Introduction and History
The 6EM5 is a miniature beam power tube originally designed by major manufacturers for use as a vertical-deflection amplifier in television receivers featuring picture tubes with diagonal deflection angles of 110 degrees. Introduced during the late 1950s and early 1960s — the golden era of wide-angle CRT television design — the 6EM5 was engineered to handle the demanding pulse waveforms and high peak currents required for vertical scanning circuits in both color and black-and-white television sets.
Manufactured by companies including RCA, Toshiba, Sylvania, and various Japanese and European producers, the 6EM5 was a workhorse of the television industry. Its robust construction, capable of withstanding peak positive-pulse plate voltages of up to 2,200 volts, made it ideally suited for the harsh operating environment of deflection circuits where tubes were subjected to large voltage swings and high peak currents.
As the television industry transitioned to solid-state designs in the late 1960s and 1970s, the 6EM5 gradually fell out of mainstream production. However, like many television-era tubes, it has found a second life in the audio community, where its unique characteristics have attracted the attention of adventurous amplifier designers and audiophiles seeking distinctive sonic qualities from unconventional tube types.
The 8EM5 is the series-heater equivalent of the 6EM5, identical in all respects except for heater ratings, designed for series-string television receiver heater chains.
Technical Specifications and Design
General Description
- Type: Beam Power Tube
- Envelope: Miniature glass (T-6 1/2)
- Base: Noval 9-pin (B9A)
- Outline Section: 6G
- Mounting: Vertical or horizontal
Heater Ratings
| Parameter | 6EM5 | 8EM5 | Unit |
|---|---|---|---|
| Heater Voltage (ac/dc) | 6.3 | 8.4 | volts |
| Heater Current | 0.8 | 0.6 | ampere |
| Heater Warm-up Time (Average) | — | 11 | seconds |
Heater-Cathode Voltage
- Peak value: ±200 max volts
- Average value: 100 max volts
Direct Interelectrode Capacitances
| Capacitance | Value | Unit |
|---|---|---|
| Grid No.1 to Plate (Cga) | 0.7 max | pF |
| Grid No.1 to Cathode, Heater, Grid No.2, Grid No.3 (Cgk) | 10 | pF |
| Plate to Cathode, Heater, Grid No.2, and Grid No.3 (Cak) | 5.1 | pF |
Maximum Ratings — Vertical-Deflection Oscillator and Amplifier (525-line, 30-frame system)
| Parameter | Value | Unit |
|---|---|---|
| DC Plate Voltage | 315 | volts |
| Peak Positive-Pulse Plate Voltage (Absolute Maximum) | 2,200 | volts |
| Grid No.2 (Screen-Grid) Voltage | 285 | volts |
| Peak Negative-Pulse Grid No.1 (Control-Grid) Voltage | 250 | volts |
| Peak Cathode Current | 210 | mA |
| Average Cathode Current | 60 | mA |
| Plate Dissipation | 10 | watts |
| Grid No.2 Input | 1.5 | watts |
| Bulb Temperature (At hottest point) | 250 | °C |
Characteristics — Class A1 Amplifier
| Parameter | Condition 1 | Condition 2 | Unit |
|---|---|---|---|
| Plate Voltage | 60 | 250 | volts |
| Grid No.2 Voltage | 250 | 250 | volts |
| Grid No.1 Voltage | 0 | −18 | volts |
| Mu Factor, Grid No.1 to Grid No.2 | — | 8.7 | — |
| Plate Resistance (Approx.) | — | 0.05 | megohm (50,000 Ω) |
| Transconductance | — | 5,100 | µmhos |
| Plate Current | 180• | 40 | mA |
| Grid No.2 Current | 30• | 3 | mA |
| Grid No.1 Voltage (Approx.) for plate current of 0.2 mA | — | −37 | volts |
• These values can be measured by a method involving a recurrent waveform such that the maximum ratings of the tube will not be exceeded.
Typical Application Data — Frame (Vertical) Output
| Parameter | Value | Unit |
|---|---|---|
| Plate Voltage | 250 | volts |
| Grid No.2 Voltage | 250 | volts |
| Grid No.1 Voltage | −18.0 | volts |
| Plate Current | 40.0 | mA |
| Grid No.2 Current | 3.0 | mA |
| Load Resistance (Ra) | 50,000 | ohms |
| Transconductance (gm) | 5.1 | mA/V (5,100 µmhos) |
Maximum Circuit Values
- Grid No.1 Circuit Resistance: 2.2 megohms
Note: Pulse duration must not exceed 15% of a vertical scanning cycle (2.5 milliseconds). Under no circumstances should the absolute peak plate voltage value of 2,200 volts be exceeded.
