1AC6 — Miniature Battery Heptode Frequency Changer

A directly heated, seven-pin miniature heptode designed for frequency conversion in battery-operated receivers, offering high conversion conductance, low oscillator drive requirements, and automatic volume control (AVC) capability.

Overview

The 1AC6 (also designated as the DK92) is a directly heated heptode, or pentagrid converter, designed primarily for use as a self-oscillating frequency changer in portable, battery-operated radio receivers. It is engineered to operate uniformly at frequencies up to 30 MHz.

Compared to older types such as the 1R5, the 1AC6 features improved short-wave performance and reduced high-tension (H.T.) current consumption. A key design feature is the provision of separate external connections for the oscillator anode (Grid 2) and the screen grid (Grid 4). This separation allows the implementation of conventional oscillator circuits and yields significantly improved oscillator stability and performance.

Physical Characteristics and Pinout

The 1AC6 is constructed in a miniature glass envelope (T5 1/2 bulb) with a standard 7-pin miniature button base (B7G). The tube is designed to operate in any mounting position.

Physical Dimensions

• Maximum Overall Length: 54.5 mm (2-1/8 inches)
• Maximum Seated Height: 47.5 mm (1-7/8 inches)
• Bulb Length (excluding tip): 1-1/2 inches ± 3/32 inch
• Maximum Diameter: 19 mm (3/4 inch)

Base Connections

Two distinct pinout configurations are specified for the 1AC6, both utilizing the 7-pin miniature base.

Standard B7G Configuration

In the standard configuration, the electrodes are mapped to the seven pins as follows (viewed from the underside of the base):

Alternative 7DH Configuration

An alternative configuration (designated under the JETEC 7DH scheme) maps the pins as follows:

Filament and Power Ratings

The 1AC6 features a directly heated, coated filament designed strictly for DC operation. The filament is highly efficient, minimizing current drain on portable battery supplies.

Maximum Ratings (Design Center Values)

To ensure reliable operation and prevent premature degradation, the operating conditions must be kept within the following absolute and design-center limits:

Electrical Characteristics and Operating Points

The 1AC6 is typically operated as a self-oscillating frequency changer. Two primary operating points are specified, corresponding to nominal battery supply voltages of 67.5 V and 90 V (reduced by the negative bias of the receiver's output stage).

Typical Operating Conditions (Converter Service)

Oscillator Section Characteristics

The characteristics of the oscillator portion of the heptode can be evaluated independently by connecting Grid 1 to the positive filament. Under these conditions, the mutual conductance and amplification factor of the oscillator triode section (Grid 1 acting as control grid, Grid 2 acting as anode) are as follows:

Interelectrode Capacitances

Interelectrode capacitances are measured without an external shield. The direct capacitances between the various electrodes are detailed below:

• R.F. Input (cg3-all): 7.5 pF
• I.F. Output (ca-all): 8.4 to 8.5 pF
• Oscillator Input (cg1-all): 3.9 to 4.0 pF
• Oscillator Output (cg2-all): 4.8 to 5.0 pF
• Grid 1 to Grid 2 (cg1-g2): 3.0 pF
• Grid 2 to Grid 3 (cg2-g3): 1.6 pF
• Grid 1 to Grid 3 (cg1-g3): 0.2 pF (maximum)
• Grid 3 to Anode (cg3-a): 0.36 to 0.4 pF (maximum)
• Plate to Grid 1 (ca-g1): 0.11 pF (maximum)
• Plate to Grid 2 (ca-g2): 0.3 pF (maximum)

Performance Curves and Behavior

The performance of the 1AC6 is highly dependent on the oscillator drive and the control grid bias. Analysis of the characteristic curves reveals several key operational behaviors:

Conversion Conductance and Noise vs. Oscillator Grid Current

• Conversion Conductance (gc): At a supply voltage of 85 V, gc rises rapidly from zero when there is no oscillator grid current (Ig1 = 0). It reaches a broad peak of approximately 325 to 330 µA/V when Ig1 is between 100 µA and 200 µA. As the grid current increases further toward 350 µA, the conversion conductance gradually declines to approximately 230 µA/V.
• Equivalent Noise Resistance (Req): The equivalent noise resistance of the converter stage reaches its minimum value of approximately 40 kΩ when the oscillator grid current is optimized near 150 µA.
• Anode Impedance (ra): The internal plate resistance drops continuously as oscillator drive increases, falling from 2.5 MΩ at low grid currents down to 0.6 MΩ when Ig1 reaches 300 µA.
• Current Consumption: Both the total cathode current (Ik) and the oscillator anode current (Ig2) rise steadily with increasing oscillator grid current. The cathode current plateaus at approximately 3.1 mA for Ig1 values exceeding 150 µA.

Characteristics vs. Control Grid Bias (Vg3)

• AVC Control and Cutoff: Applying a negative bias to Grid 3 (Vg3) provides effective automatic volume control. As Vg3 is swept from 0 V to -6 V, the conversion conductance drops exponentially from 325 µA/V to approximately 3 µA/V. The exact cutoff point is sensitive to the screen grid (Grid 4) voltage; higher screen voltages shift the cutoff point more negative (ranging from -2 V to -7 V as Grid 4 varies from 30 V to 90 V).
• Electrode Currents: Both the anode current (Ia) and the screen grid current (Ig4) decrease exponentially toward zero as Grid 3 bias approaches -6 V. Conversely, the oscillator anode current (Ig2) increases from 1.6 mA to 3.0 mA over the same bias range, reflecting a redistribution of space charge within the tube.
• Anode Impedance: As Grid 3 becomes more negative, the anode impedance increases dramatically, rising from its nominal 0.65 MΩ to over 100 MΩ at -6 V bias.

Performance vs. Oscillator Voltage

When operating from an 85 V supply, the conversion conductance peaks broadly when the oscillator grid receives an AC drive voltage between 4 V and 5 V rms. Within this region, Grid 1 current increases linearly with the applied oscillator voltage, while the plate resistance rises gradually from 0.8 MΩ to 1.2 MΩ across an oscillator drive range of 0 V to 8 V rms.