1. Introduction and History
The CV574 is a British military designation for the 6X5GT full-wave rectifier vacuum tube, manufactured by Brimar STC (Standard Telephones and Cables) during the mid-20th century. The 'CV' prefix stands for 'Common Valve,' indicating its standardization for British military and government applications. Developed during the World War II era, the CV574 was part of the widespread effort to standardize electronic components for military equipment.
Brimar, a respected British manufacturer founded in the 1930s, produced high-quality valves for both military and civilian markets. The CV574 represented their contribution to the critical rectifier tube category, essential for power supply circuits in various electronic equipment. The military specification typically demanded higher reliability and more stringent testing than civilian counterparts, making CV-designated tubes particularly robust.
2. Technical Specifications and Design
The CV574 (6X5GT) is a full-wave rectifier tube with the following specifications:
- Heater/Filament: 6.3V at 0.6A
- Envelope: GT-style glass bulb (tubular shape, smaller than ST types)
- Base: Octal (8-pin)
- Maximum DC Output Current: 70mA
- Maximum AC Voltage Per Plate: 350V RMS
- Peak Inverse Voltage: 1250V
- Voltage Drop: Approximately 20V at 70mA
- Internal Structure: Indirectly heated cathode with two separate anodes (plates)
The design of the CV574 features an indirectly heated cathode surrounded by two separate plates (anodes). The tube contains no grid structures as it functions solely as a rectifier. The octal base provides sturdy mechanical support and reliable electrical connections. The internal construction is optimized to handle the thermal stresses associated with rectification while maintaining consistent performance over an extended operational lifetime.
A notable design characteristic is the relatively small voltage drop across the tube during operation, which improves power supply efficiency compared to some alternative rectifiers of the era.
3. Applications and Usage
The CV574/6X5GT found wide application in various electronic equipment, particularly:
- Military Communications Equipment: Radio receivers, transmitters, and field telephone systems
- Radar Systems: Power supply sections for early radar equipment
- Test and Measurement Instruments: Including AVO meters and other test equipment
- Audio Amplifiers: Both military and later civilian audio equipment
- Power Supply Units: General-purpose power supplies for electronic equipment
In typical usage, the CV574 rectifies alternating current from a power transformer into direct current for the B+ (high voltage) supply in vacuum tube circuits. Its moderate current capability made it suitable for small to medium-sized equipment rather than high-power applications.
In power supply circuits, the CV574 was often paired with filter capacitors and chokes to produce smooth DC voltage. The tube's relatively low internal resistance contributed to good voltage regulation under varying load conditions, an important characteristic for sensitive electronic equipment.
4. Equivalent or Substitute Types
The CV574 has several equivalent or substitute types that can be used in most applications:
- 6X5GT: The commercial equivalent to the CV574, electrically identical
- 6X5G: Earlier version with larger ST-shaped glass envelope but identical electrical characteristics
- 6X5: Metal envelope version with the same electrical specifications
- VT-126: U.S. military designation for the same tube
- 5X4G/5Z4G: Higher current capability alternatives that may substitute in some applications
- 6X4: Miniature version with similar characteristics but different pinout
- EZ35: European equivalent with slightly different ratings
When substituting tubes, it's important to verify that the replacement's current capability, voltage ratings, and heater specifications are compatible with the original circuit design. While the above alternatives may work in many cases, circuit modifications might be necessary for optimal performance and safety.
5. Notable Characteristics
The CV574/6X5GT possesses several notable characteristics that distinguished it during its operational lifetime:
- Reliability: Military specification tubes like the CV574 underwent more rigorous testing than commercial equivalents, resulting in exceptional reliability
- Indirect Heating: Unlike some rectifiers that used direct heating, the indirectly heated cathode provided better isolation from the AC heater supply, reducing hum in sensitive applications
- Warm-up Time: The tube requires approximately 15-20 seconds to reach operational temperature, which provided a natural delay for equipment power-up sequences
- Robustness: The tube could withstand significant voltage surges and operational stress
- Compact Design: The GT envelope offered a good compromise between the larger ST glass types and more fragile miniature tubes
- Long Service Life: When operated within specifications, the CV574 typically provided thousands of hours of reliable service
The Brimar STC manufacturing quality was generally considered excellent, with consistent performance between production batches. Tubes that pass modern AVO testing (a comprehensive tube tester standard) still perform admirably despite their age, testament to the quality of their original construction.
Collectors and vintage equipment enthusiasts value the CV574 not only for its technical capabilities but also for its historical significance in British electronic equipment development. The distinctive Brimar branding and construction details make it identifiable to knowledgeable tube collectors.
