1. Introduction and History
The CV136, known in North America as the 6AM5, is a compact vacuum tube that emerged during the mid-20th century as part of the standardized tube development era. The CV designation indicates British military specification, while the 6AM5 designation follows the RCA tube numbering system. This tube represents a significant development in audio and electronic applications, designed to deliver reliable performance in demanding environments.
The 6AM5/CV136 was developed in the 1950s and became widely adopted in both consumer and professional audio equipment. Its design reflects the engineering principles of the post-war era, incorporating improvements in filament efficiency and output characteristics. The tube gained particular prominence in guitar amplifiers, where its unique sonic properties made it a favorite among musicians and engineers seeking specific tonal qualities.
2. Technical Specifications and Design
Physical Specifications
- Tube Type: Compact triode
- Filament Voltage: 6.3V AC or DC
- Filament Current: 0.45A
- Basing: Noval (9-pin) base
- Overall Height: Approximately 2.75 inches (70mm)
- Bulb Type: T6.5 or T9 envelope (glass bulb)
Electrical Characteristics
- Maximum Plate Voltage: 330V
- Maximum Plate Dissipation: 2.75W
- Amplification Factor (μ): Approximately 60
- Plate Resistance (rp): Approximately 3,000 ohms
- Transconductance (gm): Approximately 20 millimhos
- Grid Bias: -2V to -8V typical
- Output Impedance: Moderate, suitable for transformer coupling
Operational Parameters
The CV136 operates as a medium-mu triode with moderate amplification characteristics. Its plate resistance and transconductance values make it suitable for both voltage amplification and moderate power applications. The tube exhibits good linearity across its operating range, contributing to its popularity in audio circuits where low distortion is desirable.
The tube's filament design allows for either AC or DC operation, providing flexibility in circuit design. The 6.3V filament voltage is standard across many tube types, simplifying power supply design in multi-tube equipment.
3. Applications and Usage
Primary Applications
- Guitar Amplifiers: Output and power amplification stages
- Audio Preamplifiers: Voltage amplification in microphone and line preamps
- Vintage Radio Equipment: IF and detector stages
- Test Equipment: Laboratory oscillators and signal generators
- Industrial Control Circuits: Switching and amplification functions
Circuit Configurations
The CV136 functions effectively in triode configurations, commonly employed as:
- Single-ended amplifiers with transformer coupling
- Push-pull configurations for increased output
- Cathode-follower circuits for low-impedance output
- Common-cathode amplifiers for voltage gain
- Phase inverter stages in multi-stage amplifiers
4. Sound Characteristics and Audio Community Usage
Sonic Profile
The CV136/6AM5 possesses distinctive sonic characteristics that have endeared it to audio professionals and musicians:
- Midrange Presence: The tube exhibits a pronounced and articulate midrange, making it particularly valued in guitar amplifiers where clarity and definition are paramount.
- Harmonic Coloration: Unlike solid-state components, the CV136 introduces subtle harmonic distortion that many listeners describe as "warm" or "musical." This characteristic becomes more pronounced at higher drive levels.
- Dynamic Response: The tube responds quickly to transient signals, preserving attack and decay characteristics of musical instruments.
- Compression Characteristics: The CV136 exhibits natural compression at high signal levels, creating a smooth, forgiving quality that flatters vocals and instruments.
- Frequency Response: Generally linear across the audio spectrum, though the tube's output impedance influences frequency response depending on load impedance.
Audio Community Applications
The CV136 has established a dedicated following within the audio enthusiast community:
- Guitar Amplifier Design: Highly sought after for vintage amp restoration and boutique amplifier manufacturing. Musicians prize the CV136 for its ability to produce singing, responsive tone in both clean and overdriven applications.
- Tube Preamp Restoration: Original equipment manufacturers specified the CV136 in numerous vintage preamps. Audiophiles and recording engineers actively seek NOS (New Old Stock) examples for restoration projects.
- High-End Audio: Some boutique audio manufacturers incorporate the CV136 in small-signal stages of tube preamps and integrated amplifiers, valuing its midrange character.
- DIY Tube Audio: The tube's moderate specifications and availability make it popular among hobbyists building custom amplifiers and preamps.
- Recording Studios: Professional studios maintain stocks of CV136 tubes for microphone preamps and mixing console restoration.
Comparative Sonic Qualities
When compared to other small triodes, the CV136 offers:
- More robust output than dual-triodes like the 12AX7, making it suitable for driving output stages directly
- Warmer character than pentodes, with more gradual saturation characteristics
- Fuller bass response than some competitors, attributed to its moderate plate resistance
- Reduced microphony compared to some high-mu triodes, important in sensitive preamplifier applications
5. Notable Characteristics
Reliability and Longevity
The CV136 demonstrates exceptional reliability when operated within specifications. The tube's robust construction and moderate operating parameters contribute to extended service life. Many examples from the 1950s-1970s remain functional today, testament to the quality of original manufacturing.
Manufacturing Quality Variations
Original examples of the CV136/6AM5 exhibit variations based on manufacturer:
- Mullard (British): Known for warm tone and reliability; highly valued by audio professionals
- RCA (American): Consistent performance and good availability; slightly more neutral sonic character
- Telefunken (German): Premium construction; prized for extended frequency response
- Philips/Miniwatt: Reliable performers; balanced sonic characteristics
NOS Considerations
New Old Stock (NOS) CV136 tubes command premium prices in the current market. Factors affecting value include:
- Manufacturing origin and reputation
- Test data and measured parameters
- Original packaging and documentation
- Age and storage conditions
- Availability and demand from audio enthusiasts
Microphony and Noise Performance
The CV136 exhibits moderate microphony levels typical of its era. In sensitive applications, careful mechanical isolation and damping rings improve performance. The tube's relatively low noise floor makes it suitable for preamplifier applications where signal-to-noise ratio is critical.
6. Equivalent or Substitute Types
Direct Equivalents
- 6AM5: American designation; electrically and physically identical
- CV136: British military specification; same tube, different designation
- EL84 (in some applications): Similar output capabilities, though different operating characteristics
Functional Alternatives
- 12AX7/ECC83: Higher amplification factor; suitable for voltage amplification stages where the CV136 provides power output
- 6V6GT: Similar power output capabilities; more suited to power amplification
- EL91: European equivalent with similar specifications
- 6AQ5: Related tube with slightly different characteristics; not a direct substitute
Substitution Guidelines
When substituting the CV136, ensure:
- Filament voltage compatibility (6.3V required)
- Similar amplification factor for circuit stability
- Comparable plate dissipation ratings
- Physical compatibility with socket and space constraints
- Testing with circuit designer or experienced technician
Conclusion
The CV136/6AM5 represents a significant achievement in vacuum tube design, combining reliable performance with distinctive sonic characteristics. Its continued popularity in audio applications demonstrates the enduring appeal of tube technology. Whether restoring vintage equipment or designing new tube circuits, the CV136 remains a valued component in the toolkit of audio professionals and enthusiasts. The tube's moderate specifications, proven reliability, and musical tonal qualities ensure its relevance in contemporary audio applications, particularly among those who appreciate the unique character that vacuum tubes impart to audio signals.
For those seeking authentic vintage tone or restoring original equipment, sourcing genuine NOS CV136 tubes from reputable dealers ensures optimal performance and longevity. The investment in quality tubes pays dividends through superior sound quality and reliable operation.
