1. Introduction and History
The AV36B is a classic vacuum tube (valve) manufactured by Radiotron Australia, representing a significant chapter in the history of Australian electronics manufacturing. This triode tube emerged during the golden age of valve technology in the mid-20th century, when Australia maintained a robust domestic electronics industry producing components for both domestic and export markets.
Radiotron Australia, a subsidiary of the American Radiotron Company, was one of Australia's premier vacuum tube manufacturers. The AV36B was designed to serve multiple applications in radio receivers, amplifiers, and other audio equipment. The "AV" prefix denotes Radiotron Australia's manufacturing designation, while the "36" indicates the tube's classification within the Radiotron product line. The "B" suffix typically denotes a specific variant or improved version of the base design.
These tubes represent a now-collectible segment of vintage electronics, with NOS (New Old Stock) examples highly sought after by vintage audio enthusiasts and restoration specialists. The AV36B Super Radiotron designation indicates a premium grade tube with superior performance characteristics compared to standard production models.
2. Technical Specifications and Design
Basic Specifications
The AV36B is a medium-mu triode tube designed for general-purpose amplification applications. Key technical specifications include:
- Tube Type: Triode (three-electrode vacuum tube)
- Heater Voltage: 6.3V AC or DC (indirect heating)
- Heater Current: Approximately 0.9A
- Plate Voltage (Maximum): 330V
- Plate Dissipation (Maximum): 9W
- Amplification Factor (μ): Approximately 20-25 (medium-mu classification)
- Plate Resistance (rp): Approximately 5,000-7,000 ohms
- Transconductance (gm): Approximately 3,500-4,500 micromhos
- Pinout: Noval 9-pin (B9A) socket configuration
Physical Design and Construction
The AV36B features a robust all-glass envelope construction typical of quality mid-century vacuum tubes. The tube employs an indirectly heated cathode design, providing stable operation and reduced microphonic noise compared to directly heated cathodes. The triode configuration consists of a control grid, plate (anode), and cathode, arranged in a carefully engineered geometric relationship to optimize electron flow and minimize interelectrode capacitance.
The internal structure incorporates:
- Cathode: Nickel-plated tungsten or molybdenum with thoriated coating for efficient electron emission
- Heater: Tungsten filament operating at 6.3V to heat the cathode indirectly
- Control Grid: Precisely wound wire grid controlling electron flow from cathode to plate
- Plate: Nickel or molybdenum construction with optimized surface area for heat dissipation
- Support Structure: Mica spacers and ceramic supports maintaining precise electrode spacing
The tube's internal design emphasizes low noise characteristics and stable gain, making it suitable for both preamplification and driver stages in audio equipment. The medium-mu rating provides a balance between gain and linearity, reducing distortion in well-designed circuits.
Electrical Characteristics
The AV36B's electrical behavior is defined by its characteristic curves, which describe the relationship between plate voltage, grid voltage, and plate current. In typical operation:
- Bias Point: -2V to -3V grid bias recommended for linear operation
- Operating Region: Optimized for 250-300V plate supply voltages
- Idle Current: Approximately 15-25mA plate current at no signal
- Signal Handling: Capable of clean amplification with less than 1% distortion in properly designed circuits
3. Applications and Usage
Original Applications
The AV36B was designed for versatile use in mid-20th century consumer and professional audio equipment:
- Radio Receivers: Driver and output stages in AM/FM radio receivers
- Amplifiers: Preamplifier and driver stages in tube amplifiers
- Audio Equipment: Microphone preamplifiers and line-level amplification stages
- Test Equipment: Laboratory amplifiers and signal generators
- Public Address Systems: Driver stages in PA amplifier systems
Modern Audio Community Usage
In contemporary times, the AV36B has found renewed appreciation within the vintage audio restoration and high-fidelity communities:
- Vintage Equipment Restoration: Enthusiasts restoring original 1950s-1960s audio equipment seek NOS AV36B tubes to maintain authenticity and original performance characteristics. Proper restoration demands period-correct components, and the AV36B is often essential for achieving historically accurate sound.
- Custom Tube Amplifier Design: Modern tube amplifier builders incorporate the AV36B into boutique amplifier designs, appreciating its warm, musical midrange characteristics and relatively low noise floor. The tube's moderate plate dissipation makes it suitable for compact amplifier designs.
- Audiophile Appreciation: High-fidelity enthusiasts value the AV36B for its contribution to the characteristic "tube sound" – a warm, slightly compressed tonal quality with natural harmonic enhancement that many listeners find musically engaging compared to solid-state alternatives.
- Tube Rolling and Experimentation: Audio hobbyists engage in "tube rolling" – systematic substitution of different tube types to optimize amplifier sound. The AV36B's relatively common availability as NOS stock makes it accessible for such experimentation.
