1. Introduction and History
The 6AQ5 is a miniature beam power pentode vacuum tube that was introduced in the late 1940s. Developed during the post-World War II era when manufacturers were transitioning from larger glass envelope tubes to more compact designs, the 6AQ5 became a popular choice in consumer electronics. It was designed as a smaller, more efficient alternative to the octal-based 6V6GT, offering similar performance characteristics in a more compact package.
The tube was widely manufactured by companies such as RCA, Sylvania, GE, Tung-Sol, and Brimar throughout the 1950s and 1960s. It became particularly popular in television audio output stages, table radios, and small audio amplifiers where space was at a premium but reasonable power output was still required.
The 6AQ5 is part of the 7-pin miniature tube family (B7G base) that helped revolutionize electronics by allowing for smaller, more efficient device designs while maintaining good performance characteristics.
2. Technical Specifications and Design
Physical Characteristics:
- Base: 7-pin miniature (B7G)
- Envelope: Glass, miniature construction
- Height: Approximately 54mm (2.125 inches)
- Diameter: Approximately 19mm (0.75 inches)
- Filament/Heater: 6.3V at 0.45A
- Pin Configuration:
- Pin 1: Shell
- Pin 2: Heater
- Pin 3: Plate
- Pin 4: Screen Grid (G2)
- Pin 5: Control Grid (G1)
- Pin 6: Not Connected
- Pin 7: Heater
Electrical Parameters:
- Plate Voltage (max): 250V
- Screen Voltage (max): 250V
- Plate Dissipation (max): 7.5W
- Screen Dissipation (max): 2W
- Cathode Current (max): 45mA
- Transconductance: Approximately 4,100 µmhos (4.1 mA/V)
- Amplification Factor: 62.5 (approximate)
Typical Operating Conditions (Class A1 Single-Ended):
- Plate Voltage: 250V
- Screen Voltage: 250V
- Grid Voltage: -12.5V
- Plate Current: 45mA
- Screen Current: 4.5mA
- Plate Resistance: 52,000 ohms
- Load Resistance: 5,000 ohms
- Power Output: 4.5W
3. Applications and Usage
The 6AQ5 found widespread use in various electronic applications due to its compact size and good power handling capabilities:
Audio Applications:
- Single-Ended Amplifiers: Used in budget hi-fi and radio amplifier output stages
- Push-Pull Configurations: Capable of delivering 10-12W in push-pull pairs
- Guitar Amplifiers: Found in several vintage practice and small combo amplifiers
- Portable Phonographs: Common in record players where modest power was required
Television and Radio:
- TV Audio Output: Extensively used in television audio sections
- AM/FM Radios: Popular in table radios of the 1950s and 1960s
- Car Radios: Used in some automobile radio designs
Other Applications:
- Public Address Systems: Small PA amplifiers
- Intercoms: Used in office and industrial intercom systems
- Test Equipment: Signal generators and other electronic test equipment
The Brimar 6AQ5 variant mentioned ("Looks NOS but in wrong box") suggests a New Old Stock tube that may have been repackaged or misidentified. Brimar was a respected British manufacturer known for quality tubes, and their 6AQ5 variants were used in various British audio equipment.
4. Equivalent or Substitute Types
The 6AQ5 has several direct equivalents and near-equivalents that can be used as substitutes in most applications:
Direct Equivalents:
- 6005: Industrial version of the 6AQ5
- EL90: European designation for the same tube
- CV850: British military designation
- N727: Another military designation
Near Equivalents (may require minor circuit adjustments):
- 6AQ5A: Improved version with slightly higher ratings
- 6HG5: Similar characteristics but not a direct replacement
- 6BQ5/EL84: Higher power output but can substitute in some applications with bias adjustments
- 6V6GT: Larger octal-based tube that the 6AQ5 was designed to replace (not directly interchangeable due to different pin configurations)
12-Volt Heater Equivalents:
- 12AQ5: Identical to 6AQ5 but with 12.6V heater (0.225A)
5. Notable Characteristics
The 6AQ5 has several distinctive characteristics that made it popular during its era and continue to make it sought after for certain applications today:
Sonic Character:
- Warm Midrange: Known for a pleasant, warm midrange response in audio applications
- Sweet Treble: Smoother high-frequency response compared to some later solid-state designs
- Compression Characteristics: Gradual and musical clipping behavior when overdriven
Technical Advantages:
- Compact Size: Significantly smaller than octal equivalents like the 6V6
- Efficiency: Good power output for its size and power consumption
- Reliability: Generally robust and long-lasting when operated within specifications
- Low Microphonics: Relatively resistant to mechanical vibration compared to some other tube types
Modern Relevance:
- DIY Audio: Still used in hobbyist tube amplifier projects
- Vintage Equipment Restoration: Essential for authentic restoration of period electronics
- Boutique Guitar Amplifiers: Used in some modern low-wattage guitar amplifier designs
The 6AQ5's balance of compact size, reasonable power output, and pleasant sonic characteristics ensured its widespread adoption during the tube era. While no longer in production by major manufacturers, new old stock (NOS) tubes like the mentioned Brimar example remain available through specialty suppliers and are valued by audio enthusiasts and vintage equipment restorers.
For those seeking the characteristic sound of a beam power pentode in a compact package, the 6AQ5 remains a practical and historically significant choice that bridges the gap between the earlier octal tubes and later miniature designs.
