Introduction and History
The E810F, also designated as the 7788, is a Special Quality (S.Q.) sharp-cutoff pentode originally designed by Philips for use as a wide-band amplifier in professional instrumentation, telecommunications, and measurement equipment. The datasheet dates from December 1968, placing its development firmly in the golden era of European professional-grade vacuum tube engineering.
Manufactured under the Philips S.Q. (Special Quality) program, the E810F was built to exacting standards that far exceeded those of ordinary commercial tubes. The S.Q. designation guaranteed tighter parameter spreads, superior mechanical construction, gold-plated pins for low interface resistance, and rigorous life testing to 10,000 hours. The tube was produced by Philips (Amperex in the US market), as well as by Telefunken, Siemens, and other European manufacturers under license or to the same specification. Telefunken-branded E810F tubes with gold pins are particularly prized by collectors and audiophiles today.
The E810F's extraordinary transconductance of 50 mA/V — remarkably high for a small-signal pentode — combined with its low equivalent noise resistance of just 110 Ω and a quality factor of 250 MHz made it an ideal choice for wideband RF and IF amplifier stages, oscilloscope vertical amplifiers, and other applications demanding both gain and bandwidth. Its robust mechanical construction, rated for shock and vibration resistance, further suited it to military and industrial environments.
Technical Specifications and Design
General Information
| Tube Type | E810F / 7788 |
| Category | Special Quality (S.Q.) Sharp-Cutoff Pentode |
| Designed For | Wide-band amplifier service |
| Base | Noval (9-pin), Gold plated pins |
| Heating | Indirect; A.C. or D.C.; Parallel supply |
| Mechanical Quality | Shock and vibration resistant |
| Life Test | 10,000 hours |
Heater Ratings
| Heater Voltage (Vf) | 6.3 V |
| Heater Current (If) | 340 mA (nominal); range 320–360 mA |
| Heater Voltage Range | 6.0 V to 6.6 V (exceeding this range shortens tube life) |
Key Electrical Characteristics (Nominal Operating Point)
| Parameter | Symbol | Nominal | Range (Initial Spread) | End of Life | Unit |
|---|---|---|---|---|---|
| Anode Supply Voltage | Vba | 135 | — | — | V |
| Grid No.3 (Suppressor) Voltage | Vg3 | 0 | — | — | V |
| Grid No.2 (Screen) Supply Voltage | Vbg2 | 165 | — | — | V |
| Grid No.1 (Control) Supply Voltage | +Vbg1 | 12.5 | — | — | V |
| Cathode Resistor | Rk | 360 | — | — | Ω |
| Anode Current | Ia | 35 | negligible spread | — | mA |
| Grid No.2 Current | Ig2 | 5.0 | 4.4–5.6 | — | mA |
| Mutual Conductance (Transconductance) | S (gm) | 50 | 42–58 | min. 35 | mA/V |
| Internal Resistance (Plate Resistance) | Ri (rp) | 42 | — | — | kΩ |
| Amplification Factor (g2 to g1) | μg2g1 | 57 | — | — | — |
| Equivalent Noise Resistance | Req | 110 | — | — | Ω |
| Negative Grid Current | −Ig1 | — | max. 0.1 | max. 0.2 | μA |
| Hum Voltage (on g1) | Vg1 | — | max. 150 | μV | |
| Input Resistance (at 100 MHz) | rg1 | 415 | — | — | Ω |
| Quality Factor (without shield) | S / 2π(Cg1+Ca+5) | 250 | — | — | MHz |
| Quality Factor (with shield) | — | 245 | — | — | MHz |
