1. Introduction and History
The ECL86 (also marketed as 6GW8 in North America) is a multi-section vacuum tube that combines a triode and a power pentode in a single glass envelope. Introduced in the late 1950s by Philips, it became part of the "Special Quality" (SQ) series developed for European audio applications. The ECL86 was designed during the golden era of vacuum tube technology, just before the widespread adoption of transistors in consumer electronics.
The tube features a noval (9-pin) base and was primarily designed for use in audio frequency applications. Philips Miniwatt, along with other manufacturers like Mullard (a Philips subsidiary), Telefunken, and Siemens, produced significant quantities of these tubes for use in radio receivers, television sets, and audio amplifiers throughout the 1960s and early 1970s.
The ECL86's development reflected the industry trend toward space-efficient designs, combining multiple functions in a single envelope to reduce chassis size and production costs while maintaining excellent performance characteristics.
2. Technical Specifications and Design
Physical Characteristics
- Base type: Noval (B9A) 9-pin
- Envelope: Glass, typically with a tubular shape
- Heater voltage: 6.3V
- Heater current: 0.76A
- Pin configuration:
- Pin 1: Pentode cathode
- Pin 2: Pentode grid 1
- Pin 3: Pentode grid 2
- Pin 4: Pentode plate (anode)
- Pin 5: Triode plate (anode)
- Pin 6: Triode grid
- Pin 7: Triode cathode
- Pin 8: Heater
- Pin 9: Heater
Triode Section Specifications
- Plate voltage (max): 300V
- Plate dissipation: 1.8W
- Transconductance: 3.3 mA/V (3,300 μmhos)
- Amplification factor (μ): 100
- Plate resistance: 30 kΩ
Pentode Section Specifications
- Plate voltage (max): 300V
- Screen voltage (max): 300V
- Plate dissipation: 7W
- Screen dissipation: 1.8W
- Transconductance: 10.8 mA/V (10,800 μmhos)
- Typical operating conditions (Class A):
- Plate voltage: 250V
- Screen voltage: 250V
- Grid bias: -7.3V
- Plate current: 36 mA
- Screen current: 5 mA
- Load resistance: 7 kΩ
- Power output: approximately 4.5W
The ECL86's design incorporates a high-gain triode section optimized for voltage amplification paired with a power pentode capable of delivering several watts of audio output power. The internal construction features separate cathodes for each section but shares a common heater, making efficient use of the available space within the envelope.
3. Applications and Usage
The ECL86/6GW8 found widespread application in various audio equipment during the vacuum tube era:
Audio Amplifiers
The most common application for the ECL86 was in audio amplifiers, particularly in European-designed equipment. Its dual-section design made it ideal for compact amplifier circuits where the triode section served as a voltage amplifier or phase inverter, while the pentode section functioned as the power output stage. This allowed manufacturers to create complete single-channel amplifiers using just one tube, or stereo amplifiers with two ECL86s.
Radio Receivers
In radio receivers, the ECL86 was often employed in the audio section, with the triode handling audio preamplification and the pentode driving the loudspeaker. This configuration was particularly popular in European FM/AM table radios and radiograms of the 1960s.
Television Sets
In television receivers, the ECL86 typically served in the audio output stage, processing the sound signal and driving the speaker. Its compact design helped reduce the overall tube count in television chassis.
Musical Instrument Amplifiers
Some European guitar and instrument amplifiers utilized the ECL86, particularly in lower-power practice amplifiers where its 4-5 watts of output power was sufficient.
Typical Circuit Configurations
The most common circuit configuration for the ECL86 was a single-ended Class A amplifier, where the triode section functioned as a voltage amplifier directly coupled to the pentode output stage. In push-pull configurations, a pair of ECL86 tubes could be used, with the triode sections configured as a phase splitter/inverter driving the two pentode sections in push-pull, yielding approximately 8-10 watts of output power.
4. Equivalent or Substitute Types
The ECL86 has several direct equivalents and potential substitutes:
Direct Equivalents
- 6GW8 - The North American designation for the identical tube
- ECL806 - An improved version with slightly higher ratings
Potential Substitutes (with modifications)
- ECL82/6BM8 - Similar dual-section tube but with lower power output capabilities (approximately 2W)
- ECL84/6DX8 - Another triode-pentode combination, but with different characteristics requiring circuit modifications
- ECL85/6GV8 - Similar to ECL86 but with different operating parameters
When considering substitutions, it's important to note that while these tubes may physically fit in the same socket, their electrical characteristics differ, potentially requiring circuit modifications for optimal performance and to prevent damage to components. Direct replacement should only be done with the exact equivalent types (ECL86 or 6GW8).
5. Notable Characteristics
The ECL86/6GW8 exhibits several characteristics that made it popular among designers and continue to make it sought after by audio enthusiasts today:
Sound Quality
The ECL86 is renowned for its warm, detailed sound reproduction. The pentode section provides a rich harmonic content that many audiophiles find pleasing, especially for vocal and instrumental reproduction. When properly implemented in a well-designed circuit, it delivers a sound signature that many describe as natural and engaging.
Efficiency and Compact Design
By combining two active sections in one envelope, the ECL86 allowed for space-efficient amplifier designs. This was particularly valuable in an era when component size and cost were significant considerations. A complete amplifier channel could be built using just one ECL86, a rectifier tube, and a handful of passive components.
Reliability
When operated within its specifications, the ECL86 exhibits excellent reliability and longevity. Philips Miniwatt versions, in particular, are known for their robust construction and consistent performance characteristics. Many original tubes from the 1960s still function perfectly in vintage equipment today.
Collectibility
As with many vacuum tubes from this era, the ECL86 has become collectible among audio enthusiasts and vintage equipment restorers. NOS (New Old Stock) examples, particularly those from prestigious manufacturers like Philips Miniwatt, Mullard, and Telefunken, command premium prices in the current market due to their reputation for quality and the diminishing supply.
Design Considerations
When designing circuits around the ECL86, engineers needed to account for its relatively high heater current (0.76A) compared to some other audio tubes. Additionally, the pentode section's screen grid (G2) requires proper voltage regulation to ensure optimal performance and longevity. The tube also benefits from a well-designed output transformer matched to its characteristics.
Modern Usage
Today, the ECL86 continues to be used in boutique audio amplifier designs and by DIY enthusiasts. Its moderate power output makes it suitable for nearfield listening applications and headphone amplifiers. Modern implementations often feature improved power supply designs and component quality compared to vintage circuits, extracting even better performance from this classic tube.
The enduring popularity of the ECL86 in audio applications speaks to its excellent design and the pleasing sonic characteristics it imparts when properly implemented in high-quality audio circuits.