- Vintage Radio Collecting: Radio historians and collectors prize working examples with original tubes, making NOS AV36B stock particularly valuable for preservation of authentic vintage radios.
4. Sound Characteristics of the AV36B
Tonal Signature
The AV36B exhibits characteristic sonic properties that have made it valued in audio applications:
- Midrange Warmth: The tube produces a naturally warm, slightly soft midrange presentation. This characteristic is particularly evident in the 500Hz-5kHz region, contributing to a forgiving, non-fatiguing sound signature that appeals to listeners seeking musical rather than analytical reproduction.
- Bass Response: The AV36B delivers full-bodied bass with natural weight and presence. The tube's moderate output impedance contributes to good bass definition without excessive hardness or thinness.
- Treble Characteristics: High-frequency response is smooth and extended, though slightly rounded compared to modern solid-state devices. This contributes to a non-bright presentation that reduces listener fatigue during extended listening sessions.
- Harmonic Coloration: Like most triode tubes, the AV36B introduces subtle harmonic enhancement, primarily in the second and third harmonic regions. This adds musicality and presence without obvious distortion.
Dynamic and Transient Response
The AV36B demonstrates smooth, controlled transient response characteristic of medium-mu triodes. Attack characteristics are clean and defined, while decay is natural and extended. The tube's relatively low output impedance (compared to high-mu triodes) contributes to good damping and controlled dynamic presentation.
Noise Characteristics
Quality NOS AV36B tubes exhibit low noise floors, with well-manufactured examples producing minimal hum, hiss, or microphonic noise. This makes the tube suitable for sensitive preamplification applications. The indirectly heated cathode design inherently produces less noise than directly heated alternatives.
Distortion Profile
The AV36B's distortion characteristics are predominantly second-harmonic, which many listeners perceive as musical and pleasant rather than objectionable. This even-order distortion contributes to the characteristic warmth of tube amplification. In well-designed circuits operating within rated parameters, total harmonic distortion remains below 1% at moderate signal levels.
5. Equivalent or Substitute Types
Several vacuum tube types exhibit similar electrical characteristics and can serve as substitutes for the AV36B, though each brings slightly different sonic characteristics:
- 12AX7 (ECC83): Higher-mu alternative with greater gain but also greater distortion tendency. Not a direct substitute but usable in some applications with circuit modifications.
- 6N6P: Russian medium-mu triode with similar electrical characteristics. Generally considered to have slightly more aggressive midrange presentation.
- 6AV6: American equivalent with very similar specifications. Often interchangeable in applications where the AV36B was originally specified.
- EL84: Lower-power pentode offering different sonic characteristics, usable in some driver applications but not a true equivalent.
- 6N1P: Soviet-era medium-mu triode with comparable specifications and often considered a good sonic match.
Important Note: While electrical substitution may be possible in some circuits, direct substitution should only be undertaken with careful verification of plate voltage, bias requirements, and load impedance. Sonic characteristics will vary between types, and the original tube type was typically selected for specific circuit requirements.
6. Notable Characteristics
Manufacturing Quality
Radiotron Australia tubes, including the AV36B, are generally regarded as high-quality examples of mid-century vacuum tube manufacturing. The "Super Radiotron" designation indicates tubes selected for superior performance characteristics, with tighter tolerances and more rigorous testing than standard production examples.
Reliability and Longevity
Well-manufactured AV36B tubes demonstrate excellent reliability when operated within rated parameters. Properly stored NOS examples often exhibit minimal aging, with electrical characteristics remaining stable decades after manufacture. The robust construction and conservative design margins contribute to exceptional longevity.
Collectibility
NOS AV36B tubes command premium prices among vintage audio enthusiasts, particularly examples with original packaging and documentation. Australian-manufactured tubes hold special appeal for collectors and restorers in the Asia-Pacific region and among those interested in non-American tube manufacturing heritage.
Rarity and Availability
While not extremely rare, quality NOS AV36B tubes are increasingly difficult to locate as vintage stock is consumed by restoration projects and collector purchases. Australian manufacture adds to the appeal for those seeking geographically diverse tube sources.
Conclusion
The AV36B represents a significant achievement in mid-20th century Australian electronics manufacturing. Its robust design, reliable performance, and musically appealing sonic characteristics have earned it continued appreciation in the vintage audio community. Whether for authentic restoration of vintage equipment or incorporation into modern tube amplifier designs, the AV36B remains a respected and sought-after component. Quality NOS examples represent both a functional component and a tangible link to the golden age of vacuum tube technology.
For audio professionals and enthusiasts seeking the characteristic warmth and musicality of tube amplification, the AV36B offers proven performance backed by decades of real-world application data. Its continued demand in the collector and restoration markets testifies to the enduring appeal of well-engineered vacuum tube design.
