1. Introduction and History
The CV140 is a British military designation for what is commercially known as the 6AL5 vacuum tube, a miniature dual diode valve developed during the World War II era. First introduced in the early 1940s, the CV140 was part of the Common Valve (CV) numbering system used by British military services to standardize electronic components.
The tube was manufactured by several companies, including AEI (Associated Electrical Industries) and Thron, as indicated in the product information. The CV140 was designed as a more compact and reliable replacement for earlier dual diode tubes like the 6H6. Its development coincided with the broader miniaturization trend in vacuum tube technology that emerged during and after World War II.
As a military specification component, the CV140 was built to withstand more rigorous operating conditions than its civilian counterparts, with tighter tolerances and more robust construction. These tubes were extensively used in British radar systems, communications equipment, and various military electronic devices.
2. Technical Specifications and Design
The CV140/6AL5 is a miniature twin diode vacuum tube with the following specifications:
- 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:
- Heater Voltage: 6.3 volts
- Heater Current: 0.3 amperes
- Maximum Peak Inverse Voltage: 330 volts
- Maximum DC Output Current (per plate): 9 mA
- Maximum Peak Plate Current (per plate): 30 mA
- Plate to Cathode Capacitance: Approximately 3.0 pF
- Plate to Plate Capacitance: Approximately 0.8 pF
- Pin Configuration:
- Pin 1: Plate (Diode 1)
- Pin 2: Cathode
- Pin 3: Heater
- Pin 4: Heater
- Pin 5: Plate (Diode 2)
- Pin 6: Not Connected
- Pin 7: Not Connected
The CV140 features two independent diodes with a common cathode, housed in a compact glass envelope. The design emphasizes low capacitance and high reliability, with the diode sections carefully shielded from each other to minimize crosstalk. The tube's construction allows for efficient heat dissipation despite its small size.
3. Applications and Usage
The CV140/6AL5 found widespread use in various electronic applications:
- Military Applications:
- Radar systems for signal detection and processing
- Communications equipment for signal demodulation
- Electronic countermeasure systems
- Navigation equipment
- Civilian Applications:
- FM detector circuits (ratio detectors and discriminators)
- AM signal detection
- Phase detectors in television receivers
- Automatic Volume Control (AVC) circuits
- Clipping and limiting circuits
- Signal rectification in test equipment
- Audio equipment (particularly in high-fidelity systems)
The tube's low capacitance and good high-frequency response made it particularly suitable for FM detection circuits and other applications requiring efficient operation at radio frequencies. Its dual diode configuration was ideal for balanced detector circuits, where both diodes needed to have closely matched characteristics.
4. Equivalent or Substitute Types
The CV140 has several equivalent and substitute types that can be used in most applications:
- Direct Equivalents:
- 6AL5 (Commercial American designation)
- EAA91 (European designation)
- D77 (British commercial designation)
- 5726 (Special quality version)
- CV4025 (Alternative British military designation)
- Potential Substitutes (with circuit modifications):
- 6H6 (Older octal-based dual diode, requires socket adapter)
- EB91 (Similar characteristics but may require circuit adjustments)
- 6AL5W (Ruggedized version with enhanced reliability)
- Solid-state replacements (Using silicon diodes with appropriate voltage dropping resistors)
When substituting these tubes, it's important to consider not only the electrical characteristics but also the physical dimensions and pin configurations, as these can vary between different types.
5. Notable Characteristics
The CV140/6AL5 has several distinctive characteristics that made it a preferred choice for many applications:
- Excellent High-Frequency Performance: The tube's low interelectrode capacitance and compact design allowed it to operate efficiently at high frequencies, making it suitable for FM and television circuits.
- Matched Diode Sections: Military specification CV140 tubes typically had closely matched diode sections, ensuring balanced operation in detector circuits.
- Reliability: The military-grade CV140 was designed for enhanced reliability under adverse conditions, with more robust construction than civilian equivalents.
- Low Noise: The tube exhibits relatively low noise characteristics, making it suitable for audio and sensitive detection applications.
- Long Service Life: When operated within specifications, these tubes often demonstrated exceptional longevity, with many NOS (New Old Stock) examples still functioning perfectly decades after manufacture.
- Warm-Up Time: The tube has a relatively quick warm-up time compared to larger vacuum tubes, allowing equipment to become operational faster.
The CV140's compact size represented an important step in the miniaturization of electronic equipment, bridging the gap between the larger octal-based tubes of the pre-war era and the even smaller tubes that would follow. Despite being superseded by semiconductor technology in most applications, the CV140/6AL5 remains valued by vintage equipment enthusiasts and audiophiles who appreciate its unique audio characteristics and historical significance.
For collectors and restorers of vintage British military equipment, original AEI or Thron-manufactured CV140 tubes are particularly sought after for their authenticity and typically high build quality.