Pin Configuration (Noval 9-pin, Base 9HN)
| Pin | Connection |
|---|---|
| 1 | Grid No.2 (Screen Grid, G2) |
| 2 | No Connection (NC) |
| 3 | Grid No.1 (Control Grid, G1) |
| 4 | Heater (H) |
| 5 | Heater (H) |
| 6 | Grid No.1 (G1) — see note |
| 7 | Cathode, Grid No.3 (K, G3) |
| 8 | Internal Connection (IC) — do not connect externally |
| 9 | Plate (P) |
Important Note: Pin 8 is an internal connection and must not be connected to any external circuit. This is a critical consideration when designing circuits or selecting sockets for the 6EM5.
Derived Parameters
- Amplification Factor (µ): Approximately 8.7 (Grid No.1 to Grid No.2 mu factor at Vp=250V, Vg2=250V)
- Plate Resistance (rp): Approximately 50,000 ohms (0.05 megohm)
- Transconductance (gm): 5,100 µmhos (5.1 mA/V)
Applications and Usage
Original Television Applications
The 6EM5 was specifically designed for vertical-deflection output service in television receivers employing wide-angle (110-degree diagonal) picture tubes. In this role, the tube served as the power output stage of the vertical scanning circuit, generating the sawtooth current waveform necessary to deflect the electron beam vertically across the CRT screen.
The vertical deflection circuit demanded a tube capable of:
- High peak plate voltage tolerance: During the retrace (flyback) portion of the vertical scan, the plate voltage could spike to extremely high values — hence the 2,200-volt peak positive-pulse plate voltage rating.
- High peak cathode current: The tube needed to deliver up to 210 mA peak cathode current during the active scan period.
- Substantial plate dissipation: At 10 watts maximum, the 6EM5 could handle the continuous power demands of deflection service.
- Reliable operation at elevated temperatures: With a maximum bulb temperature rating of 250°C, the tube was designed for the hot environment inside television chassis.
Vertical-Deflection Oscillator and Amplifier Service
In many television designs, the 6EM5 was paired with an oscillator tube (or a dual-function tube like the 6EM7/6EA7) to form a complete vertical deflection system. The RCA datasheet specifies detailed operating conditions for this service in a 525-line, 30-frame system (the NTSC standard), with the tube operating at DC plate voltages up to 315 volts and screen voltages up to 285 volts.
Audio Amplifier Applications
While never intended for audio service by its original designers, the 6EM5 has been adopted by the DIY audio community and boutique amplifier builders for use in single-ended triode (SET) and pentode amplifier configurations. Its relatively high plate dissipation of 10 watts, combined with good transconductance and reasonable plate resistance, makes it a viable candidate for small to medium power audio output stages. More details on audio applications are provided in the sections below.
Sound Characteristics
The 6EM5, when pressed into audio service, exhibits a distinctive sonic character that has earned it a dedicated following among adventurous audiophiles and DIY amplifier builders. As a beam power tube originally designed for deflection service rather than audio, its sonic signature differs noticeably from purpose-built audio output tubes like the EL84 or 6V6.
Pentode/Beam Power Mode
When operated in its native pentode (beam power) configuration, the 6EM5 delivers a sound that is often described as:
- Dynamic and punchy: The tube's ability to handle high peak currents (up to 210 mA peak cathode current) translates into excellent transient response and dynamic headroom in audio applications. Percussive instruments and plucked strings benefit from this characteristic snap and authority.
- Forward and present midrange: The 6EM5 tends to produce a midrange that is slightly forward and detailed, giving vocals and lead instruments a sense of immediacy and presence. This quality is appreciated by listeners who favor an engaging, "front-row" presentation.
- Extended but slightly grainy treble: In pentode mode, the high-frequency response is well-extended but can exhibit a slight graininess or edge compared to more refined audio pentodes. This characteristic can be tamed with careful circuit design, appropriate feedback, and quality output transformers.
- Solid bass foundation: With its relatively low plate resistance of approximately 50,000 ohms (for a beam power tube) and good current delivery capability, the 6EM5 provides a reasonably tight and controlled bass response, though it may not match the deep bass authority of larger output tubes.
Triode-Connected Mode
Many audio enthusiasts prefer to operate the 6EM5 in triode-connected mode (screen grid tied to plate), which significantly alters its sonic character:
- Smoother and more refined: Triode connection dramatically reduces the plate resistance and lowers distortion, producing a smoother, more liquid sound with less of the pentode "edge."
- Warmer tonal balance: The triode-connected 6EM5 shifts toward a warmer, more harmonically rich presentation with a gentle emphasis on even-order harmonics that many listeners find musically pleasing.
- Reduced power output: The trade-off for improved sonic refinement is a reduction in available output power, typically yielding around 1–2 watts in single-ended triode configuration — sufficient for high-efficiency speakers but inadequate for less sensitive designs.
