1AK5 — Subminiature Diode-Pentode

The 1AK5 is a subminiature, filament-type diode-pentode designed for applications requiring extreme economy of space, weight, and battery drain. It combines a sharp-cutoff pentode and a single diode within a compact glass envelope, making it highly suitable for portable or compact equipment.

Physical Construction and Lead Configuration

The 1AK5 is constructed in a subminiature T-2X3 glass envelope. It features flexible terminal leads that can be soldered or welded directly into circuits, or cut to a length of 0.20" for use with standard subminiature sockets.

Physical Dimensions and Mounting

• Envelope Type: T-2X3 Glass
• Mounting Position: Any
• Maximum Width: 0.285" × 0.385"
• Maximum Height: 1.50"
• Lead Length: 1.5" minimum
• Lead Spacing: 0.048" center-to-center
• Lead Identification: A red dot is located adjacent to Lead 1.

Terminal Connections and Pinout

The 1AK5 is documented with two closely related lead/pin connection schemes.

Under the 1AJ5 base connection diagram, the pinout is:

• Pin 1: Filament Negative (F−), Grid 3 (G3)
• Pin 2: Plate (P)
• Pin 3: Grid 2 (G2)
• Pin 4: Diode Plate (DP)
• Pin 5: Grid 1 (G1)
• Pin 6: Filament Positive (F+)

Under the alternative flexible lead configuration, the connections are:

• Lead 1: Pentode Plate
• Lead 2: Grid #2 (Screen Grid)
• Lead 3: Diode Plate
• Lead 4: Filament, Negative; Grid #3 (comprised of two separate deflector plates, one connected to Lead 4, the other to Lead 6)
• Lead 5: Grid #1 (Control Grid)
• Lead 6: Filament, Positive; Grid #3

Filament and Electrical Ratings

The filament is designed for direct-current (DC) operation. The ratings reflect its low-power design:

• Filament Voltage (DC): 1.25 V
• Filament Current: 20 mA (also documented as 0.2 A in some specifications)

Maximum Ratings

The maximum ratings for the 1AK5 are established under both Absolute Maximum and Design Center systems:

• Maximum Plate Voltage: 90 V
• Maximum Screen (Grid #2) Voltage: 90 V
• Maximum Total Cathode Current: 1.0 mA
• Maximum Continuous Diode Current: 0.75 mA

The diode plate is physically located at the negative end of the filament.

Interelectrode Capacitances

The direct interelectrode capacitances of the 1AK5 vary slightly depending on whether an external shield is employed.

Measured Without External Shield

• Input: 2.0 pF
• Output: 2.7 pF
• Grid-to-Plate: 0.10 pF (with '+' indicating plate-to-plate capacitance)

Measured With Close-Fitting Shield (Connected to Lead 4)

• Grid to Plate ($g_1$ to $p$): 0.10 pF maximum
• Input ($g_1$ to $f + g_2 + g_3$): 2.0 pF (approximate)
• Output ($p$ to $f + g_2 + g_3$): 2.7 pF (approximate)

Typical Operating Conditions

Class A / Class A1 Amplifier

When operated as a standard Class A amplifier, the typical operating parameters are as follows:

• Filament Voltage: 1.25 V DC
• Filament Current: 20 mA
• Plate Voltage: 45 V
• Grid #2 (Screen) Voltage: 45 V
• Grid #1 (Control Grid) Voltage: 0 V (returned through a 5 MΩ resistor)
• Plate Current: 0.5 mA
• Screen Current: 0.2 mA
• Plate Resistance: 0.4 MΩ (400,000 Ω)
• Transconductance: 280 µmhos
• Minimum Diode Current (with 10 V DC): 1.5 mA

Resistance-Coupled Amplifier

The 1AK5 can be configured as a high-gain resistance-coupled amplifier. A typical operating point yields an approximate voltage gain of 40:

• Plate Supply Voltage: 45 V
• Grid #2 Supply Voltage: 45 V
• Grid #1 Voltage: 0 V (with a 5 MΩ grid resistor)
• Load Resistance: 1 MΩ
• Series Grid #2 Resistor: 5 MΩ
• Grid #2 Bypass Condenser: 0.1 µF
• Grid #1 Resistor (following stage): 10 MΩ
• Approximate Voltage Gain: 40

Characteristic Curves and Behavior

The average transfer and plate characteristics of the 1AK5 illustrate its performance as a sharp-cutoff pentode:

• Transconductance vs. Grid Bias: At a filament voltage of 1.25 V and screen voltage of 45 V, the transconductance ($g_m$) rises from near 0 µmhos at a control grid voltage ($E_{[c1](/tubes/C1)}$) of −4.0 V to approximately 300 µmhos at $E_{[c1](/tubes/C1)} = 0\text{ V}$.
• Plate Current vs. Grid Bias: Under the same conditions, the plate current ($I_b$) rises to approximately 500 µA (0.5 mA) at $E_{[c1](/tubes/C1)} = 0\text{ V}$.
• Plate Characteristics: The plate current curves plotted for grid voltages from 0 V to −3.0 V (in 0.5 V steps) display a typical pentode knee occurring around 15 V to 20 V, above which the plate current remains relatively constant with increasing plate voltage.

Technical Summary

The 1AK5 is a subminiature, filament-type diode-pentode designed for low-power, space-constrained applications. Operating with a 1.25 V DC filament, it functions as a sharp-cutoff pentode and a single diode. With typical plate and screen voltages of 45 V, it provides a transconductance of 280 µmhos and a plate resistance of 0.4 MΩ, making it capable of achieving a voltage gain of approximately 40 in resistance-coupled amplifier circuits. Its flexible leads and tiny T-2X3 glass envelope make it highly versatile for compact, battery-operated designs.