1. Introduction and History
The 6U8-A is a miniature compound vacuum tube containing a sharp-cutoff pentode and a medium-mu triode within a single glass envelope. Each section has its own separate cathode and is electrically independent, making the tube exceptionally versatile for circuit designers. Originally developed for use as a combined triode oscillator and pentode mixer in television and FM radio receivers, the 6U8-A quickly found its way into a wide variety of general-purpose applications.
The tube was manufactured by General Electric (documented in datasheet ET-T1270A, dated March 1960), as well as by numerous other manufacturers worldwide including RCA, Sylvania, Mullard (as the ECF82), Toshiba (Tokyo Shibaura Electric), and various Eastern European factories. The 5U8 is identical to the 6U8-A except for heater ratings (4.7V heater instead of 6.3V), making it suitable for series-string heater configurations common in AC/DC television receivers.
The 6U8-A represents an improvement over the original 6U8, with tighter specifications and improved reliability. It became one of the most widely produced triode-pentode combination tubes of the miniature 9-pin era, and its European equivalent, the ECF82, enjoyed equally widespread use across the continent.
2. Technical Specifications and Design
General Electrical Data
| Parameter | 6U8-A | 5U8 |
|---|---|---|
| Cathode Type | Coated Unipotential | |
| Heater Voltage (AC or DC) | 6.3 V | 4.7 V |
| Heater Current | 0.45 A ±6% | 0.6 A ±6% |
| Heater Warm-up Time | 11 seconds | 11 seconds |
Maximum Ratings
| Parameter | Pentode Section | Triode Section |
|---|---|---|
| Plate Voltage | 330 V | 330 V |
| Screen Supply Voltage | 330 V | — |
| Positive DC Grid-Number 1 Voltage | 0 V | 0 V |
| Plate Dissipation | 3.0 W | 2.5 W |
| Screen Dissipation | 0.55 W | — |
| Heater-Cathode Voltage (DC positive, component) | 100 V | 100 V |
| Heater-Cathode Voltage (DC positive, total DC and peak) | 200 V | 200 V |
| Heater-Cathode Voltage (DC negative, total DC and peak) | 200 V | 200 V |
| Grid-Number 1 Circuit Resistance (Fixed Bias) | 0.5 MΩ | — |
| Grid-Number 1 Circuit Resistance (Cathode Bias) | 1.0 MΩ | — |
Characteristics and Typical Operation — Class A1 Amplifier
Pentode Section
| Parameter | Condition 1 | Condition 2 |
|---|---|---|
| Plate Voltage | 100 V | 125 V |
| Screen Voltage | 70 V | 110 V |
| Grid-Number 1 Voltage | 0 V | −1.0 V |
| Plate Resistance (approx.) | — | 200,000 Ω |
| Transconductance (gm) | 5,500 µmhos | 5,000 µmhos |
| Plate Current | — | 9.5 mA |
| Screen Current | — | 3.5 mA |
| Grid-Number 1 Voltage (approx.) at Ib = 20 µA | — | −8 V |
Triode Section
| Parameter | Value |
|---|---|
| Plate Voltage | 125 V |
| Grid Voltage | −1.0 V |
| Amplification Factor (µ) | 40 |
| Plate Resistance (approx.) | 5,300 Ω |
| Transconductance (gm) | 7,500 µmhos |
| Plate Current | 13.5 mA |
| Grid Voltage (approx.) at Ib = 20 µA | −9 V |
Direct Interelectrode Capacitances
| Parameter | With Shield | Without Shield |
|---|---|---|
| Pentode Section | ||
| Grid-Number 1 to Plate (Pg1 to Pp), max | 0.007 µµF | 0.015 µµF |
| Input: Pg1 to (h+Pk+Pg2+Pg3+i.s.) | 5.0 µµF | 5.0 µµF |
| Output: Pp to (h+Pk+Pg2+Pg3+i.s.) | 3.5 µµF | 2.6 µµF |
| Heater to Cathode: h to (Pk+Pg3+i.s.) | 3.0 µµF | 3.0 µµF |
| Triode Section | ||
| Grid to Plate (Tg to Tp) | 1.8 µµF | 1.8 µµF |
| Input: Tg to (Tk+h+Pk+Pg3+i.s.) | 2.8 µµF | 2.8 µµF |
| Output: Tp to (Tk+h+Pk+Pg3+i.s.) | 2.0 µµF | 1.5 µµF |
| Heater to Cathode: h to Tk | 3.0 µµF | 3.0 µµF |
| Cross-Section | ||
| Pentode Grid-Number 1 to Triode Plate (max) | 0.2 µµF | 0.2 µµF |
| Pentode Plate to Triode Plate (max) | 0.02 µµF | 0.1 µµF |
Physical Construction
- Envelope: T-6½, Glass
- Base: E9-1, Small Button 9-Pin (Noval / B9A)
- Mounting Position: Any
- Basing Designation: EIA 9AE
Pin Connections (Bottom View)
| Pin | Connection |
|---|---|
| 1 | Triode Plate |
| 2 | Pentode Grid Number 1 |