- Intimate soundstage: SET amplifiers built around the triode-connected 6EM5 tend to produce an intimate, three-dimensional soundstage with good image specificity, particularly in the midrange.
Ultralinear Mode
When operated in ultralinear configuration (screen grid connected to an appropriate tap on the output transformer), the 6EM5 offers a compromise between pentode and triode characteristics — retaining much of the pentode's power and dynamics while gaining some of the triode's smoothness and lower distortion.
General Sonic Observations
Compared to mainstream audio output tubes, the 6EM5 is generally characterized as having a slightly raw, energetic quality that some listeners find exciting and others may consider less polished. It excels with rock, jazz, and acoustic music where its dynamic capabilities and midrange presence are assets. For classical music and large-scale orchestral works, more refined audio tubes may be preferred, though this is ultimately a matter of personal taste and system synergy.
The sonic character can vary significantly between manufacturers — Toshiba-made 6EM5 tubes, for example, are often noted for their clean construction and consistent performance, while other manufacturers may exhibit different tonal shadings.
Equivalent or Substitute Types
Direct Equivalents
| Type | Relationship | Notes |
|---|---|---|
| 8EM5 | Series-heater equivalent | Identical to 6EM5 except for heater ratings: 8.4V at 0.6A. Designed for series-string television heater chains. Directly interchangeable if heater supply is adjusted accordingly. |
Possible Substitutes (with caveats)
The 6EM5 does not have a large number of direct drop-in substitutes due to its specialized design for deflection service. However, the following tubes share some characteristics and may be considered in certain applications:
- 6BQ5/EL84: While not a direct substitute (different pinout, different ratings), the EL84 occupies a similar power class and is sometimes compared to the 6EM5 in audio applications. The EL84 has lower plate dissipation (12W max) but was purpose-designed for audio service and has more refined audio characteristics. Not pin-compatible.
- 6GW8/ECL86: A triode-pentode combination tube where the pentode section has somewhat similar characteristics. Not a direct substitute.
Important: There is no widely recognized direct pin-for-pin audio equivalent of the 6EM5. When substituting tubes in existing circuits, always verify pinout compatibility, heater requirements, and maximum ratings. The 6EM5's pin 8 internal connection is a particular concern — circuits designed for other noval-based tubes may have connections to pin 8 that could cause problems with the 6EM5.
Notable Characteristics
Exceptional Peak Voltage Rating
Perhaps the most striking specification of the 6EM5 is its extraordinary peak positive-pulse plate voltage rating of 2,200 volts. While this rating applies specifically to the short-duration pulses encountered in vertical deflection service (pulse duration must not exceed 15% of a vertical scanning cycle, or 2.5 milliseconds), it speaks to the robust construction of the tube's internal elements and the quality of its insulation. This rating is far beyond what most miniature tubes can withstand.
Pin 8 Internal Connection
A critical design detail of the 6EM5 is that pin 8 is an internal connection and must not be connected to any external circuit. This is an important consideration for circuit designers and a potential source of confusion when adapting circuits designed for other noval-based tubes. Failure to observe this restriction could result in damage to the tube or circuit malfunction.
High Peak Cathode Current Capability
The 6EM5 can deliver peak cathode currents of up to 210 mA, with an average cathode current limit of 60 mA. This high peak-to-average current ratio reflects its deflection-service heritage, where the tube must deliver large current pulses during the active scan period while maintaining a moderate average current for thermal management.
Generous Grid Circuit Resistance
The maximum grid No.1 circuit resistance of 2.2 megohms is notably generous, allowing the use of high-impedance grid circuits without risk of grid current-induced distortion or thermal runaway. This is advantageous in audio applications where high-impedance coupling networks may be employed.
Mu Factor and Gain Characteristics
With a mu factor (Grid No.1 to Grid No.2) of approximately 8.7, the 6EM5 has a moderate amplification factor that places it between low-mu power pentodes and high-mu types. This moderate mu, combined with a transconductance of 5,100 µmhos, provides a good balance of gain and linearity for audio applications.
Thermal Robustness
The 250°C maximum bulb temperature rating indicates that the 6EM5 was designed to operate reliably in thermally demanding environments. This thermal robustness translates to long service life and consistent performance in audio applications, where operating temperatures are typically far below the tube's thermal limits.
Low Screen Current
At the typical operating point of 250V plate and 250V screen with −18V grid bias, the 6EM5 draws only 3 mA of screen current against 40 mA of plate current. This favorable plate-to-screen current ratio indicates efficient beam forming and suggests good linearity characteristics, as excessive screen current is often associated with non-linear behavior in beam power tubes.
