1. Introduction and History
The 6BW6 is a beam power tetrode vacuum tube that was developed in the late 1940s and became widely used during the 1950s and 1960s. It was designed primarily for audio applications and found extensive use in radio receivers, audio amplifiers, and television sets of that era. The tube was manufactured by several companies including Mullard, STC (Standard Telephones and Cables), and various American manufacturers.
The 6BW6 is the American designation, while in Europe it was often known by its CV (Common Valve) military designation CV2136. The tube was part of the post-World War II generation of vacuum tubes that offered improved performance in more compact envelopes. The STC-manufactured CV2136/6BW6 became particularly well-regarded among audio enthusiasts for its reliable performance and sound characteristics.
2. Technical Specifications and Design
The 6BW6 is a beam power tetrode with the following key specifications:
- Physical Characteristics:
- Envelope: Glass, miniature
- Base: 9-pin miniature (B9A)
- Height: Approximately 2.25 inches (57mm)
- Diameter: Approximately 0.75 inches (19mm)
- Electrical Parameters:
- Heater Voltage: 6.3V
- Heater Current: 0.75A
- Maximum Plate Voltage: 300V
- Maximum Screen Voltage: 300V
- Maximum Plate Dissipation: 12W
- Maximum Screen Dissipation: 2.5W
- Transconductance: Approximately 7.5 mA/V
- Typical Operating Conditions (Class A1 Amplifier):
- Plate Voltage: 250V
- Screen Voltage: 250V
- Grid Voltage: -12.5V
- Plate Current: 45mA
- Screen Current: 4.5mA
- Plate Resistance: Approximately 22kΩ
- Load Resistance: 5kΩ
- Power Output: Approximately 4.5W
- Pin Configuration:
- Pin 1: No connection
- Pin 2: Heater
- Pin 3: Plate (Anode)
- Pin 4: Screen Grid (Grid 2)
- Pin 5: No connection
- Pin 6: No connection
- Pin 7: Heater
- Pin 8: Cathode and Grid 3
- Pin 9: Control Grid (Grid 1)
The internal structure of the 6BW6 features beam-forming plates that direct the electron flow from the cathode to the anode in concentrated beams, enhancing efficiency and power handling capability. This design allows for higher power output with relatively low distortion compared to pentodes of similar size.
3. Applications and Usage
The 6BW6 was employed in a variety of electronic equipment, with its primary applications including:
- Audio Amplifiers: Commonly used as the output tube in hi-fi amplifiers and radio receivers, particularly in British designs. It could deliver approximately 4.5 watts in single-ended configurations and up to 10-12 watts in push-pull arrangements.
- Television Sets: Used in the audio output stages of television receivers.
- Radio Receivers: Employed in the audio output stages of AM/FM radio sets, where its moderate power output was sufficient for home listening.
- Public Address Systems: Used in smaller PA amplifiers where moderate power was required.
- Guitar Amplifiers: Although not as common as the EL84/6BQ5, some guitar amplifier manufacturers used the 6BW6 in smaller practice amplifiers.
In audio applications, the 6BW6 was often preferred for its warm sound character and ability to provide reasonable power output from relatively low plate voltages, making it suitable for compact amplifier designs. The tube was typically biased for Class A operation in high-fidelity applications, while push-pull Class AB configurations were used where more power was required.
4. Equivalent or Substitute Types
The 6BW6 has several equivalents and near-equivalents that can be used as substitutes in most circuits:
- Direct Equivalents:
- CV2136 (UK military designation)
- N727 (European designation)
- EL91 (European designation)
- Near Equivalents (may require minor circuit adjustments):
- 6AQ5/EL90 (slightly different characteristics but often usable)
- 6V6GT (higher power rating, may require bias adjustment)
- 6F6G (older equivalent with octal base)
- 6AM5 (similar characteristics)
When substituting tubes, it's important to consider not only the electrical characteristics but also the physical dimensions and pin configuration. While the tubes listed as direct equivalents can generally be swapped without circuit modifications, those listed as near equivalents may require adjustments to bias voltages or other circuit parameters for optimal performance and tube longevity.
5. Notable Characteristics
The 6BW6 possesses several characteristics that made it popular during its heyday and continue to make it sought after by audio enthusiasts and vintage equipment restorers:
- Audio Quality: The 6BW6 is known for producing a warm, rich sound with a pleasant harmonic character when operated in Class A, making it popular for hi-fi applications where sound quality was prioritized over maximum power output.
- Efficiency: As a beam power tetrode, the 6BW6 offers better efficiency than triodes of similar size, allowing it to produce reasonable power output from modest plate voltages.
- Reliability: Particularly the STC-manufactured CV2136 variants were known for their durability and long service life when operated within specifications.
- Low Microphonics: Good specimens of the 6BW6 exhibit relatively low microphonic tendencies compared to some other output tubes, making them suitable for sensitive audio applications.
- Compact Size: The miniature envelope allowed for more compact equipment designs compared to the larger octal-based tubes that preceded it.
- Moderate Power Output: While not a high-power tube, its approximately 4-5 watts of output in single-ended configuration was adequate for many home audio applications of the era.
The 6BW6/CV2136 manufactured by STC (as mentioned in the related product information) is particularly sought after by collectors and audio enthusiasts for restoration of vintage British equipment. NOS (New Old Stock) examples, which are unused tubes from original production runs, command premium prices due to their scarcity and the perceived superior quality of vintage manufacturing processes.
In modern times, while no longer in production, the 6BW6 remains popular in vintage audio equipment restoration and among audiophiles who appreciate the distinctive sound characteristics of valve amplification. Their relatively modest power output aligns well with the current trend toward lower-power, high-quality audio systems, particularly for near-field listening environments.