Second Operating Point (Lower Voltage)
| Anode Supply Voltage (Vba) | 120 V |
| Grid No.3 Voltage (Vg3) | 0 V |
| Grid No.2 Supply Voltage (Vbg3) | 150 V |
| Cathode Resistor (Rk) | 47 Ω |
| Anode Current (Ia) | 35 mA (range 31–39 mA) |
Absolute Maximum Ratings
| Parameter | Symbol | Maximum | Unit |
|---|---|---|---|
| Anode Voltage (no current) | Vao | 400 | V |
| Anode Voltage (operating) | Va | 250 | V |
| Anode Dissipation | Wa | 5 | W |
| Grid No.2 Voltage (no current) | Vg2o | 400 | V |
| Grid No.2 Voltage (operating) | Vg2 | 200 | V |
| Grid No.2 Dissipation | Wg2 | 1 | W |
| Grid No.1 Voltage (negative) | −Vg1 | 25 | V |
| Grid No.1 Peak Voltage (negative) | −Vg1p | 50 | V |
| Grid No.1 Peak Voltage (positive) | +Vg1p | 50 | V |
| Grid No.1 Dissipation | Wg1 | 10 | mW |
| Cathode Current | Ik | 50 | mA |
| Cathode Current (1000 hr life) | Ik | 65 | mA |
| Voltage Between Cathode and Heater | Vkf | 100 | V |
| Bulb Temperature | tbulb | 200 | °C |
| Bulb Temperature (1000 hr life) | tbulb | 220 | °C |
Grid No.1 Resistor Limits
| With fixed bias | Rg1 max. 0.2 MΩ |
| With automatic bias, Rk = 47 Ω | Rg1 max. 0.6 MΩ |
| With automatic bias, Rk = 360 Ω | Rg1 max. 3.5 MΩ |
Interelectrode Capacitances
| Capacitance | Symbol | Without Shield (Nom.) | With Shield (Nom.) | Unit |
|---|---|---|---|---|
| Anode to g3, g2, cathode, heater, screen | Ca/g3g2kfs | 3.5 | 4.1 | pF |
| Grid No.1 to g3, g2, cathode, heater, screen (Ik=0) | Cg1/g3g2kfs | 14.5 | 14.5 | pF |
| Grid No.1 to g3, g2, cathode, heater, screen (Ik=40mA, f=100 Mc/s) | Cg1/g3g3kfs | 24 | 24 | pF |
| Anode to Grid No.1 | Cag1 | — | max. 36 / max. 32 | mpF |
| Anode to Cathode | Cak | 60 | 33 | mpF |
| Anode to Heater | Caf | 31 | 20 | mpF |
| Grid No.1 to Heater | Cg1f | 60 | 55 | mpF |
| Cathode to Heater | Ckf | 5.2 | — | pF |
Class A Output Stage Operating Characteristics
| Anode Supply Voltage (Vba) | 155 V |
| Grid No.3 Voltage (Vg3) | 0 V |
| Grid No.2 Supply Voltage (Vbg2) | 165 V |
| Grid No.1 Supply Voltage (+Vbg1) | 12.5 V |
| Cathode Resistor (Rk) | 360 Ω |
| Cathode Capacitor (Ck) | 1000 μF |
| Anode Resistor (Ra) | 560 Ω |
| Anode Current (Ia) | 35 mA |
| Anode Current, Peak to Peak (Iap) | 40 mA |
| Total Distortion (dtot) | 7.5% |
Life Test Conditions (10,000 Hours)
| Anode Supply Voltage (Vba) | 165 V |
| Anode Resistor (Ra) | 820 Ω |
| Grid No.3 Voltage (Vg3) | 0 V |
| Grid No.2 Supply Voltage (Vbg2) | 165 V |
| Grid No.1 Supply Voltage (+Vbg1) | 12.5 V |
| Cathode Resistor (Rk) | 360 Ω |
| Anode Current (Ia) | 35 mA |
| Voltage Between Cathode and Heater (Vkf) | 100 V |
Physical Dimensions
- Maximum diameter: 22 mm
- Maximum seated height: 49.2 mm
- Maximum overall height: 55.6 mm
Pin Configuration (Noval Base, Bottom View)
| Pin | Connection |
|---|---|
| 1 | Cathode (k) |
| 2 | Grid No.1 (g1) |
| 3 | Cathode (k) |
| 4 | Heater (f) |
| 5 | Heater (f) |
| 6 | Grid No.2 (g2) |
| 7 | Anode (a) |
| 8 | Grid No.3, Screen (g3, s) |
| 9 | Grid No.2 (g2) |
Note: Grid No.2 (screen grid) is connected to both pins 6 and 9. The cathode is connected to both pins 1 and 3. The suppressor grid (g3) is brought out to pin 8 along with the internal screen/shield.
