1AB6 — Directly Heated Miniature Pentagrid Converter

The 1AB6 (also designated as IAB6 or DK96) is a directly heated miniature heptode, or pentagrid converter, designed primarily for frequency changer service in battery-operated radio receivers. It features a low-drain, coated filament designed for efficient direct-current (DC) operation, allowing it to be powered from series or parallel filament supplies.

Construction, Filament, and Physical Dimensions

The 1AB6 is constructed in a miniature glass envelope with a standard 7-pin miniature button base (B7G). The tube is designed to operate in any mounting position.

The physical dimensions of the tube are compact, making it suitable for portable equipment:

• Maximum overall length: 54.5 mm (2-5/32 inches) or 2.205 inches
• Maximum seated height: 47.5 mm (1-7/8 inches) or 1.955 inches
• Bulb length (excluding tip): 1.580 inches ± 0.094 inches
• Maximum diameter: 19 mm (3/4 inch)

The filament is coated and optimized for low power consumption. It can be operated under two different supply configurations:

• Series Supply: Nominal filament voltage of 1.3 V at a current of 24 mA.
• Parallel Supply: Nominal filament voltage of 1.4 V at a current of 25 mA (0.025 A).
• Filament Voltage Limits: The permissible filament voltage ranges from a minimum of 1.1 V to a maximum of 1.6 V.

Pinout and Electrode Connections

The 1AB6 uses the 7DH basing arrangement. Viewed from the underside of the base, the pin connections are assigned as follows:

Absolute Maximum Ratings

The following maximum ratings are established under the Design Center and Design-Maximum rating systems. These limits must not be exceeded to ensure reliable operation and longevity of the tube:

Typical Operating Points and Converter Service

In typical frequency changer (converter) service, the 1AB6 operates with separate excitation and no impedance in the Grid No. 2 lead. The operating characteristics are documented at two primary supply levels (64 V and 85 V), which are typically derived from standard battery voltages decreased by the negative bias of the receiver's output stage.

Oscillator Section Characteristics

The characteristics of the oscillator portion of the heptode are evaluated with Grid No. 1 connected directly to the positive side of the filament:

• Plate Voltage: 64 V or 85 V
• Grid No. 4 Voltage: 64 V
• Grid No. 3 Voltage: 0 V
• Grid No. 2 (Oscillator Anode) Voltage: 35 V
• Grid No. 2 Current: 1.7 mA
• Mutual Conductance ($g_1$ to $g_2$): 0.6 mA/V (600 µmhos)
• Amplification Factor ($\mu_{g1-g2}$): 7.5

Interelectrode Capacitances

The interelectrode capacitances of the 1AB6, measured without an external shield, are detailed below:

• Plate to all other electrodes ($C_{a-\text{all}}$): 8.1 pF to 8.4 pF
• Grid No. 3 to all other electrodes ($C_{g3-\text{all}}$): 7.4 pF to 7.6 pF
• Grid No. 2 to all other electrodes ($C_{g2-\text{all}}$): 4.8 pF
• Grid No. 1 to all other electrodes ($C_{g1-\text{all}}$): 3.9 pF
• Plate to Grid No. 3 ($C_{a-g3}$): maximum 0.36 pF
• Plate to Grid No. 2 ($C_{a-g2}$): maximum 0.3 pF
• Plate to Grid No. 1 ($C_{a-g1}$): maximum 0.11 pF
• Grid No. 2 to Grid No. 3 ($C_{g2-g3}$): 1.6 pF
• Grid No. 1 to Grid No. 3 ($C_{g1-g3}$): maximum 0.2 pF
• Grid No. 1 to Grid No. 2 ($C_{g1-g2}$): 3.0 pF

Qualitative Performance and Curve Interpretation

The performance curves of the 1AB6 reveal several key operational behaviors:

• Control Grid (Grid No. 3) Bias Effects: Both plate current and conversion conductance decrease smoothly as the Grid No. 3 bias becomes more negative. The plate current drops in a semi-logarithmic fashion. Applying higher Grid No. 4 (screen) voltages (ranging from 30 V to 90 V) shifts both the plate current and conversion conductance curves upward and to the left, requiring a more negative bias to achieve cutoff.
• Oscillator Voltage Effects: At a supply voltage of 85 V, the conversion conductance peaks when the oscillator voltage is between 4 V and 5 V r.m.s. Below this range, conversion efficiency drops; above it, the conversion conductance decreases slightly. The plate resistance of the tube increases gradually as the oscillator voltage rises, while the Grid No. 1 current increases in an almost linear relationship with the oscillator voltage.

Technical Summary

The 1AB6 is a highly efficient, directly heated miniature pentagrid converter designed for battery-operated receivers. Operating with a low-drain filament (1.4 V, 25 mA), it combines a mixer and oscillator in a single 7-pin miniature envelope. Under typical operating conditions with an 85 V supply, it provides a conversion conductance of 300 µmhos and a high plate resistance of up to 1.0 MΩ, while maintaining a total cathode current draw of only 2.4 mA. This makes it an exceptionally optimized frequency changer for portable radio applications.