| 3 | Pentode Grid Number 2 (Screen) |
| 4 | Heater |
| 5 | Heater |
| 6 | Pentode Plate |
| 7 | Pentode Cathode, Grid Number 3, and Internal Shield |
| 8 | Triode Cathode |
| 9 | Triode Grid |
3. Applications and Usage
The 6U8-A was primarily designed as a combined oscillator-mixer tube for television and FM radio receivers. In this classic application, the triode section functions as a local oscillator while the pentode section serves as the mixer (frequency converter). The electrical independence of the two sections — each having its own cathode — provides excellent isolation between the oscillator and mixer functions, reducing pulling effects and improving frequency stability.
Beyond its original television and FM receiver application, the 6U8-A found use in:
- VHF Tuners: The tube was a staple in television front-end tuners throughout the 1950s and 1960s, where its low interelectrode capacitances (particularly the pentode's grid-to-plate capacitance of just 0.007 µµF with shield) made it well-suited for high-frequency operation.
- IF Amplifiers: The pentode section's sharp cutoff characteristic and high transconductance (5,000–5,500 µmhos) made it useful in intermediate frequency amplifier stages.
- General-Purpose Amplification: Both sections could be used independently as voltage amplifiers in various signal processing circuits.
- Instrument Circuits: Test equipment and laboratory instruments employed the 6U8-A in oscillator, amplifier, and buffer stages.
- Audio Preamplifiers and Phono Stages: The combination of a medium-mu triode and a sharp-cutoff pentode in one envelope made the 6U8-A attractive for compact audio preamplifier designs, particularly phono stages where the pentode could provide high gain for the RIAA equalization network.
The pentode section's high plate resistance of approximately 200,000 Ω and the triode section's relatively low plate resistance of 5,300 Ω give the designer considerable flexibility. The triode section, with its amplification factor of 40 and transconductance of 7,500 µmhos, is a capable medium-mu triode comparable in performance to half of a 12AT7, while the pentode section offers the high gain and high output impedance characteristic of a well-designed sharp-cutoff pentode.
4. Sound Characteristics
The 6U8-A occupies an interesting niche in the sonic landscape of vacuum tubes. Because it was designed primarily for RF and television service rather than audio, its sound character has been explored mainly by experimenters and DIY audio enthusiasts rather than mainstream hi-fi manufacturers. Nevertheless, those who have used it in audio circuits report distinctive qualities:
Triode Section
The triode section, with its medium amplification factor of 40 and relatively low plate resistance of 5,300 Ω, produces a sound that sits between the warmth of a 12AU7 (µ=17) and the brightness of a 12AX7 (µ=100). Users describe it as having a clear, articulate midrange with good transient response. The moderate mu provides enough gain for most preamplifier stages without the tendency toward harshness that can accompany very high-gain triodes. The triode section's 7,500 µmhos transconductance gives it a lively, dynamic quality — it responds quickly to signal changes, lending a sense of immediacy and presence to the sound.
Pentode Section
The pentode section, when used in audio applications, offers a different flavor. With its high plate resistance (200,000 Ω) and sharp-cutoff characteristic, it can deliver very high voltage gain in a single stage. In pentode mode, the sound tends to be more detailed and extended in the upper frequencies, with a slightly leaner tonal balance compared to the triode section. Some listeners describe a certain "sparkle" or "air" in the treble that can be very appealing in phono stages and line-level preamplifiers. When triode-strapped (screen tied to plate), the pentode section takes on a warmer, more rounded character with reduced gain but lower distortion.