Applications and Usage
The E810F was originally designed for demanding professional applications requiring high gain, wide bandwidth, and low noise:
- Wide-band amplifiers: The tube's primary intended application, as stated on the datasheet. Its quality factor of 250 MHz and transconductance of 50 mA/V made it exceptional for video and IF amplifier chains.
- Oscilloscope vertical amplifiers: Many high-end oscilloscopes from Philips, Tektronix, and Hewlett-Packard used the E810F or its 7788 equivalent in their vertical deflection amplifier stages, where bandwidth and linearity were critical.
- Nuclear instrumentation: The low noise and tight parameter spreads made the E810F suitable for pulse amplifiers in nuclear counting and spectroscopy equipment.
- Telecommunications: Used in carrier telephony equipment and broadband repeater amplifiers where consistent gain and low noise were essential.
- RF and IF amplifiers: The tube's input resistance of 415 Ω at 100 MHz and its excellent shielding characteristics made it suitable for VHF receiver front-ends.
- Regulated power supplies: The high transconductance and current capability allowed use as a series pass element or error amplifier in precision voltage regulators.
- Audio amplification: Though not its original purpose, the E810F has found a devoted following in high-end audio, particularly in headphone amplifiers and preamplifier stages (discussed in detail below).
Sound Characteristics
The E810F has earned a distinctive reputation in the audiophile community for its sonic qualities, which are a direct consequence of its extraordinary electrical parameters. Here is how experienced listeners and audio engineers typically characterize its sound:
- Exceptional detail and resolution: The E810F's very high transconductance of 50 mA/V and low equivalent noise resistance of 110 Ω translate directly into an ability to resolve micro-details and subtle textural information that lesser tubes simply cannot reproduce. Low-level ambient cues, the decay of reverb tails, and the fine grain of instrumental timbre are rendered with unusual clarity.
- Speed and transient response: Designed for wideband service up to 250 MHz, the E810F exhibits lightning-fast transient response in audio applications. Leading edges of notes — the attack of a piano hammer, the pluck of a guitar string, the snap of a snare drum — are reproduced with startling immediacy and precision. This gives the tube a sense of "speed" that is often compared favorably to solid-state amplification, but without the associated hardness.
- Low noise floor: The S.Q. construction and 110 Ω equivalent noise resistance yield an exceptionally black background, allowing music to emerge from silence with a purity that is immediately noticeable, especially in headphone listening where noise is more apparent.
- Neutral to slightly warm tonal balance: Unlike some pentodes that can sound thin or clinical, the E810F is generally described as having a neutral tonal balance with a slight warmth in the midrange. It does not impose a heavy coloration on the signal, making it appealing to listeners who want tube amplification without excessive euphonic editorializing.
- Excellent dynamics: The tube's high current capability (35 mA anode current, up to 50 mA cathode current) and 5 W anode dissipation give it dynamic headroom that is unusual for a small-signal pentode. Macro-dynamics are powerful and authoritative, while micro-dynamics — the subtle volume shadings within a musical phrase — are preserved with finesse.
- Spatial presentation: Listeners frequently note the E810F's ability to create a wide, deep, and precisely defined soundstage. Instrument placement is stable and three-dimensional, with excellent separation between voices in complex orchestral passages.
- Triode-strapped character: When wired as a triode (screen tied to anode), the E810F takes on a somewhat different character — warmer, with lower output impedance and reduced gain, but with even lower distortion. Many audio designers offer switchable pentode/triode operation to allow listeners to choose their preferred sonic signature.
