1AD4 — Sharp-Cutoff Subminiature Pentode

A directly heated, fully shielded subminiature pentode designed for radio-frequency (RF) and audio-frequency (AF) amplification in battery-operated and portable electronic equipment.

Overview

The 1AD4 (also designated as the DF 62) is a subminiature sharp-cutoff pentode featuring a directly heated filament. It is specifically engineered for low-power, portable, and battery-operated applications, serving as an RF or AF amplifier. The tube is constructed with a fully shielded glass envelope and flexible leads, allowing it to be soldered directly into circuits or used with standard inline subminiature sockets.

With a nominal filament supply of 1.25 V at 100 mA, the 1AD4 provides efficient operation from single-cell batteries. Its design incorporates a built-in metallic shield and a dual-plate suppressor grid configuration to optimize high-frequency performance and minimize interelectrode coupling.

Construction, Physical Dimensions, and Installation

The 1AD4 is housed in a subminiature glass envelope (such as the T-2x3 or A 7x 10.1 outline) with a flat-press base. The bulb is entirely coated with a conductive metallic shield to provide built-in electrical shielding.

Physical Dimensions

• Maximum Body Length: 38.1 mm (1.5 in.)
• Maximum Width: 9.78 mm (0.385 in.)
• Maximum Thickness / Diameter: 7.24 mm (0.285 in.)
• Minimum Lead Length: 38.1 mm (1.5 in.)

Installation Precautions

The 1AD4 features five flexible, tinned leads (0.016-inch diameter) spaced 0.048 inches center-to-center. These leads can be soldered or welded directly to circuit components. When soldering, great care must be taken to avoid overheating the glass-to-metal seal. Additionally, leads should not be bent closer than 2 mm from the glass-to-metal seal to prevent mechanical stress and potential envelope failure. The tube can be mounted in any position.

Suppressor Grid and Shielding Configuration

The suppressor grid (Grid 3) consists of two separate plates. To optimize shielding and electron flow, one plate is internally connected to the negative filament terminal (which also connects to the external conductive envelope coating), while the other plate is connected to the positive filament terminal.

Pinout and Terminal Connections

Depending on the manufacturer and the specific standard applied (such as the 5A or 7L7 base configurations), two distinct pinout arrangements exist for the 1AD4. In both configurations, a red or colored dot on the glass envelope indicates the position of Lead 1.

Standard Configuration (DF 62 / 1AD4)

In this layout, the leads are arranged in-line from 1 to 5:

• Lead 1: Anode (Plate) — indicated by the red dot
• Lead 2: Grid 2 (Screen Grid)
• Lead 3: Negative Filament ($-f$), Suppressor Grid (Grid 3 plate), and Conductive Shield Coating
• Lead 4: Grid 1 (Control Grid)
• Lead 5: Positive Filament ($+f$) and Suppressor Grid (Grid 3 plate)

Alternative Configuration (5-Lead Flat-Press)

An alternative terminal connection scheme is documented for certain subminiature flat-press versions:

• Lead 1: Negative Filament ($-f$)
• Lead 2: Grid 1 (Control Grid)
• Lead 3: Grid 2 (Screen Grid)
• Lead 4: Grid 3 (Suppressor Grid)
• Lead 5: Plate (Anode)

Filament Ratings

The 1AD4 utilizes a directly heated filament designed for DC operation. The filament can be operated in parallel supply configurations.

Electrical Characteristics and Operating Points

The 1AD4 operates as a Class A (or Class A1) amplifier. It exhibits high transconductance and high plate resistance at low operating voltages. The published typical operating conditions are divided into 45 V and 90 V regimes:

Typical Operating Conditions

Maximum Ratings

The maximum ratings of the 1AD4 vary depending on the rating system applied (Absolute Maximum Rating System vs. Design Centre Rating System) and the specific manufacturer specifications.

Note on Voltage Limits: Under certain military specifications (such as MIL-E-1B), standard clauses permitting maximum plate and screen voltage ratings to be exceeded are explicitly designated as non-applicable to the 1AD4.

Interelectrode Capacitances

The direct interelectrode capacitances of the 1AD4 depend on whether an external shield is utilized. The internal conductive coating connected to Lead 3 provides significant shielding when grounded.

• With External Shield / Grounded Conductive Coating:
  • Input Capacitance ($C_{in}$): 4.0 pF
  • Output Capacitance ($C_{out}$): 4.0 pF
  • Grid 1 to Anode ($C_{g1/a}$): < 0.01 pF (10 mpF max)
• Without External Shield:
  • Input Capacitance ($C_{in}$): 4.5 pF
  • Output Capacitance ($C_{out}$): 4.5 pF
  • Grid 1 to Anode ($C_{g1/a}$): 0.01 pF

Qualitative Curve Observations

Transfer Characteristics

At an operating voltage of $V_a = V_{[g2](/tubes/G2)} = 45\text{ V}$, the transfer curves show a smooth, continuous reduction of transconductance ($g_m$), anode current ($I_a$), and screen current ($I_{[g2](/tubes/G2)}$) as the grid bias ($V_{g1}$) sweeps from $0\text{ V}$ down to $-4\text{ V}$. The sharp-cutoff behavior is clearly defined, with the tube reaching near-complete cutoff around $-3.5\text{ V}$ to $-4.0\text{ V}$.

Plate Characteristics (Pentode Connection)

The pentode plate curves (plotted up to 140 V with $V_{[g2](/tubes/G2)} = 45\text{ V}$) display typical pentode knee behavior occurring below $15\text{ V}$ of anode voltage. Above this knee, the plate current curves ($I_b$) form flat plateaus extending up to $120\text{ V}$ and higher, for grid biases ranging from $0\text{ V}$ down to $-2.5\text{ V}$, as well as positive grid biases up to $+5\text{ V}$. The screen current ($I_{[g2](/tubes/G2)}$) decreases as the plate voltage increases, which is characteristic of partition noise and current distribution changes between the screen grid and anode at low plate potentials.

Plate Characteristics (Triode Connection)

When triode-connected, the average plate characteristics show typical triode behavior. The plate current increases smoothly with rising plate voltage, reaching up to 6.0 mA at a filament voltage of 1.25 V DC, as the grid voltage ($V_{[c1](/tubes/C1)}$) is stepped from $0\text{ V}$ down to $-8.0\text{ V}$.

Technical Summary

The 1AD4 (DF 62) is a subminiature, directly heated sharp-cutoff pentode designed for low-voltage RF and AF amplification in portable, battery-powered equipment. Operating with a 1.25 V, 100 mA filament, it delivers a transconductance of 2000 µmhos to 2200 µmhos at a typical plate and screen potential of 45 V, which rises to 2600 µmhos at 90 V.

Its physical construction features a fully shielded glass envelope with a conductive coating connected to the negative filament, and a suppressor grid split into two plates connected across the filament supply. With its flexible leads, the 1AD4 can be soldered directly into compact assemblies, offering reliable high-frequency performance, low interelectrode capacitance (grid-to-plate capacitance $< 0.01\text{ pF}$), and sharp-cutoff characteristics.