1. Introduction and History
The CV1988 is a British military designation for the 6SN7GT dual triode vacuum tube. The "CV" prefix stands for "Common Valve" and was used by British military services for standardization purposes during and after World War II. The CV1988 was manufactured at the Tungsram Tottenham factory in the United Kingdom, making tubes produced at this facility particularly sought after by collectors and audio enthusiasts.
Tungsram, originally a Hungarian company, established manufacturing facilities in the UK to supply vacuum tubes for military and civilian applications. The Tottenham factory produced these valves under strict military specifications, which typically meant higher quality control standards than their civilian counterparts.
The CV1988 was primarily used in military communication equipment, radar systems, and other electronic devices where reliability under harsh conditions was essential. Post-war, these tubes found their way into high-fidelity audio equipment, contributing to their enduring legacy in the audio community.
2. Technical Specifications and Design
The CV1988 is a medium-mu dual triode vacuum tube with the following specifications:
- Structure: Dual triode (two identical triode sections in one envelope)
- Base: 8-pin octal
- Heater Voltage: 6.3V
- Heater Current: 0.6A
- Plate Voltage (max): 300V
- Plate Dissipation (per section): 2.5W
- Amplification Factor (μ): Approximately 20
- Transconductance: Around 2600 μmhos (2.6 mA/V)
- Plate Resistance: Approximately 7700 ohms
The design features two completely independent triode sections sharing only the heater/cathode connection. The tube is housed in a glass envelope (bulb) with a standard octal base. British military versions often featured robust construction with additional reinforcement to withstand vibration and shock in field operations.
The British Tungsram-manufactured CV1988 tubes are recognizable by their distinctive construction details and factory markings, typically including the CV number, factory code, and date code indicating the manufacturing period.
3. Applications and Usage
The CV1988 found widespread use in various applications:
Military Applications:
- Radar systems
- Radio communications equipment
- Signal processing circuits
- Oscilloscopes and test equipment
- Computing devices
Audio Applications:
- Preamplifier stages in high-fidelity systems
- Phase inverter circuits in push-pull amplifiers
- Line stages in audio equipment
- Driver stages for power amplifier tubes
- Cathode follower applications
The tube's balanced characteristics make it particularly suitable for differential amplifier circuits and phase splitters. Its relatively low noise characteristics and good linearity have made it a favorite among audio designers and enthusiasts, especially in vacuum tube amplifiers for musical instruments and high-end audio reproduction.
In modern usage, the CV1988 is primarily found in boutique audio equipment, particularly in the driver stages of high-end vacuum tube amplifiers where its sonic characteristics are prized.
4. Equivalent or Substitute Types
The CV1988 is directly equivalent to several other tube designations:
- 6SN7GT: The American designation for the same tube
- 6SN7GTB: An improved version with slightly different ratings
- B65: Marconi-Osram Valve (MOV) designation
- ECC32: Similar European tube, though not an exact equivalent (has higher amplification factor)
- CV1988A: Updated military specification version
- VT-231: U.S. military designation for the 6SN7GT
While these tubes can be used as substitutes in most applications, there may be slight differences in characteristics that could affect performance in critical circuits. The ECC32, for instance, has a higher amplification factor and different plate resistance, which might require circuit adjustments when used as a replacement.
5. Notable Characteristics
The CV1988, particularly those manufactured by Tungsram at the Tottenham factory, has several distinctive characteristics:
Construction Quality:
Military specification tubes like the CV1988 were built to higher standards than commercial equivalents. They typically feature:
- Reinforced mica spacers
- More robust internal structures
- Higher quality control standards
- Extended reliability testing
Sonic Characteristics:
In audio applications, the CV1988 is known for:
- Warm, balanced midrange reproduction
- Good detail and transparency
- Low microphonics (compared to many other dual triodes)
- Consistent performance between the two triode sections
- Good linearity when properly biased
Collectibility:
The British Tungsram Tottenham factory CV1988 tubes are particularly rare and sought after by collectors and audiophiles for several reasons:
- Limited production numbers compared to American equivalents
- Historical significance as military components
- Perceived superior sound quality in audio applications
- Distinctive construction details that differentiate them from other manufacturers
The rarity of genuine Tungsram Tottenham CV1988 tubes has made them valuable collector's items, with prices significantly higher than standard 6SN7GT tubes. Audio enthusiasts often seek these tubes for their purported superior sonic characteristics in high-end vacuum tube amplifiers.
Due to their age and rarity, when purchasing these tubes, careful verification of authenticity is recommended, as counterfeits or misidentified tubes occasionally appear in the marketplace.
