operation. It can be powered by either a parallel or series supply:

• Parallel Supply: 1.4 V @ 25 mA
• Series Supply: 1.3 V @ 25 mA

Direct Interelectrode Capacitances

The direct interelectrode capacitances vary depending on whether the tube is operated in a standard pentode configuration or a triode configuration (where Grid No. 2 and Grid No. 3 are connected directly to the plate).

Maximum Ratings (Design Center Values)

To ensure reliable operation and prevent premature degradation, the 1AN5 must be operated within the following design center limits:

• Supply Voltage: 120 V (Absolute maximum: 150 V)
• Plate Voltage: 120 V
• Plate Dissipation: 0.25 W
• Grid No. 2 Voltage: 90 V
• Grid No. 2 Dissipation: 0.15 W
• Cathode Current: 2.5 mA
• Grid No. 1 Circuit Resistance: 3 MΩ
• Grid No. 3 Circuit Resistance: 1.5 MΩ

Operating Characteristics

The 1AN5 can be configured in several ways depending on the receiver design requirements.

Intermediate Frequency (I.F.) Amplifier Service

In I.F. amplifier applications, Grid No. 3 is connected to 0 V. The operating characteristics are specified for supply voltages of 85 V, 64 V, and 45 V, referenced to the negative filament. The 85 V and 64 V supplies represent typical battery voltages (90 V and 67.5 V, respectively) minus the bias of the output tube.

85 V Supply Conditions

At an 85 V supply, the tube can be operated with either a 33,000 Ω or a 47,000 Ω series resistor for Grid No. 2:

• With 33,000 Ω Grid No. 2 Resistor:
  • Grid No. 1 Bias: 0 V (normal) to -5 V (cutoff)
  • Grid No. 2 Voltage: 62 V (at 0 V bias)
  • Plate Current: 1.7 mA (at 0 V bias)
  • Grid No. 2 Current: 700 µA (at 0 V bias)
  • Transconductance: 940 µmhos (at 0 V bias) / 10 µmhos (at -5 V bias)
  • Plate Resistance: 0.45 MΩ (at 0 V bias) / >10 MΩ (at -5 V bias)
  • Amplification Factor (g2 to g1): 20
• With 47,000 Ω Grid No. 2 Resistor:
  • Grid No. 1 Bias: 0 V (normal) to -5 V (cutoff)
  • Grid No. 2 Voltage: 57 V (at 0 V bias)
  • Plate Current: 1.5 mA (at 0 V bias)
  • Grid No. 2 Current: 595 µA (at 0 V bias)
  • Transconductance: 900 µmhos (at 0 V bias) / 10 µmhos (at -5 V bias)
  • Plate Resistance: 0.525 MΩ (at 0 V bias) / >10 MΩ (at -5 V bias)
  • Amplification Factor (g2 to g1): 20

64 V Supply Conditions

At a 64 V supply, the tube can be operated with either a 1,500 Ω or a 4,700 Ω series resistor for Grid No. 2:

• With 1,500 Ω Grid No. 2 Resistor:
  • Grid No. 1 Bias: 0 V (normal) to -3.8 V (cutoff)
  • Grid No. 2 Voltage: 63 V (at 0 V bias)
  • Plate Current: 1.7 mA (at 0 V bias)
  • Grid No. 2 Current: 780 µA (at 0 V bias)
  • Transconductance: 880 µmhos (at 0 V bias) / 10 µmhos (at -3.8 V bias)
  • Plate Resistance: 0.25 MΩ (at 0 V bias) / >10 MΩ (at -3.8 V bias)
  • Amplification Factor (g2 to g1): 20
• With 4,700 Ω Grid No. 2 Resistor:
  • Grid No. 1 Bias: 0 V (normal) to -3.8 V (cutoff)
  • Grid No. 2 Voltage: 61 V (at 0 V bias)
  • Plate Current: 1.6 mA (at 0 V bias)
  • Grid No. 2 Current: 725 µA (at 0 V bias)
  • Transconductance: 870 µmhos (at 0 V bias) / 10 µmhos (at -3.8 V bias)
  • Plate Resistance: 0.27 MΩ (at 0 V bias) / >10 MΩ (at -3.8 V bias)
  • Amplification Factor (g2 to g1): 20

45 V Supply Conditions

At a 45 V supply, Grid No. 2 is connected directly to the supply (0 Ω series resistor):

• Grid No. 1 Bias: 0 V (normal) to -2.7 V (cutoff)
• Grid No. 2 Voltage: 45 V
• Plate Current: 0.9 mA (at 0 V bias)
• Grid No. 2 Current: 445 µA (at 0 V bias)
• Transconductance: 700 µmhos (at 0 V bias) / 10 µmhos (at -2.7 V bias)
• Plate Resistance: 0.28 MΩ (at 0 V bias) / >10 MΩ (at -2.7 V bias)
• Amplification Factor (g2 to g1): 20

Multiplicative Mixer Service

When used as a multiplicative mixer, the local oscillator voltage is applied to Grid No. 3. The circuit uses a Grid No. 3 resistor of 0.3 MΩ and an oscillator voltage of 12 V (rms). The operating characteristics under these conditions are:

Additive Mixer Service

In additive mixer service, the 1AN5 is operated in a triode connection with Grid No. 2 and Grid No. 3 connected directly to the plate. A grid resistor of 1 MΩ is used across all operating points. The plate series resistor must be bypassed.

Application Considerations

When designing receiver circuits utilizing the 1AN5 in the short wave range, special attention must be paid to transit time effects. If automatic gain control (AGC) is applied to the mixer stage, a transit-time-induced current flows to Grid No. 1. This current must be accounted for in the input circuit design to prevent detuning, impedance mismatch, or a reduction in overall receiver sensitivity.

Technical Summary

The 1AN5 is a highly versatile, low-power variable-mu pentode designed for battery-operated AM/FM receivers. It features a dual-rated filament (1.4 V parallel / 1.3 V series) drawing only 25 mA, making it ideal for portable equipment. The tube can be configured as a high-impedance pentode for I.F. amplification and multiplicative mixing, or as a low-impedance triode for additive mixing. Its variable-mu characteristics provide effective AGC action, with transconductance dropping to 10 µmhos at cutoff. Designers must consider transit-time effects on Grid No. 1 when applying AGC in short wave mixer applications.