1. Introduction and History
The EF70 is a subminiature high-slope radio frequency pentode manufactured by Mullard, one of the most respected British valve manufacturers. Designed during the post-war era of miniaturization in electronics, the EF70 represents a class of compact, wire-lead subminiature valves that were developed for professional, military, and specialized communications equipment where space and weight were at a premium.
The EF70 is described by Mullard as a "subminiature high slope r.f. pentode with a short suppressor grid base." A notable design feature is that a diode is connected internally to the suppressor grid (g3) to prevent this grid from locking at a positive voltage — a clever engineering solution to ensure stable operation in demanding RF circuits.
The valve uses a B8D/F flying-lead base configuration rather than a conventional pin base, with direct soldered connections to the circuit. This construction was typical of subminiature valves intended for ruggedized or space-constrained applications, including military communications receivers, airborne electronics, and portable instrumentation. The EF70 was assigned the military CV designation CV467, confirming its use in British military equipment.
With its high transconductance of 2.5 mA/V and compact form factor, the EF70 offered excellent RF gain performance in a package far smaller than conventional miniature valves of the era.
2. Technical Specifications and Design
Heater
| Heater Voltage (Vh) | 6.3 V |
| Heater Current (Ih) | 200 mA |
Interelectrode Capacitances (measured with external shield)
| Ca–g1 (Anode to Grid 1) | < 0.025 pF |
| Cin (Input Capacitance) | 4.5 pF |
| Cout (Output Capacitance) | 4.7 pF |
Typical Operating Characteristics
| Anode Voltage (Va) | 100 V |
| Screen Grid Voltage (Vg2) | 100 V |
| Suppressor Grid Voltage (Vg3) | 0 V |
| Anode Current (Ia) | 3.0 mA |
| Screen Grid Current (Ig2) | 2.25 mA |
| Grid 1 Voltage (Vg1) | −2.0 V |
| Transconductance (gm) | 2.5 mA/V |
| Anode Resistance (ra) | 100 kΩ |
| Amplification Factor (μg1–g2) | 38 |
| Vg3 at Ia = 100 μA | −8.0 V |
Limiting Values (Absolute Maximum Ratings)
| Va(b) max. (Anode Voltage, boost) | 300 V |
| Va max. (Anode Voltage) | 175 V |
| Pa max. (Anode Dissipation) | 750 mW |
| Vg2(b) max. (Screen Voltage, boost) | 300 V |
| Vg2 max. (Screen Voltage) | 175 V |
| Pg2 max. (Screen Dissipation) | 400 mW |
| Ik max. (Cathode Current) | 10 mA |
| Rg1–k max. (Grid 1 to Cathode Resistance) | 500 kΩ |
| Rh–k max. (Heater to Cathode Resistance) | 20 kΩ |
| Vh–k max. (Heater to Cathode Voltage) | 100 V |
Physical Construction
| Base Type | B8D/F (Flying Leads) |
| Envelope | Subminiature glass, all-glass construction |
| Mounting Position | Any |
| Overall Diameter | 9.3–10.16 mm max. |
| Overall Length (body) | 29–32 mm (body), 38.1 mm max. overall |
| Lead Length (tinned) | 38 mm min. |
| Soldering Clearance | Minimum 5 mm from seal; bending minimum 1.5 mm from seal |
Pin Configuration (B8D/F Base — viewed from lead end)
The EF70 uses a flying-lead base with the following wire connections (as shown in the Mullard datasheet, viewed from the base):
- a — Anode
- g1 — Control Grid (Grid 1)
- g2 — Screen Grid (Grid 2)
- g3 — Suppressor Grid (Grid 3) — internally connected to a diode
- k — Cathode
- h, h — Heater (two leads)
Note: The suppressor grid (g3) has an internal diode connection to prevent it from locking at a positive voltage.
Cooling Requirements
Mullard's datasheet specifically warns that "in operation this valve may become very hot" and recommends adequate cooling. The preferred mounting method is a metal clip that conducts heat away from the chassis, targeting a bulb temperature of no more than 100°C. This is an important consideration for any application, particularly in enclosed chassis designs.