The sonic differences between manufacturers are also noted by experienced listeners. Telefunken E810F tubes with diamond-bottom markings and gold pins are often described as having the most refined and detailed presentation, while Philips/Amperex versions are considered slightly warmer and more full-bodied. Siemens-branded examples are said to fall somewhere between the two. These distinctions, while subtle, are consistently reported by critical listeners in controlled comparisons.
Equivalent or Substitute Types
| Type | Compatibility | Notes |
|---|---|---|
| 7788 | Direct equivalent | The American RETMA designation for the same tube. The 7788 and E810F are fully interchangeable with identical pinout and specifications. The 7788 was manufactured by Amperex (Philips' US subsidiary) and by GE, Sylvania, and others. |
| E810F from different manufacturers | Direct equivalent | Philips, Telefunken, Siemens, Valvo, and Mullard all produced E810F tubes to the same specification. All are interchangeable, though sonic characteristics may vary subtly between manufacturers. |
Important notes on substitution:
- The E810F/7788 is a unique type with no close equivalent from a different tube family. Its combination of very high transconductance, low noise, and specific operating voltages means that circuits designed for the E810F generally cannot accept other pentode types without significant redesign.
- Some sources suggest the EF86 or 6267 as a vague functional alternative (both are low-noise audio pentodes on a noval base), but these have vastly different characteristics — the EF86 has a transconductance of only about 1.8 mA/V versus the E810F's 50 mA/V, completely different operating points, and a different pinout. They are not interchangeable.
- The E130L (pentode) has sometimes been compared to the E810F in audio circles, but it is a different tube with different specifications and pinout, and is not a drop-in substitute.
Notable Characteristics
- Extraordinary transconductance: At 50 mA/V, the E810F has one of the highest transconductances of any small-signal receiving tube ever manufactured. This is roughly 25–30 times higher than a typical audio pentode like the EF86 and approaches the territory of some power pentodes, despite the E810F's modest 5 W plate dissipation.
- Gold-plated pins: The noval base features gold-plated pins as standard, ensuring low and stable contact resistance over the tube's lifetime. This was a hallmark of the S.Q. tube program and contributes to long-term reliability.
- Shock and vibration resistance: The E810F was tested to withstand 500 g shock acceleration and 2.5 g vibration at 50 Hz for 32 hours, making it one of the most mechanically robust small-signal tubes available. This ruggedness translates to low microphonic sensitivity in audio applications.
- Tight parameter control: The S.Q. specification guaranteed that key parameters like anode current (35 mA with "negligible spread") and transconductance (42–58 mA/V initial, minimum 35 mA/V at end of life) would remain within narrow bounds. This made tube matching straightforward and ensured consistent performance across production lots.
- Positive grid 1 bias: Unusually for a pentode, the E810F operates with a positive grid No.1 supply voltage of +12.5 V, with the actual bias set by the cathode resistor (360 Ω). This operating regime is a consequence of the tube's very high transconductance and requires careful circuit design to maintain stability.
- Low noise: The equivalent noise resistance of 110 Ω at 45 MHz, combined with a maximum hum voltage of only 150 μV on the control grid, makes the E810F exceptionally quiet. The maximum leakage current between cathode and heater is specified at only 10 μA initially and 20 μA at end of life.
- Dual cathode and screen grid pins: The cathode is brought out to pins 1 and 3, and the screen grid to pins 6 and 9, reducing lead inductance and improving high-frequency performance. This dual-pin arrangement is essential for maintaining the tube's wideband characteristics.
- Insulation resistance: A minimum of 100 MΩ (initial) and 40 MΩ (end of life) between the anode and all other electrodes, measured at 250 V, ensures excellent isolation and low leakage.
Usage in the Audio Community
Despite being designed for instrumentation and wideband amplification, the E810F has become one of the most celebrated tubes in the high-end audio world, particularly in the headphone amplifier and preamplifier segments. Its adoption by the audio community represents one of the most successful "repurposings" of a professional tube type for audiophile use.