Overall Character
The 6U8-A is generally described as a clean, neutral-sounding tube without the pronounced coloration of some audio-specific types. It lacks the lush, syrupy warmth of a 12AX7 or the bold, forward presentation of a 6DJ8, instead offering a balanced, transparent sound that lets the character of the source material come through. The separate cathodes for each section help minimize crosstalk and interaction between stages when both sections are used in the same circuit, contributing to a cleaner overall presentation. Because these tubes were produced in enormous quantities for the television industry, they tend to be very consistent in their sonic characteristics from sample to sample — a practical advantage for builders seeking repeatable results.
5. Equivalent and Substitute Types
Direct / Identical Substitutes
| Type | Notes |
|---|---|
| ECF82 | European designation for the same tube. Identical pinout and specifications. Manufactured by Mullard, Philips, Valvo, Siemens, Telefunken, and many others. Direct drop-in replacement. |
| CV9858 | British military (CV) designation. Identical to the 6U8-A. Direct drop-in replacement. |
Different Rating Substitutes (NOT Direct Drop-In)
| Type | Notes |
|---|---|
| 6U8 | The original version before the "A" suffix improvement. Similar but may have slightly different specifications and tolerances. Generally usable in 6U8-A sockets but may not meet the tighter 6U8-A specifications. |
| 6678 | A premium/ruggedized version with different ratings. Not a direct drop-in; verify specifications before substituting. |
| CV5065 | British military designation with different ratings from the 6U8-A. Not a direct drop-in replacement. |
| 5U8 | Identical to the 6U8-A except for heater voltage (4.7V vs. 6.3V) and heater current (0.6A vs. 0.45A). NOT interchangeable without heater circuit modification. |
Important Note: While the ECF82 and CV9858 are true drop-in replacements for the 6U8-A, the 6U8, 6678, and CV5065 have different ratings and should be used with caution. Always verify the specific requirements of your circuit before substituting.
6. Notable Characteristics
- Independent Cathodes: Unlike many combination tubes where sections share a common cathode, the 6U8-A features completely separate cathodes for the triode (pin 8) and pentode (pin 7) sections. This electrical independence allows each section to be biased independently and minimizes interaction between the two halves.
- Internal Shield: The tube includes an internal shield connected to pin 7 (pentode cathode), which significantly reduces the grid-to-plate capacitance of the pentode section from 0.015 µµF to just 0.007 µµF. An external shield (EIA 315) can be used for further isolation.
- Sharp-Cutoff Pentode: The pentode section features a sharp-cutoff (remote cutoff is NOT present) characteristic, with cutoff occurring at approximately −8V on grid number 1 (at Ib = 20 µA). This makes it suitable for applications requiring clean signal handling without the variable-mu compression effects of remote-cutoff pentodes.
- Low Interelectrode Capacitances: The cross-section capacitance between the pentode plate and triode plate is extremely low (0.02 µµF with shield), enabling the tube to be used effectively in circuits where the two sections must operate at different frequencies without mutual interference.
- Versatile Mounting: The tube can be mounted in any position, giving designers maximum flexibility in chassis layout.
- Heater Warm-up Time: The 11-second warm-up time is defined as the time for the heater voltage to reach 80% of rated value when applying 4 times the rated heater voltage through a series resistance equal to 3 times the rated heater voltage divided by the rated heater current.
- Screen Rating Chart: The GE datasheet provides a detailed screen rating chart showing the permissible area of operation as a function of screen voltage and screen dissipation, which is critical for ensuring reliable operation in pentode-mode circuits. The maximum screen dissipation of 0.55 W must be carefully observed.
- Abundant Supply: Due to massive production volumes for the television industry, NOS (New Old Stock) 6U8-A and ECF82 tubes remain readily available from manufacturers including GE, RCA, Sylvania, Toshiba, Mullard, Philips, Siemens, Telefunken, and many others. This abundance keeps prices reasonable compared to more sought-after audio tubes.
