1AD5 — Subminiature Sharp-Cutoff RF Pentode

The 1AD5 is a subminiature, directly heated sharp-cutoff radio frequency (RF) pentode. Designed for low-power applications, it is primarily utilized as an RF pentode and Class A amplifier in compact electronic equipment where space and power consumption are critical constraints.

Physical Construction and Filament Ratings

The 1AD5 features a subminiature envelope with a physical configuration defined by the 3-5 outline drawing. It utilizes a directly heated filamentary cathode designed for low-voltage DC operation, making it highly suitable for battery-operated devices.

The calculated filament power of 0.05 W (50 mW) is derived directly from the nominal filament voltage of 1.25 V and the nominal current of 0.04 A ($1.25\text{ V} \times 0.04\text{ A}$).

Base Connection and Pinout

The tube utilizes an 8CP base connection. The pinout configuration provides dual connections for both the control grid (Grid 1) and the screen grid (Grid 2). The pin assignments, viewed from the bottom of the tube, are as follows:

Interelectrode Capacitances

The internal shielding and electrode geometry of the 1AD5 result in very low grid-to-plate capacitance, which is essential for stable RF amplification. The typical direct interelectrode capacitances are:

• Input Capacitance: 1.9 µµF
• Output Capacitance: 3.0 µµF
• Grid-to-Plate Capacitance: 0.009 µµF

Maximum Ratings

To ensure reliable operation and prevent premature failure, the operating voltages of the 1AD5 must be maintained within the specified maximum limits. These maximum ratings are absolute values:

• Maximum Plate Voltage: 67.5 V
• Maximum Screen (Grid 2) Voltage: 67.5 V

Typical Operation and Electrical Characteristics

The 1AD5 is characterized for Class A amplifier service under two distinct operating conditions, representing high-voltage and low-voltage configurations. In both typical operating points, the control grid (Grid 1) is operated at zero bias.

Under both operating conditions, the plate resistance remains approximately 700,000 Ω, while the transconductance scales from 430 µmhos at 30 V up to 735 µmhos at 67.5 V.

Technical Summary

The 1AD5 is a subminiature, directly heated sharp-cutoff RF pentode featuring an 8CP base and a highly efficient 1.25 V, 0.04 A filamentary cathode. With maximum plate and screen ratings of 67.5 V, it is optimized for low-voltage Class A RF amplification. Its dual-pin connections for Grid 1 and Grid 2, combined with an extremely low grid-to-plate capacitance of 0.009 µµF, make it a robust choice for high-frequency applications within its power class.