1AH4 — Subminiature RF Pentode

A filament-type, fully shielded subminiature pentode designed for radio-frequency applications where space, weight, and battery power drain must be minimized.

Overview

The 1AH4 is a subminiature pentode designed primarily for radio-frequency (RF) amplification in portable, battery-operated equipment. It features a coated filament and a specialized suppressor grid (Grid #3) construction consisting of two separate deflector plates. To ensure stable high-frequency operation in compact layouts, the tube is fully shielded with an integral metallic envelope coating.

Mechanical Construction and Base Configuration

The 1AH4 is constructed in a T-2x3 subminiature glass bulb. The entire exterior of the bulb is coated with a metallic shield that is internally connected to lead 3, eliminating the need for an external shielding canister.

The tube features a flat subminiature base with five tinned flexible leads, each measuring 0.016 inches in diameter. The leads are arranged with a center-to-center spacing of 0.05 inches. A red indexing dot is located on the bulb adjacent to lead 1 to identify the pin sequence.

Physical Dimensions

• Maximum Bulb Length: 1.5 inches (1-1/2")
• Minimum Lead Length: 1.5 inches (1-1/2")
• Maximum Width: 0.385 inches
• Maximum Thickness: 0.285 inches

Installation and Mounting

The flexible leads are designed to be soldered or welded directly to circuit components, which is ideal for ultra-compact assemblies. For applications utilizing standard subminiature sockets, the leads can be cut to a length of 0.20 inches. The tube can be mounted and operated in any physical position.

Pinout and Electrode Connections

The terminal connections, viewed from the bottom of the tube, correspond to the 5A or 1AD4 base connection scheme:

The suppressor grid (Grid #3) is unique in that it consists of two separate deflector plates rather than a single tied structure. One plate is connected to the negative filament terminal (Lead 3) along with the metallic envelope shield, while the other plate is connected to the positive filament terminal (Lead 5).

Filament and Absolute Maximum Ratings

The 1AH4 is designed for low-power battery operation. Its filament is a coated type designed for DC or AC operation.

Filament Specifications

• Filament Voltage: 1.25 V (nominal)
• Filament Current: 0.04 A (40 mA)
• Filament Supply Type: AC or DC

Design Center Maximum Ratings

These ratings are interpreted according to the RMA Standard MB-210 (or M8-210) convention:

• Maximum Plate Voltage: 90 V
• Maximum Grid #2 (Screen) Voltage: 90 V
• Maximum Total Cathode Current: 2.0 mA

Direct Interelectrode Capacitances

The capacitances of the 1AH4 are extremely low, reflecting its design as an RF amplifier. These values are measured with the integral metallic shield connected to lead 3:

• Grid to Plate (Maximum): 0.01 ̷̷F
• Input Capacitance: 3.5 ̷̷F
• Output Capacitance: 4.5 ̷̷F

These low capacitances, particularly the very small grid-to-plate feedback capacitance, are critical for stable high-frequency amplification without self-oscillation.

Typical Operating Conditions and Characteristics

The 1AH4 operates as a Class $A_1$ amplifier. The specifications outline two typical operating conditions: one with a fixed screen voltage and another utilizing a screen-dropping resistor. In both cases, the control grid ($G_1$) is returned to ground or the negative filament terminal through a high-value grid resistor ($R_{g1}$) of 5 Megohms, resulting in a nominal grid bias of 0 V.

Class $A_1$ Amplifier Operating Parameters

Analysis of Operating Conditions

In Condition I, the screen grid is fed directly from a 45 V source. In Condition II, a higher supply voltage of 67.5 V is used for both the plate and the screen circuit, with a 0.1 MΩ series resistor dropping the screen voltage. Under both conditions, the tube draws a very low plate current of 0.75 mA and a screen current of 0.2 mA. This results in a total cathode current of 0.95 mA, which is well within the design center maximum limit of 2.0 mA.

The transconductance remains constant at 750 ̷mhos in both configurations, while the plate resistance increases from 1.5 MΩ to 2.0 MΩ when operating at the higher plate voltage of 67.5 V. The higher plate resistance in Condition II provides a slight advantage in stage gain and selectivity when coupled to high-impedance RF tuned circuits.

Technical Summary

The 1AH4 is a highly specialized, subminiature RF pentode featuring a coated filament that operates at 1.25 V and 40 mA. Its physical design includes an integrated metallic envelope shield and a flat five-lead base, allowing for direct soldering into compact, lightweight, battery-powered equipment.

Electrically, the 1AH4 is characterized by its low power consumption, drawing less than 1 mA of total cathode current under typical Class $A_1$ operating conditions. It provides a transconductance of 750 ̷mhos and a high plate resistance of 1.5 to 2.0 MΩ at plate voltages of 45 to 67.5 V. With a maximum plate and screen rating of 90 V and an exceptionally low grid-to-plate capacitance of 0.01 ̷̷F, the 1AH4 serves as an efficient and stable voltage amplifier for portable radio-frequency applications.