3. Applications and Usage
The EF70 was designed primarily as an RF amplifier pentode for use in:
- Radio frequency amplifier stages — The very low anode-to-grid capacitance (< 0.025 pF) makes the EF70 exceptionally well-suited for high-frequency RF amplification with minimal feedback and excellent stability.
- IF amplifier stages — The high transconductance and favorable capacitance characteristics allow for efficient intermediate frequency amplification in superheterodyne receivers.
- Military communications receivers — The CV467 designation confirms its deployment in British military equipment, where the subminiature form factor was essential for portable and airborne applications.
- Portable and airborne electronics — The compact size, low heater power consumption (1.26 W), and ability to mount in any position made the EF70 ideal for equipment subject to vibration and space constraints.
- Instrumentation — High-impedance input stages in measurement equipment benefited from the EF70's low capacitances and high gain.
- AGC-controlled stages — The suppressor grid cutoff characteristic (Vg3 = −8.0 V at Ia = 100 μA) allows for gain control via the suppressor grid, a useful feature in receiver AGC systems.
The maximum grid-to-cathode resistance of 500 kΩ allows for high-impedance input circuits, while the 100 V heater-to-cathode voltage rating permits use in series heater strings common in AC/DC equipment.
4. Sound Characteristics
While the EF70 was designed as an RF pentode rather than an audio amplifier, subminiature pentodes of this class have attracted attention from adventurous audiophiles and DIY builders who appreciate their unique sonic qualities when pressed into audio service:
- Clarity and detail — The high transconductance (2.5 mA/V) relative to the modest operating currents gives the EF70 a lively, detailed presentation. In audio preamplifier circuits, users report excellent resolution of fine musical detail and transient information.
- Low microphonics — Subminiature construction with short, rigid internal structures and the absence of a conventional socket (direct soldered connections) results in inherently low microphonic sensitivity. This is a significant advantage in high-gain phono preamplifier and microphone preamp applications.
- Neutral to slightly bright tonal balance — As an RF pentode, the EF70 tends toward a clean, transparent, and slightly forward presentation compared to larger audio-specific pentodes. The sound is often described as articulate and precise rather than warm and lush.
- Tight, controlled bass — The relatively low plate resistance (100 kΩ) for a pentode of this type contributes to good damping and bass control when used in audio voltage amplifier stages.
- Excellent signal-to-noise ratio — The subminiature construction and low capacitances contribute to a quiet noise floor, making the EF70 attractive for low-level signal amplification in audio applications.
It should be noted that the EF70's maximum anode dissipation of 750 mW and maximum cathode current of 10 mA limit its use to voltage amplifier and preamplifier stages. It is not suitable for output stage duty. The sonic character is best appreciated in high-gain, low-level applications where its inherent quietness and detail retrieval can shine.
5. Equivalent or Substitute Types
| Type | Relationship | Notes |
|---|---|---|
| CV467 | Direct equivalent (military designation) | Identical to EF70 in all respects. British military CV number. Drop-in replacement with no differences in ratings or pinout. |
| 6847 | Related type — NOT a drop-in substitute | Different ratings from the EF70. Should not be used as a direct replacement without circuit modification. Verify specifications against the 6847 datasheet before substitution. |
Important: Due to the subminiature flying-lead construction, physical interchangeability is generally straightforward between the EF70 and CV467, as both use the same B8D/F base with identical lead configurations. However, the 6847 has different electrical ratings and should only be considered as a substitute after careful review of its datasheet and any necessary circuit adjustments.
Other subminiature RF pentodes from the same era (such as the EF72, EF71, or various American subminiature types) may share similar physical form factors but will have different electrical characteristics and are not recommended as direct substitutes without thorough verification.
6. Notable Characteristics
- Internal suppressor grid diode: The most distinctive feature of the EF70 is the internal diode connected to the suppressor grid (g3). This prevents the suppressor from locking at a positive potential — a condition that could occur due to secondary emission or charge accumulation, which would disrupt normal pentode operation. This is an elegant design solution that enhances operational reliability.
