1. Introduction and History
The 6G8G is a multi-function vacuum tube (valve) that was manufactured by AWV (Amalgamated Wireless Valve Company) under the Super Radiotron brand in Australia. This tube was developed during the golden era of vacuum tube technology in the 1930s to early 1940s. The 6G8G belongs to the octal base family of tubes, as indicated by the 'G' suffix in its designation, which refers to the glass envelope with an octal base.
AWV was the Australian counterpart to RCA in the United States, producing Radiotron tubes under license. The 6G8G was particularly popular in Australian-made radio receivers of the era, providing multiple functions in a single envelope to reduce component count and manufacturing costs.
2. Technical Specifications and Design
The 6G8G is a dual-purpose tube combining a diode and a pentode section in a single glass envelope. Here are its key specifications:
Physical Characteristics:
- Base Type: Octal (8-pin)
- Envelope: ST-shaped glass (bulbous)
- Height: Approximately 3.5-4 inches (89-102mm)
- Diameter: Approximately 1.25 inches (32mm) at widest point
Electrical Characteristics:
- Heater Voltage: 6.3V
- Heater Current: 0.3A
- Pentode Section:
- Plate Voltage (max): 250V
- Screen Grid Voltage (max): 100V
- Plate Dissipation: 2.5W
- Transconductance: Approximately 1600-1800 μmhos
- Plate Resistance: Approximately 1 megohm
- Diode Section:
- Maximum DC Output Current: 8-10mA
Pin Configuration:
- Heater
- Plate (Pentode)
- Screen Grid
- Suppressor Grid (connected to cathode internally)
- Diode Plate
- Pentode Cathode and Grid 3
- Control Grid (Pentode)
- Heater
3. Applications and Usage
The 6G8G was primarily designed for use in radio receivers, where its dual-function design made it particularly valuable. Common applications included:
Radio Receivers:
- IF Amplification: The pentode section was typically used as an intermediate frequency (IF) amplifier.
- Detection: The diode section served as the detector for demodulating AM signals.
- AVC (Automatic Volume Control): The diode section could also be used to generate AVC voltage to maintain consistent audio output regardless of signal strength.
Audio Applications:
- First Audio Stage: The pentode section could also function as a first audio amplifier stage following detection.
- Phase Inverter: In some designs, the tube was used as a phase inverter to drive push-pull output stages.
The 6G8G was commonly found in Australian-designed radio sets from the late 1930s through the 1950s, particularly in models made by AWA (Amalgamated Wireless Australasia), Astor, and other local manufacturers.
4. Equivalent or Substitute Types
Several tubes can serve as direct or near equivalents to the 6G8G:
- 6B8G: The closest American equivalent, with very similar characteristics.
- 6B8: Metal version of the 6B8G with identical electrical characteristics.
- 6B8GT: Smaller glass envelope version with the same electrical specifications.
- EBF80/6N8: A later miniature tube with similar functions but requiring a different socket.
- 6G8: Metal version of the 6G8G (if it exists - less common).
When substituting, care should be taken to ensure the pinout is compatible or the circuit is modified accordingly. The 6G8G has some Australian-specific design elements that may differ slightly from its international counterparts.
5. Notable Characteristics
The 6G8G has several characteristics that made it notable during its era:
Design Features:
- Multi-function Capability: By combining a diode and pentode in one envelope, the 6G8G reduced component count and simplified chassis design.
- Remote Cutoff Characteristic: The pentode section typically featured a variable-mu (remote cutoff) design, making it suitable for use in circuits with AVC.
- Australian Production: As a Super Radiotron product from AWV, it represents an important part of Australian electronics manufacturing history.
Performance Characteristics:
- Gain: The pentode section provides good voltage amplification, typically around 100-200 times depending on circuit configuration.
- Stability: Generally stable in operation, though like most pentodes of the era, it requires proper shielding to prevent unwanted oscillation.
- Microphonics: The large glass envelope can make the tube somewhat susceptible to microphonic effects if subjected to mechanical vibration.
Collectibility:
Today, the 6G8G is sought after by vintage radio collectors and restorers, particularly those working on Australian radio equipment. NOS (New Old Stock) examples, as mentioned in the related product information, are especially valuable as they provide original performance characteristics without the degradation seen in used tubes.
The 6G8G's distinctive ST-shaped glass envelope with its characteristic getter flash (silvery deposit) at the top makes it an attractive display piece for tube collectors as well as a functional component for vintage equipment restoration.
