1. Introduction and History
The 1S5 is a miniature pentode vacuum tube that was developed in the 1940s during the era of portable battery-operated radio receivers. Manufactured by various companies including AWV (Amalgamated Wireless Valve Company) in Australia under their "Super Raditron" brand, the 1S5 was part of the popular miniature battery tube series that included the 1R5, 1T4, and 3V4. This family of tubes was designed specifically to address the growing demand for compact, energy-efficient vacuum tubes that could operate with low battery voltages in portable equipment.
The 1S5 was widely adopted during the post-World War II period when portable radio receivers became increasingly popular among consumers. Its efficient design and low power consumption made it particularly suitable for battery-powered applications where power efficiency was crucial. The tube remained in production through the 1950s and into the early 1960s before gradually being replaced by semiconductor devices.
2. Technical Specifications and Design
The 1S5 is a miniature sharp-cutoff pentode with a diode section in the same envelope, making it a dual-purpose tube. The pentode section typically functions as an audio frequency amplifier, while the diode section can be used for detection or AVC (Automatic Volume Control) purposes.
Physical Characteristics:
- Envelope: Miniature glass (T-5½)
- Base: 7-pin miniature (B7G)
- Height: Approximately 1.75 inches (44.5 mm)
- Diameter: Approximately 0.75 inches (19 mm)
- Pin Configuration:
- Pin 1: Filament
- Pin 2: Plate (Pentode section)
- Pin 3: Screen Grid (G2)
- Pin 4: Control Grid (G1)
- Pin 5: Diode Plate
- Pin 6: No connection
- Pin 7: Filament
Electrical Characteristics:
- Filament Voltage: 1.4 volts
- Filament Current: 50 mA
- Maximum Plate Voltage: 90 volts
- Maximum Screen Voltage: 90 volts
- Typical Plate Voltage: 67.5 volts
- Typical Screen Voltage: 67.5 volts
- Grid Bias Voltage: 0 volts
- Plate Resistance: Approximately 1 megohm
- Transconductance: Approximately 575 micromhos (0.575 mA/V)
- Plate Current: Approximately 0.8 mA at typical operating conditions
- Screen Current: Approximately 0.2 mA at typical operating conditions
The 1S5 features a direct-heated filament (cathode) design, which contributes to its low power consumption. The pentode section includes a suppressor grid that is internally connected to the filament, which helps reduce secondary emission effects and improves performance.
3. Applications and Usage
The 1S5 was primarily designed for and used in portable battery-operated equipment. Its most common applications included:
Portable Radio Receivers:
The 1S5 was frequently employed in portable AM radio sets, typically serving as a combined detector (using the diode section) and first audio amplifier (using the pentode section). This dual functionality allowed manufacturers to reduce the tube count in their designs, making radios more compact and energy-efficient.
Audio Amplification:
The pentode section of the 1S5 provides moderate gain for audio frequency signals, making it suitable as a voltage amplifier in the audio stages of receivers. It typically drives a power output tube like the 3V4 or 3Q4.
Detection and AVC:
The diode section can be used for signal detection (demodulation) in AM receivers and also to generate an Automatic Volume Control (AVC) voltage to maintain consistent audio output despite variations in signal strength.
Typical Circuit Configuration:
In a standard portable radio circuit, the 1S5 would follow an IF amplifier tube (often a 1T4) and precede the audio output tube. The diode section would demodulate the AM signal and provide AVC voltage, while the pentode section would amplify the resulting audio signal to drive the output stage.
4. Equivalent or Substitute Types
Several tubes can serve as direct replacements or close substitutes for the 1S5:
Direct Equivalents:
- DK91: European designation for the 1S5
- DAF91: Very similar characteristics, can often be substituted
Near Equivalents (may require minor circuit adjustments):
- 1AH5: Similar characteristics but with different pinout
- 1AN5: Similar function but with different specifications
- 1U5: Later version with improved characteristics
When substituting tubes, it's important to verify the pin connections and operating parameters to ensure compatibility with the existing circuit. While these tubes may be electrically similar, differences in pin configurations or internal characteristics might require circuit modifications.
5. Notable Characteristics
The 1S5 exhibits several distinctive characteristics that made it popular during its era:
Low Power Consumption:
With a filament current of only 50 mA at 1.4 volts, the 1S5 was designed for maximum battery efficiency. This low power consumption was crucial for portable applications where battery life was a significant concern.
Dual Functionality:
The combination of a pentode and diode in a single envelope allowed for space-efficient designs. This dual functionality was particularly valuable in compact portable radios where chassis space was limited.
Reliability:
The 1S5, especially those manufactured by reputable companies like AWV under their Super Raditron brand, was known for its reliability and consistent performance. Many of these tubes continue to function well decades after manufacture, testament to their quality construction.
Microphonics:
Like many miniature tubes, the 1S5 can be somewhat susceptible to microphonic effects (mechanical vibrations causing unwanted electrical signals) due to its compact internal structure. In sensitive audio applications, careful mounting was sometimes necessary to minimize these effects.
Warm-up Time:
The direct-heated filament design allows for nearly instantaneous operation, with no significant warm-up time required. This was a desirable feature for portable equipment where immediate operation was expected.
Collectibility:
Today, NOS (New Old Stock) 1S5 tubes, such as the AWV Super Raditron mentioned, are sought after by vintage radio enthusiasts and collectors for restoration projects. Their increasing scarcity has made well-preserved examples valuable to collectors and restorers of vintage battery-operated radios.
In conclusion, the 1S5 represents an important chapter in vacuum tube development, particularly in the evolution of portable electronics. Its efficient design and dual functionality made it a staple in battery-operated radios during the golden age of vacuum tube technology.
