1A7GT — Battery Heptode Pentagrid Converter

The 1A7GT (also designated as 1A7G, 1A7-G, and 1A7-GT) is a battery-operated heptode (pentagrid) self-oscillating frequency changer designed for use in battery-powered all-wave radio receivers. It functions as a pentagrid converter, combining the roles of mixer and local oscillator within a single envelope to perform frequency conversion at frequencies up to 15 Mc/s.

Physical Construction and Filament Ratings

The 1A7GT features a tubular (T-type) glass envelope topped with a metal cap (top cap) for the control grid connection. On the 1A7GT variant, Pin 1 is internally connected to the metal shell of the base to provide shielding.

The physical dimensions of the 1A7GT are:

• Maximum overall height: 2-7/8 inches
• Maximum diameter: 1-5/16 inches

The tube is equipped with a directly heated filament designed for low-power battery operation. The filament is rated for DC operation with the following specifications:

Base Configurations and Pinouts

The 1A7GT is designed with an octal base, but two distinct pinout configurations are documented depending on the specific base connection standard used.

Standard Octal Base Configuration

In this configuration, the pin connections viewed from the bottom of the tube are as follows:

7Z Base Connection Configuration

An alternative 7Z base connection configuration defines the pinout as follows:

In both configurations, Grids 3 and 5 ($g_3$ and $g_5$) are internally connected to function as the screen grid, Grid 4 ($g_4$) serves as the signal-input control grid, Grid 1 ($g_1$) acts as the oscillator grid, and Grid 2 ($g_2$) functions as the oscillator anode (or oscillator plate / anode grid).

Electrical Characteristics and Operating Points

The 1A7GT can be operated under different typical conditions depending on whether the screen grids are supplied through a series dropping resistor or directly from a fixed voltage source.

Operating Point A: Screen Series Resistor Supply

Under this operating condition, the screen grids are supplied from a 90 V source through a series resistor:

• Anode Voltage: 90 V
• Anode Current: 0.55 mA
• Screen Supply Voltage: 90 V
• Screen Series Resistor: 70,000 Ω
• Screen Current: 0.6 mA
• Oscillator Anode Voltage: 90 V
• Oscillator Anode Current: 1.2 mA
• Oscillator Grid ($g_1$) Resistor: 200,000 Ω (0.2 MΩ)
• Oscillator Grid Current: 0.035 mA
• Control Grid ($g_4$) Voltage: 0 V
• Anode Impedance: 0.6 MΩ
• Conversion Conductance: 0.25 mA/V (250 µmhos)

Operating Point B: Direct Screen Voltage Supply

Under this operating condition, the screen grids are connected directly to a lower fixed voltage:

• Plate (Anode) Voltage: 90 V
• Screen Voltage: 45 V
• Negative Grid Voltage (Control Grid): 0 V
• Plate Current: 0.6 mA
• Screen Current: 0.7 mA
• Plate Resistance ($R_p$): Approximately 600,000 Ω (0.6 MΩ)
• Conversion Transconductance ($g_c$): 250 µmhos
• Oscillator Plate Voltage: 90 V
• Oscillator Grid Current: 1.2 mA

Maximum Ratings

To ensure reliable operation and prevent premature failure, the operating voltages and currents must be kept within the following maximum limits:

The variation in maximum anode and screen voltage limits reflects different rating standards, with anode voltage limits ranging from 90 V to 110 V, and screen voltage limits ranging from 55 V to 60 V.

Interelectrode Capacitances

The interelectrode capacitances of the 1A7GT, measured with a close-fitting shield connected to the negative filament terminal (Pin 7), are as follows:

• R.F. Input (Control Grid to all other electrodes): 7.5 pF
• I.F. Output (Anode to all other electrodes): 10 pF
• Oscillator Input (Oscillator Grid to all except Oscillator Anode): 3.2 pF
• Oscillator Output (Oscillator Anode to all except Oscillator Grid): 4.0 pF
• Control Grid to Oscillator Grid: 0.12 pF
• Control Grid to Anode: 0.4 pF
• Control Grid to Oscillator Anode: 0.25 pF
• Oscillator Grid to Oscillator Anode: 1.5 pF

These capacitances are critical for determining the tuning range and stability of the local oscillator and mixer stages in all-wave receiver designs.

Performance Curves and Application Notes

The conversion conductance ($g_c$) of the 1A7GT is highly dependent on the oscillator grid ($g_1$) current. When operated with a 1.4 V filament supply, an anode voltage of 90 V, a screen voltage of 45 V, and an oscillator anode voltage of 90 V supplied via a 70,000 Ω resistor through a 200,000 Ω grid leak, the conversion conductance exhibits the following behavior:

• At a very low oscillator grid current of 0.01 mA, the conversion conductance starts at approximately 0.16 mA/V (160 µmhos).
• As the grid current increases, the conversion conductance rises and reaches a peak of approximately 0.27 mA/V (270 µmhos) when the grid current is between 0.04 mA and 0.05 mA.
• Beyond this peak, further increases in grid current cause a gradual decline in conversion conductance, dropping to approximately 0.21 mA/V (210 µmhos) at a grid current of 0.1 mA.

This characteristic demonstrates that optimizing the local oscillator drive to maintain the oscillator grid current between 0.04 mA and 0.05 mA is essential for achieving maximum conversion gain.

The 1A7GT is designed specifically for battery-operated receivers and provides satisfactory performance across all broadcast and shortwave bands up to a frequency of 15 Mc/s.

Technical Summary

The 1A7GT is an octal-based, directly heated pentagrid converter designed for battery-operated all-wave radio receivers. Operating with a highly efficient 1.4 V, 0.05 A DC filament, it combines the functions of a mixer and local oscillator in a single envelope. It features a typical conversion conductance of 0.25 mA/V (250 µmhos) and a high plate resistance of 0.6 MΩ under typical operating conditions with an anode voltage of 90 V. With its low power consumption, compact physical dimensions, and reliable operation up to 15 Mc/s, the 1A7GT was a standard frequency-changer solution for portable and farm radio designs of its era.