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1A5G

Directly heated battery power pentode for very-low-consumption receiver audio output, producing roughly 0.1 W.

Directly heatedCold cathodeCommon cathodeDissimilar

Sections: 1
Base: -
Socket code: IO
Base pins: 8
Nominal heater: 1.4 V / 50 mA

Description

1A5G — Low-Drain Battery Power Pentode

The 1A5G (also designated as 1A5G/GT) is a low-drain, low-consumption battery-operated power output pentode. It is designed for highly economical operation in battery-operated receivers. Due to its special filament construction, it can operate directly from a 1.5 V dry cell without requiring a ballast resistor. The tube was registered by the Philco Radio & Television Corporation under RMA release number 140 on June 10, 1938. The tube is housed in a glass envelope with an octal base and can be mounted in any position.

Mechanical Construction, Filament, and Pinout

The 1A5G features a filament designed for low-current battery operation. It has the following filament ratings:

Filament Voltage: 1.4 V (nominal, capable of direct operation on a 1.5 V dry cell)
Filament Current: 0.05 A (50 mA)

The tube is housed in a T-9, T9-B, or glass tubular (GT outline) envelope. The physical dimensions are:

Maximum Diameter: 1-5/16 inches
Maximum Overall Length: 4 inches (or 3-5/16 inches depending on the specific envelope outline)
Seated Height: 2-5/8 inches

The mounting position is unrestricted (any position). The tube uses a small shell octal 7-pin base (designation 6-X). The pin connections are as follows:

Pin	Connection
Pin 1	No connection (NC)
Pin 2	Positive Filament (F+)
Pin 3	Plate / Anode (P)
Pin 4	Screen Grid ( $G_2$ )
Pin 5	Control Grid ( $G_1$ )
Pin 6	Omitted
Pin 7	Negative Filament / Suppressor Grid ( $F-$ , $G_3$ )
Pin 8	No connection (NC)

Electrical Characteristics and Operating Points

The 1A5G can be operated under two typical operating conditions depending on the plate and screen voltages (85 V or 90 V maximum):

Parameter	85 V Operation	90 V Operation (Maximum)
Plate (Anode) Voltage	85 V	90 V
Screen Grid ( $G_2$ ) Voltage	85 V	90 V
Control Grid ( $G_1$ ) Voltage	-4.5 V	-4.5 V
Plate Current	3.5 mA	3.5 mA to 4.0 mA
Screen Grid Current	0.7 mA	0.8 mA
Plate Resistance (Anode Impedance)	0.3 MΩ	0.3 MΩ
Transconductance (Mutual Conductance)	800 µmhos	850 µmhos (0.85 mA/V)
Amplification Factor	240	255
Load Impedance (Optimum Load)	25,000 Ω	25,000 Ω
Power Output	100 mW	115 mW to 117 mW
Total Harmonic Distortion	10%	7%

Circuit-Specific Applications and Bias Configurations

The 1A5G is optimized for low-consumption battery-operated receivers. Several circuit configurations are documented for its application:

Bias Configurations

Automatic Bias: This is the preferred biasing method because the grid bias voltage will scale down proportionally as the 'B' battery voltage drops over time, maintaining balanced operation.
C-Battery: A separate C-battery is not required for receivers using the 1.4 V series. The grid returns for RF and second detector stages should be connected directly to the negative filament.

Parallel Operation

For applications requiring higher power output, two 1A5G tubes can be operated in parallel:

Load Impedance: 12,500 Ω (half of the single-tube optimum load)
Power Output: Approximately doubled, yielding around 200 mW to 230 mW.
Distortion: Remains approximately the same as that of a single tube (7% to 10%).

Technical Summary

The 1A5G is an octal-based, low-drain power pentode designed for highly economical battery-operated receivers. Operating with a filament supply of 1.4 V at 50 mA, it can run directly off a 1.5 V dry cell. It delivers a power output of 100 mW to 117 mW under typical single-tube Class A conditions with plate and screen voltages of 85 V to 90 V. Its design supports automatic bias and parallel operation, making it a versatile and efficient output stage for portable battery-powered equipment.

Socket description

International Octal

General specifications

Tube and section characteristics

Properties common to every published source and operating point.

Heater and base

Base: -
Socket code: IO
Base pins: 8
Nominal heater voltage: 1.4 V
Nominal heater current: 50 mA
Parallel heater voltage: -
Parallel heater current: -
Series capable: No

Terminal pinout

Electrical assignments shared by every tube using this base designation.

No verified pinout is recorded for this base designation.

Section ratings

1 physical section, each with unique specifications.

Section 1

Pentode

X-ray radiationInterelectrode capacitances

Plate resistance: 300000 Ω
Transconductance: 0.00085 S
Amplification factor: 255
Max plate voltage: 90 V
Max screen voltage: 90 V

Published values promoted where clearly attributable to this section.

Source-dependent data

Operating conditions by source

Each block preserves the test conditions and published values from one reference. Values from different sources are intentionally kept separate.

Data set 01

1948 Brimar Radio Valve Manual

Published operating points and test conditions from this source.

SECTION 1

Pentode

Other

Power output
0.117 W

Data set 02

1950 Sylvania

Published operating points and test conditions from this source.

SECTION 1

Pentode

Other

Typical plate current
3.5 mA

Data set 03

1958 Brimar Radio Valve No 7

Published operating points and test conditions from this source.

SECTION 1

Pentode

Other

Plate Voltage
90 V
Plate Current
4 mA
Screen Voltage
90 V
Screen Current
0.8 mA
Control Grid Voltage
-4.5 V
Optimum Load
25000 ohm
Power Output
0.115 W

Characteristic

Plate Resistance
300000 ohm
Transconductance
850 µS
Amplification Factor
255