1A7G — Battery Pentagrid Converter Vacuum Tube

The 1A7G (and its physically compact variant, the 1A7GT) is a filament-type battery heptode designed for use as a self-oscillating frequency changer (pentagrid converter). It is engineered specifically for low-power consumption in portable, battery-operated, or accumulator-operated radio receivers. The tube functions as a combined mixer and oscillator, providing satisfactory performance in all-wave receivers at frequencies up to 15 megacycles per second (Mc/s).

Physical Construction and Variants

The 1A7G and 1A7GT share identical electrical characteristics but differ in their physical dimensions, envelope shapes, and certain interelectrode capacitances.

• 1A7G: Features a T-9 (or T9-C) glass bulb with a small 8-pin octal base and a skirted miniature top cap.
  • Maximum Diameter: 1-3/16 inches to 1-9/16 inches.
  • Maximum Overall Height: 4-1/16 inches to 4-1/8 inches.
  • Maximum Seated Height: 3-1/2 inches to 3-9/16 inches.
• 1A7GT: Features a more compact glass envelope.
  • Maximum Diameter: 1-5/16 inches.
  • Maximum Overall Height: 2-7/8 inches.

Both variants can be mounted in any position. A key mechanical difference is that Pin 1 on the 1A7GT is connected to its metal shell, whereas Pin 1 on the 1A7G is a No Connection (NC) pin.

Filament Ratings

The tube features a coated filament designed for direct-current (DC) operation. Due to its specialized construction, it can operate directly from a 1.5 V dry cell without requiring a ballast resistor.

Pinout and Electrode Nomenclature

The 1A7G uses an octal 8-pin base (designation 7-Z or 7Z) and a top cap. Because different historical standards describe the internal grids of this pentagrid structure differently, the pin connections are documented under two primary nomenclatures:

Nomenclature System A

• Pin 1: No Connection (NC) (Note: Connected to metal shell on 1A7GT)
• Pin 2: Filament positive [f(+)]
• Pin 3: Anode [a]
• Pin 4: Grids 3 and 5 [g3, g5] (Screen)
• Pin 5: Grid 1 [g1] (Oscillator Grid)
• Pin 6: Grid 2 [g2] (Oscillator Anode)
• Pin 7: Filament negative [f(-)]
• Pin 8: No Connection (NC)
• Top Cap: Grid 4 [g4] (Control Grid)

Nomenclature System B

• Pin 1: No Connection (NC) (Note: Connected to metal shell on 1A7GT)
• Pin 2: Filament positive (F+)
• Pin 3: Anode (A)
• Pin 4: Oscillator Anode (G2)
• Pin 5: Oscillator Grid (G1)
• Pin 6: Screen (G3, G5)
• Pin 7: Filament negative (F-)
• Pin 8: No Connection (NC)
• Top Cap: Control Grid (G4)

(Note: Another historical classification refers to Grids 2 and 4 as the screen, and Grid 3 as the signal-input grid).

Maximum Ratings

The maximum electrical limits for the 1A7G vary slightly depending on the rating standard applied:

Typical Operating Conditions (Converter Service)

In typical frequency converter service, the 1A7G operates under the following conditions:

• Plate Voltage: 90 V
• Screen (G3, G5) Voltage: 45 V
• Anode-Grid / Oscillator Anode (G2) Voltage: 90 V
• Control Grid (G4) Voltage: 0 V
• Oscillator Grid (G1) Resistor: 200,000 ohms (0.2 MΩ)
• Plate Resistance (Approximate): 0.6 MΩ (600,000 ohms)
• Conversion Conductance (at 0 V Grid): 250 µmhos (0.25 mA/V)
• Conversion Conductance (at -2 V Grid): 50 µmhos
• Conversion Conductance (at -3 V Grid): 5 µmhos (or ~5.0 µmhos)
• Plate Current: 0.55 mA to 0.6 mA
• Screen Current: 0.6 mA to 0.7 mA
• Oscillator Anode (G2) Current: 1.2 mA
• Oscillator Grid (G1) Current: 0.035 mA
• Total Cathode Current: 2.4 mA to 2.5 mA

Circuit-Specific Operating Notes

• Screen Grid Supply: The 45 V screen potential is preferably obtained from a 90 V or 110 V supply using a series dropping resistor. Recommended values range from 45,000 to 75,000 ohms (typically 70,000 ohms when operating from a 90 V B-battery).
• Grid Return: Although a control grid bias is not required (0 V typical), the control grid return must be connected either to the automatic volume control (AVC) line or through a resistor of at least 1.0 megohm (suitably bypassed) to the negative filament (Pin 7).

Oscillator Characteristics (Non-Oscillating)

When the oscillator portion of the tube is evaluated in a non-oscillating state, it exhibits the following characteristics:

• Transconductance: 550 µmhos
• Test Conditions: Plate Voltage = 90 V, Screen Voltage = 45 V, Control Grid Voltage = 0 V, Anode-Grid Voltage = 90 V, Oscillator Grid Voltage = 0 V.

Conversion Conductance Curves

The conversion conductance of the 1A7G is highly dependent on the oscillator grid (G1) current. Under typical operating conditions (Filament = 1.4 V, Anode = 90 V, Screen = 45 V, and Oscillator Anode = 90 V via a 70,000-ohm resistor) with a 200,000-ohm grid leak:

• The conversion conductance curve peaks at approximately 0.27 mA/V (270 µmhos) when the oscillator grid current is between 0.04 mA and 0.06 mA.
• If the oscillator grid current drops to 0.01 mA, the conductance falls off to about 0.18 mA/V.
• If the oscillator grid current increases to 0.1 mA, the conductance falls off to approximately 0.20 mA/V.

Direct Interelectrode Capacitances

Interelectrode capacitances vary slightly depending on the shielding configuration and the specific physical variant (1A7G vs. 1A7GT).

Measured with a close-fitting shield connected to Pin 7 (or negative filament):

• R.F. Input (Control Grid to all other electrodes): 6.5 pF (or 7.0 pF max under some standards)
• I.F. Output (Anode to all other electrodes): 11.0 pF (or 10 pF under some standards)
• Oscillator Input (Oscillator Grid to all except Oscillator Anode): 4.0 pF (or 3.4 pF under some standards)
• Oscillator Output (Oscillator Anode to all except Oscillator Grid): 4.6 pF (or 4.4 pF under some standards)
• Control Grid to Oscillator Grid: 0.12 pF (or 0.2 pF max under some standards)
• Control Grid to Anode: 0.3 pF (or 0.5 pF max under some standards)
• Control Grid to Oscillator Anode: 0.26 pF (or 0.4 pF max under some standards)
• Oscillator Grid to Oscillator Anode: 0.9 pF

Related Designations and Comparisons

The 1A7G and 1A7GT are electrically identical except for physical dimensions and the specific interelectrode capacitances noted above. In application, the 1A7G is similar to types 1C7G and 1D7G, though it operates under different characteristics and operating conditions, meaning they are not direct drop-in equivalents without circuit modification.

Technical Summary

The 1A7G is an octal-based, filament-type pentagrid converter designed for low-power, battery-operated receiver applications. Operating with a highly efficient 1.4 V, 50 mA filament, it combines the functions of a mixer and oscillator in a single envelope. At a typical plate voltage of 90 V, it provides a conversion conductance of 250 µmhos with a total cathode current draw of approximately 2.4 to 2.5 mA. Its design allows for reliable frequency conversion up to 15 Mc/s, making it a staple component in portable all-wave radio receivers of its era.