1. Introduction and History
The CV449 is a specialized vacuum tube developed in the United Kingdom during the mid-20th century as part of the Common Valve (CV) numbering system used for British military and government specifications. This tube is commercially known as the 85A2, with the CV449 designation specifically referring to the military-grade version that met stringent quality and reliability standards.
Developed in the late 1940s to early 1950s, the CV449/85A2 was manufactured by several British companies including English Electric Valve Company (EEV), Mullard, and GEC. The tube was designed to meet the growing need for stable voltage reference sources in precision electronic equipment during the post-World War II era.
The CV449 belongs to the family of gas-filled voltage reference tubes (also called voltage regulator or stabilizer tubes), which were critical components in electronic equipment before the widespread adoption of semiconductor voltage references.
2. Technical Specifications and Design
The CV449/85A2 is a cold-cathode, gas-filled voltage reference tube with the following specifications:
- Physical Characteristics:
- Base: B7G (miniature 7-pin)
- Envelope: Glass, typically with a distinctive purple-blue glow when operating
- Height: Approximately 55-60mm
- Diameter: Approximately 19-22mm
- Electrical Characteristics:
- Nominal operating voltage: 85 volts (±1V typical tolerance)
- Current range: 1-10 mA (typically 5-8 mA for optimal regulation)
- Minimum operating current: 1 mA
- Maximum operating current: 10 mA
- Starting voltage: Approximately 115-125 volts
- Internal impedance: Approximately 180-250 ohms
- Temperature coefficient: Typically less than 10mV over operating temperature range
- Gas Filling: Neon with trace elements (typically argon and/or mercury)
The design of the CV449 is based on the physics of gas discharge. When sufficient voltage is applied across the tube, the gas ionizes, creating a plasma that conducts current while maintaining a nearly constant voltage drop across the tube. This property makes it ideal for voltage reference applications.
The tube contains two main electrodes: an anode and a cathode. Unlike amplifying vacuum tubes, the CV449 doesn't require a heated filament for operation, making it a "cold-cathode" device. This design contributes to its long operational life and stability.
3. Applications and Usage
The CV449/85A2 was extensively used in various electronic applications requiring stable voltage references:
- Precision Test Equipment: Used in voltmeters, oscilloscopes, and signal generators where stable reference voltages were critical
- Military Communications: Found in radar systems, radio equipment, and electronic warfare devices
- Scientific Instruments: Employed in laboratory equipment requiring precise voltage references
- Industrial Control Systems: Used in process control equipment and monitoring systems
- Audio Equipment: Found in high-end audio amplifiers and studio equipment
In typical circuit applications, the CV449 would be connected in series with a current-limiting resistor. The value of this resistor would be calculated to ensure that:
- The tube receives sufficient voltage to start (≥115V)
- The operating current stays within the 1-10mA range
A common formula for calculating the series resistor value is:
R = (Supply Voltage - 85V) / Desired Current
For example, with a 250V supply and desired current of 5mA:
R = (250V - 85V) / 0.005A = 33,000 ohms
4. Equivalent or Substitute Types
The CV449 has several direct and near equivalents that can be used as substitutes in most applications:
- Direct Equivalents:
- 85A2 (commercial version of the same tube)
- QS85/2 (another British designation)
- 5651 (American equivalent, though with 87V nominal voltage)
- Near Equivalents:
- 0A2 (150V stabilizer tube, would require circuit modifications)
- 0B2 (108V stabilizer tube, would require circuit modifications)
- VR75 (75V stabilizer tube, close enough for some applications)
- ZA1000 (modern solid-state replacement designed to mimic voltage reference tubes)
When substituting, it's important to consider not only the nominal voltage but also the operating current range and physical dimensions. The 5651, while similar, has slightly different characteristics and may require minor circuit adjustments for optimal performance.
5. Notable Characteristics
The CV449/85A2 has several distinctive characteristics that made it valuable in its applications:
- Exceptional Stability: The tube provides excellent voltage stability over time, with minimal drift once warmed up
- Low Temperature Coefficient: The voltage reference maintains stability across a wide temperature range
- Long Operational Life: Typical service life exceeds 10,000 hours when operated within specifications
- Distinctive Visual Indication: The characteristic purple-blue glow provides a visual confirmation of proper operation
- Noise Characteristics: The tube exhibits very low noise compared to some other reference technologies of its era
- Radiation Resistance: The military-spec CV449 offers good resistance to radiation effects, making it suitable for aerospace and nuclear applications
One notable quirk of the CV449 is its "running-in" period. When new or after long storage, the tube may require several hours of operation before achieving its optimal stability characteristics. This was a common practice in precision equipment, where reference tubes would be kept powered continuously to maintain maximum stability.
The English Electric Valve Co. version of the CV449 is particularly sought after by collectors and vintage equipment restorers for its build quality and reliability. New Old Stock (NOS) examples are increasingly rare but still available through specialized tube suppliers and auction sites.
While largely superseded by semiconductor references in modern designs, the CV449 remains in use in some specialized vintage equipment and among enthusiasts who appreciate its unique characteristics and historical significance in the development of precision electronics.
