1AG5 — Subminiature Diode-Pentode Vacuum Tube

A subminiature, filament-type diode-pentode designed for detector-amplifier applications requiring extreme economy of space, weight, and battery drain.

Overview

The 1AG5 is a subminiature, filament-type diode-pentode vacuum tube. It is engineered for applications where space, weight, and battery power consumption must be minimized. The tube features a coated filament designed for low-voltage DC operation and can be operated in any mounting position. It is typically employed as a detector-amplifier in compact, portable electronic equipment.

Mechanical and Physical Characteristics

The 1AG5 is housed in a T-2X3 subminiature glass envelope. Its physical dimensions are characterized by:

• Maximum overall length: 1.5 inches (1-1/2")
• Maximum width: 0.385 inches
• Maximum thickness: 0.285 inches
• Minimum lead length: 1.5 inches (1-1/2")

The tube features tinned flexible leads with a diameter of 0.016 inches, spaced 0.048 inches center-to-center. A colored dot is located adjacent to Lead 1 to identify the lead sequence. The flexible leads can be soldered or welded directly to circuit components. Alternatively, standard subminiature sockets can be used if the leads are cut to a length of 0.020 inches.

Lead Connections and Pinout Configurations

The 1AG5 is documented with two distinct lead configurations, depending on the specific construction:

7-Lead Inline Configuration

In this arrangement, the leads are connected as follows (from left to right, with Lead 1 adjacent to the colored dot):

• Lead 1: Filament Negative ($F_-$) and Diode Plate ($D_p$) (the diode is located at the negative end of the filament)
• Lead 2: Grid #1 ($G_1$)
• Lead 3: Filament Positive ($F_+$)
• Lead 4: Grid #3 ($G_3$) — Deflector Plate 1
• Lead 5: Grid #2 ($G_2$)
• Lead 6: Grid #3 ($G_3$) — Deflector Plate 2
• Lead 7: Plate ($P$)

8-Lead Configuration

In this arrangement, the filament and diode plate connections are separated:

• Lead 1: Diode Plate ($D_p$)
• Lead 2: Grid #1 ($G_1$)
• Lead 3: Filament Negative ($F_-$)
• Lead 4: Grid #3 ($G_3$) — Deflector Plate 1
• Lead 5: Grid #2 ($G_2$)
• Lead 6: Grid #3 ($G_3$) — Deflector Plate 2
• Lead 7: Plate ($P$)
• Lead 8: Filament Positive ($F_+$)

In both configurations, Grid #3 consists of two separate deflector plates connected individually to Lead 4 and Lead 6. Another documented terminal connection standard designates the pinout as 6B.

Filament and Absolute Maximum Ratings

The 1AG5 is designed for battery-operated equipment, utilizing a coated filament with the following ratings:

• Filament Voltage (DC): 1.25 V (with an absolute maximum tolerance of $\pm 20\%$, i.e., 1.0 V to 1.5 V)
• Filament Current: 0.03 A (30 mA)

The absolute maximum ratings for the tube must not be exceeded:

• Maximum Plate Voltage: 50 V
• Maximum Grid #2 Voltage: 50 V
• Maximum Total Cathode Current: 0.5 mA
• Maximum Diode Current (Continuous Operation): 0.25 mA

Direct Interelectrode Capacitances

Approximate capacitances, measured with a close-fitting shield connected to Lead 4 (Grid #3 / Deflector Plate 1), are:

• Grid to Plate ($G_1$ to $P$): 0.10 $\mu\mu\text{f}$
• Input ($G_1$ to $[F + G_2 + G_3]$): 1.7 $\mu\mu\text{f}$
• Output ($P$ to $[F + G_2 + G_3]$): 2.4 $\mu\mu\text{f}$ (Note: $F$ represents the filament, which may include $F_+$ depending on the lead configuration).

Typical Operating Conditions and Characteristics

Class $A_1$ Amplifier Service

The 1AG5 can be operated as a Class $A_1$ amplifier under several typical operating points.

Resistance-Coupled Amplifier Service

For resistance-coupled voltage amplifier applications, the following operating conditions are specified:

Qualitative Curve Observations

Pentode Connection

• Plate Characteristics ($E_{c2} = 45\text{ V}$): The plate current ($I_b$) curves exhibit a sharp knee below 15 V plate voltage. At $E_{[c1](/tubes/C1)} = 0\text{ V}$, the plate current saturates near 0.8 mA to 0.85 mA. The Grid #2 current ($I_{c2}$) curves show a downward slope as plate voltage increases, leveling off above 25 V.
• Plate Characteristics ($E_{c2} = 22.5\text{ V}$): The knee occurs below 10 V plate voltage. At $E_{[c1](/tubes/C1)} = 0\text{ V}$, the plate current saturates near 0.28 mA to 0.3 mA.
• Transfer Characteristics ($E_b = 45\text{ V}, E_{c2} = 45\text{ V}$): The transconductance ($G_m$) drops smoothly from approximately 300 $\mu\text{mhos}$ at $E_{[c1](/tubes/C1)} = 0\text{ V}$ to near 0 $\mu\text{mhos}$ at a cutoff voltage of approximately $-4.5 ext{ V}$.
• Transfer Characteristics ($E_b = 22.5\text{ V}, E_{c2} = 22.5\text{ V}$): The transconductance drops from approximately 240 $\mu\text{mhos}$ at $E_{[c1](/tubes/C1)} = 0\text{ V}$ to near 0 $\mu\text{mhos}$ at a cutoff voltage of approximately $-2.0 ext{ V}$ to $-2.2 ext{ V}$.

Triode Connection

When triode-connected (with Grid #2 tied to the plate, operating at $E_f = 1.25 ext{ V}$), the plate current curves exhibit typical triode-like exponential behavior. The plate current reaches approximately 1.3 mA to 1.5 mA at $E_b = 50 ext{ V}$ and $E_{[c1](/tubes/C1)} = 0\text{ V}$.

Technical Summary

The 1AG5 is a subminiature, filamentary diode-pentode designed for low-power, space-constrained applications. Operating with a nominal 1.25 V, 30 mA DC filament, it combines a signal diode and a high-gain pentode in a single T-2X3 glass envelope. The pentode section provides a transconductance of 235 to 350 $\mu\text{mhos}$ depending on the operating point, with plate resistance values up to 2.5 $ext{M} riangle$ (or 2.5 $ext{M}\Omega$). In resistance-coupled amplifier service, the tube is capable of delivering voltage gains of 50 to 70 with plate supply voltages of 45 V to 67.5 V. Its flexible leads allow direct soldering or welding into circuits, making it highly suitable for compact, battery-operated detector-amplifier designs.