1. Introduction and History
The 1U5 is a miniature battery-operated pentode vacuum tube that was developed in the late 1940s as part of the post-World War II miniaturization trend in electronics. Manufactured by several companies including RCA, Sylvania, and Australian Wireless Valve (AWV) under their Super Radiotron brand, the 1U5 was designed primarily for portable radio applications where battery power and compact size were essential requirements.
This tube belongs to the "1" series of battery tubes, which operated with a filament voltage of 1.4 volts, making them suitable for portable equipment powered by dry cells. The 1U5 was widely used throughout the 1950s and early 1960s until transistors began to replace vacuum tubes in consumer electronics.
2. Technical Specifications and Design
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)
Electrical Parameters
- Tube Type: Sharp-cutoff pentode
- Filament Voltage: 1.4 volts DC
- Filament Current: 50 mA
- Maximum Plate Voltage: 90 volts
- Screen Grid Voltage: 90 volts (maximum)
- Plate Resistance: Approximately 0.8 megohms
- Transconductance: 750 micromhos (0.75 mA/V) typical at plate voltage of 90V
- Plate Current: 1.6 mA typical at rated voltages
- Screen Current: 0.4 mA typical at rated voltages
Pin Configuration
- Pin 1: Plate
- Pin 2: Screen Grid (G2)
- Pin 3: Not Connected
- Pin 4: Filament Negative
- Pin 5: Filament Positive
- Pin 6: Control Grid (G1)
- Pin 7: Suppressor Grid (G3) and Internal Shield (connected internally to filament negative)
3. Applications and Usage
The 1U5 was primarily designed for and used in the following applications:
Primary Applications
- IF Amplifier: Most commonly used as an intermediate frequency (IF) amplifier in battery-operated superheterodyne receivers
- Audio Voltage Amplifier: Served as a voltage amplifier in audio stages of portable radios
- RF Amplifier: Sometimes utilized as a radio frequency amplifier in simpler receiver designs
Common Equipment
- Portable AM radios (particularly in "All American Five" battery variants)
- Battery-powered communication receivers
- Field equipment and military portable communications devices
- Early portable test equipment
The 1U5 was often used in combination with other tubes from the same series, such as the 1R5 (converter), 1T4 (RF pentode), and 3V4 or 3Q4 (audio output) to create complete battery-powered radio receivers.
4. Equivalent or Substitute Types
Several tubes can be considered equivalent or suitable substitutes for the 1U5:
Direct Equivalents
- DK91: European designation under the Mullard-Philips tube designation system
- 1T4: While not identical, can often be substituted with minor circuit adjustments (has higher transconductance)
- 1L4: Similar characteristics, can be substituted in many applications
Potential Substitutes (with circuit modifications)
- DAF91/1S5: A diode-pentode that can sometimes replace the 1U5 if the diode section is not needed
- DF91: Another European equivalent
- 1AG4: A later subminiature tube with similar characteristics
When substituting tubes, adjustments to bias voltages or other circuit parameters may be necessary to accommodate differences in characteristics.
5. Notable Characteristics
The 1U5 has several distinctive characteristics that made it popular in its applications:
Strengths
- Low Power Consumption: The 1.4V filament operating at only 50mA made it extremely economical for battery operation
- Good Gain: Provided sufficient amplification for IF stages in portable receivers
- Low Microphonics: Relatively resistant to mechanical vibration causing unwanted electrical signals
- Compact Size: The miniature envelope allowed for smaller portable equipment
- Instant Operation: Unlike many tubes, the 1U5 required no warm-up time due to its directly heated filament
Limitations
- Limited Power Handling: Not suitable for high-power applications
- Sensitivity to Supply Voltage: Performance could vary significantly as batteries depleted
- Filament Fragility: The low-voltage filament could be sensitive to voltage surges
AWV Super Radiotron Version
The AWV (Amalgamated Wireless Valve) Super Radiotron version of the 1U5, as mentioned in the related product information, was the Australian-manufactured variant. AWV was the principal vacuum tube manufacturer in Australia, producing tubes under license from RCA. Their Super Radiotron brand was well-regarded for quality and reliability, and NOS (New Old Stock) examples are sought after by collectors and vintage radio restorers for their durability and consistent performance.
In conclusion, the 1U5 represents an important part of vacuum tube history, bridging the gap between larger, more power-hungry tubes and the transistor era. Its efficient design made portable electronics more practical and helped pave the way for the mobile electronic devices we take for granted today.