Headphone Amplifiers
The E810F's most prominent audio application is in headphone amplifiers, where its high transconductance, low noise, and substantial current capability make it ideally suited to driving low-impedance headphones. Several acclaimed commercial and DIY headphone amplifier designs feature the E810F:
- Eddie Current designs: Craig Uthus of Eddie Current has used the E810F in several of his highly regarded headphone amplifiers, where the tube's speed and resolution are showcased to excellent effect.
- DIY community projects: The E810F is a popular choice in the Head-Fi and DIYAudio communities, where builders have developed numerous amplifier topologies around this tube. Common configurations include single-ended pentode stages, triode-strapped designs, and SRPP (Shunt Regulated Push-Pull) circuits.
- Bottlehead and similar kit manufacturers: Some boutique kit manufacturers have offered E810F-based designs, recognizing the tube's appeal to the enthusiast market.
Preamplifiers
The E810F's low noise and high gain make it an excellent candidate for phono preamplifier and line-stage applications. Its 110 Ω equivalent noise resistance is competitive with the quietest audio tubes available, and its high transconductance allows for generous feedback to reduce distortion while maintaining adequate gain. Some high-end preamplifier designers have used the E810F in pentode mode for phono stages, where its gain and noise performance are particularly advantageous for moving-coil cartridge amplification.
DAC Output Stages and Buffers
The E810F's ability to deliver significant current (up to 50 mA cathode current) and its wide bandwidth make it suitable for use as an output buffer following digital-to-analog converters, where it can drive long interconnect cables and low-impedance loads without difficulty.
Tube Rolling and Collecting
The E810F has become a sought-after collector's item, with prices for NOS (New Old Stock) examples rising significantly over the past two decades. The hierarchy of desirability among collectors and audiophiles generally follows this pattern:
- Telefunken E810F with diamond-bottom logo and gold pins: The most prized version, commanding premium prices. These are considered to offer the finest sonic performance and the tightest manufacturing tolerances.
- Philips SQ E810F (Heerlen factory): The original Philips production from the Netherlands, highly regarded for consistent quality.
- Amperex 7788 (US-made or Heerlen-made): Amperex tubes made at the Philips Heerlen factory are essentially identical to Philips-branded examples. US-made Amperex tubes are also well-regarded.
- Siemens E810F: German production with excellent quality, sometimes preferred for a slightly different tonal character.
- Valvo E810F: The Philips subsidiary brand, typically manufactured at the same Philips factories.
- GE, Sylvania, and other US 7788 production: Generally less sought-after than European examples but still capable performers.
Design Considerations for Audio Use
Engineers and DIY builders working with the E810F in audio circuits should be aware of several important considerations:
- Stability: The E810F's very high transconductance and wide bandwidth make it prone to parasitic oscillation if layout and grounding are not carefully managed. Short, direct signal paths, proper grounding, and sometimes grid-stopper resistors are essential.
- Heater supply: For lowest noise in audio applications, a regulated DC heater supply is recommended, though the tube's specified hum voltage of max. 150 μV with AC heaters (midtap grounded) is already very low.
- Operating point: The positive grid bias arrangement (+12.5 V supply with 360 Ω cathode resistor) is unusual and must be carefully implemented. The cathode resistor should be bypassed with a large capacitor (the datasheet specifies 1000 μF for the Class A output stage) for full gain.
- Power supply requirements: The relatively modest plate voltages (135 V typical, 250 V maximum) simplify power supply design compared to many audio tubes, but the significant heater current of 340 mA must be accommodated, especially in multi-tube designs.
- Triode strapping: Connecting the screen grid (pins 6 and 9) to the anode (pin 7) converts the E810F to triode operation, reducing gain but also reducing output impedance and distortion. Many audio designers prefer this configuration for its more linear transfer characteristics.
The E810F stands as a remarkable example of how a tube designed for one purpose can find an entirely new life in another application. Its combination of extraordinary transconductance, low noise, mechanical robustness, and S.Q. manufacturing quality has made it a favorite among discerning audiophiles and audio engineers who demand the highest performance from their vacuum tube equipment. While NOS supplies are finite and prices continue to rise, the E810F remains one of the most rewarding tubes to listen to and to design around in the world of high-end audio.