7. Usage in the Audio Community
The 6U8-A has carved out a modest but dedicated following in the audio community, particularly among DIY builders and experimenters who appreciate its versatility, low cost, and ready availability. While it was never a mainstream audio tube in the way that the 12AX7 or EL34 became, it offers genuine advantages in certain applications.
Phono Preamplifiers
One of the most popular audio applications for the 6U8-A is in phono preamplifier circuits. The combination of a high-gain pentode section and a medium-mu triode section in a single envelope is ideal for RIAA phono stages. A common topology uses the pentode section as the first gain stage (taking advantage of its high transconductance of 5,000 µmhos and high plate resistance for effective RIAA equalization), followed by the triode section as a second gain stage or cathode follower output. This approach yields a complete phono stage in a single tube, minimizing parts count and keeping the signal path short.
Line-Level Preamplifiers
The triode section alone, with its amplification factor of 40 and transconductance of 7,500 µmhos, makes an excellent line-level preamplifier stage. Its characteristics fall in a useful middle ground — enough gain for most line-level applications without the excess gain (and associated noise) of a 12AX7. Some builders use both sections in cascade for a high-gain line preamplifier, with the pentode providing the first stage of amplification and the triode serving as a low-impedance driver.
Headphone Amplifiers
The triode section's relatively low plate resistance of 5,300 Ω makes it a reasonable candidate for driving headphones through an output transformer or as a driver stage in a headphone amplifier. The moderate output impedance provides good damping characteristics for dynamic headphones.
Guitar Amplifiers
While not a traditional guitar amp tube, some boutique builders and modifiers have experimented with the 6U8-A in guitar amplifier preamp stages. The pentode section can provide a different flavor of overdrive compared to the ubiquitous 12AX7, with a sharper, more aggressive clipping characteristic due to its sharp-cutoff design. The triode section can serve as a clean gain stage or phase inverter.
Hybrid Amplifiers
The 6U8-A appears in some hybrid (tube/solid-state) amplifier designs where the tube serves as a voltage amplifier or buffer stage ahead of a solid-state power amplifier. The separate cathodes allow creative circuit topologies where the pentode and triode sections can be used in different parts of the signal chain without ground-loop concerns.
Manufacturer Preferences
Among audio enthusiasts, certain manufacturers' versions of the 6U8-A/ECF82 are preferred:
- Mullard ECF82: Often considered the most "musical" sounding, with a warm, rich midrange characteristic of Mullard tubes.
- Telefunken ECF82: Prized for clarity and detail, with the characteristic Telefunken precision and low microphonics.
- Siemens ECF82: Known for a neutral, transparent sound with excellent dynamics.
- GE 6U8-A: Widely available American-made tubes with consistent quality and a clean, balanced sound.
- Toshiba 6U8-A: Japanese-made NOS tubes that are readily available and offer good performance at reasonable prices.
Practical Considerations for Audio Use
Builders should be aware of several practical considerations when using the 6U8-A in audio circuits:
- The pentode section's maximum screen dissipation of only 0.55 W requires careful attention to screen supply design. A dropping resistor from the B+ supply is essential.
- The pentode section's plate dissipation limit of 3.0 W and the triode section's limit of 2.5 W are adequate for small-signal applications but must be respected.
- The internal shield (connected to pin 7) should be properly grounded for minimum noise and hum in audio circuits.
- An external tube shield (EIA 315) is recommended for phono stage applications to minimize hum pickup, particularly for the pentode section where the shield reduces grid-to-plate capacitance from 0.015 µµF to 0.007 µµF.
- The separate cathodes mean that each section can be optimally biased for its specific role in the circuit, a significant advantage over shared-cathode combination tubes.
- Due to the high availability of NOS stock, matched pairs or selected tubes can be obtained at very reasonable cost compared to premium audio tubes.
The 6U8-A remains an underappreciated gem in the audio tube world — a versatile, well-engineered combination tube that offers genuine sonic merit at a fraction of the cost of more fashionable types. For the DIY builder willing to design around its characteristics rather than simply dropping it into an existing 12AX7 circuit, the 6U8-A can deliver surprisingly refined audio performance.