Usage in the Audio Community
The Appeal of "Orphan" Tubes
The 6EM5 belongs to a category that audio enthusiasts sometimes call "orphan" tubes — types that were produced in large quantities for television service but have no established audio pedigree. These tubes are attractive to the audio community for several reasons:
- Availability and cost: Large quantities of NOS (New Old Stock) 6EM5 tubes remain available from various sources, often at prices significantly lower than popular audio tubes like the EL34 or 6L6. Toshiba NOS examples, in particular, are frequently encountered in the marketplace.
- Unexplored sonic territory: Using unconventional tubes allows builders and listeners to discover unique sonic characteristics not available from mainstream audio tubes.
- Technical challenge: Designing an amplifier around an unconventional tube type presents an engaging engineering challenge that appeals to the DIY community.
Single-Ended Triode (SET) Amplifiers
The most common audio application for the 6EM5 is in single-ended triode amplifiers, where the tube is operated with its screen grid tied to the plate (triode connection). In this configuration, the 6EM5 can deliver approximately 1–2 watts of clean single-ended power — enough to drive high-efficiency speakers (typically 95 dB/W/m or higher) to satisfying listening levels in small to medium-sized rooms.
Typical SET circuit designs using the 6EM5 feature:
- A high-mu triode or pentode driver stage (such as a 12AX7 or 12AT7)
- Cathode bias with a bypass capacitor for maximum gain
- A quality single-ended output transformer with appropriate primary impedance (typically in the range of 3,000–7,000 ohms for triode-connected operation)
- Careful power supply design with adequate filtering, as single-ended amplifiers are sensitive to power supply noise
Pentode and Ultralinear Amplifiers
Some builders operate the 6EM5 in its native pentode mode or in ultralinear configuration to extract more power. In pentode mode with a plate voltage of 250V and screen voltage of 250V, the tube can deliver several watts of output power with appropriate load matching. The specified load resistance of 50,000 ohms for the frame output application provides a starting point, though audio designers may optimize this value for minimum distortion rather than maximum power transfer.
Push-Pull Configurations
While less common than single-ended applications, pairs of 6EM5 tubes can be used in push-pull configurations to deliver higher power output with reduced even-order harmonic distortion. Push-pull designs using the 6EM5 can potentially deliver 5–8 watts in Class A or AB1 operation, sufficient for a wider range of speaker sensitivities.
Guitar Amplifiers
The 6EM5's dynamic, punchy character and its ability to handle high peak voltages make it an interesting candidate for small guitar amplifiers. Its slightly raw, energetic sound quality can be desirable in guitar applications where a degree of harmonic coloration is considered a feature rather than a flaw. Practice amplifiers and recording amplifiers built around the 6EM5 can produce distinctive tones at manageable volume levels.
Headphone Amplifiers
The moderate power output of the 6EM5 in triode-connected mode makes it suitable for driving high-impedance headphones (300–600 ohms) through an appropriate output transformer. The tube's good transconductance and moderate plate resistance contribute to adequate bandwidth and damping for headphone applications.
Design Considerations for Audio Use
Engineers and hobbyists working with the 6EM5 in audio circuits should be aware of several important considerations:
- Pin 8 clearance: Ensure that pin 8 (internal connection) is not connected to any external circuit. Some noval socket wiring schemes may inadvertently connect to this pin.
- Bias point selection: The datasheet operating point of −18V grid bias at 250V plate and 250V screen provides a good starting point for audio experimentation, yielding 40 mA plate current and 3 mA screen current.
- Output transformer selection: For pentode operation, a primary impedance in the range of 4,000–7,000 ohms is a reasonable starting point. For triode-connected operation, lower primary impedances (2,000–5,000 ohms) may be more appropriate. Quality single-ended output transformers with adequate inductance for low-frequency extension are essential.
- Power supply requirements: A well-regulated B+ supply of 200–300V is typical. The tube's 0.8A heater current at 6.3V requires a heater supply capable of delivering adequate current, and DC heater supplies are recommended for lowest noise in audio applications.
- Cutoff characteristics: The grid voltage for plate current cutoff (approximately −37V at 250V plate, 250V screen) provides useful information for determining the maximum signal swing and output power capability.
Community Resources
The 6EM5 has been discussed in various online forums dedicated to DIY audio, including the DIYAudio forum, Audiokarma, and various Japanese audio enthusiast sites (where Toshiba-made examples are particularly popular). Circuit designs, listening impressions, and construction details can be found in these communities, providing valuable resources for builders considering this tube for audio projects.
Availability
NOS 6EM5 tubes remain available from various vintage tube dealers and online marketplaces. Toshiba NOS examples are among the most commonly encountered and are generally well-regarded for their consistent quality and construction. Prices remain moderate compared to sought-after audio tubes, making the 6EM5 an economical choice for experimentation and project building.