- Exceptionally low anode-to-grid capacitance: At less than 0.025 pF, the feedback capacitance is remarkably low, enabling stable high-frequency amplification without neutralization in many applications.
- High thermal dissipation for its size: Mullard's explicit warning about heat generation and the recommendation for metal clip mounting with thermal conduction to the chassis underscores that the EF70 operates at significant power density for its diminutive size. The 100°C maximum bulb temperature must be respected.
- Flying lead construction: The B8D/F base requires direct soldering rather than socket mounting. While this provides superior mechanical and electrical connections (lower contact resistance, reduced microphonics, better high-frequency performance), it makes tube replacement more involved. Soldered connections must be at least 5 mm from the glass seal, and any lead bending must occur at least 1.5 mm from the seal to prevent cracking.
- Versatile mounting: The valve can be mounted in any position, providing maximum flexibility in equipment layout.
- Amplification factor of 38: The μg1–g2 of 38 provides a good balance between gain and linearity for RF and IF amplifier applications.
- Robust heater-to-cathode insulation: The 100 V Vh–k rating allows the EF70 to be used in series heater strings and in circuits where the cathode operates at elevated potentials relative to the heater.
7. Usage in the Audio Community
The EF70, while not originally designed for audio applications, has found a niche following among the more experimental and adventurous segments of the audio community:
Phono Preamplifiers
The EF70's combination of high gain, low noise, and extremely low microphonic sensitivity makes it an attractive candidate for moving-magnet and even moving-coil phono preamplifier stages. The subminiature construction with its rigid, compact electrode structure and direct-soldered connections eliminates the mechanical resonances that plague larger valves in high-gain phono stages. Several boutique and DIY phono preamp designs have emerged using the EF70 or similar subminiature pentodes.
Microphone Preamplifiers
For similar reasons — low noise, low microphonics, and high gain — the EF70 has been used in studio-quality microphone preamplifier designs. The high input impedance capability (Rg1–k up to 500 kΩ) is well-suited to condenser microphone capsule interfaces.
Headphone Amplifiers
Some DIY headphone amplifier designs use the EF70 as a voltage amplifier stage driving a separate output stage. The compact size of the subminiature valve appeals to builders creating desktop headphone amplifiers with a small footprint.
Hybrid Amplifier Designs
The EF70 is sometimes used as a tube voltage gain stage in hybrid tube/solid-state amplifier designs, where its high transconductance provides ample gain while a solid-state output stage handles the current delivery. The subminiature form factor integrates well into compact hybrid designs.
Practical Considerations for Audio Use
Audio builders working with the EF70 should be aware of several practical considerations:
- No socket: The flying-lead construction means tubes must be soldered directly into the circuit. Some builders create custom adapter boards or use small terminal strips to facilitate easier tube changes, though this somewhat defeats the purpose of the direct-connection design.
- Heat management: Adequate cooling must be provided, especially in enclosed chassis. Metal mounting clips that conduct heat to the chassis are recommended per the Mullard datasheet.
- Operating point selection: For audio use, the EF70 is typically operated at or near its rated conditions (Va = 100 V, Vg2 = 100 V, Vg1 = −2.0 V) or at reduced voltages for lower distortion. Some builders experiment with triode-strapping (connecting the screen grid to the anode) for a more linear, lower-gain triode-like characteristic.
- Availability: The EF70 is available as NOS (New Old Stock) from various vintage component dealers, typically sourced from Mullard production. As a relatively specialized subminiature type, supply is limited and prices can vary. The CV467 military equivalent may sometimes be found at lower cost.
- Pentode vs. triode operation: In pentode mode, the EF70 offers maximum gain but with the characteristic pentode harmonic spectrum (stronger odd-order harmonics). Triode-strapped operation reduces gain significantly but produces a more even-order harmonic spectrum preferred by many audiophiles for its more "musical" character.
Community Reception
The EF70 occupies a small but enthusiastic niche in the audio community. It appeals primarily to experienced DIY builders who enjoy working with unusual valve types and who appreciate the engineering elegance of subminiature construction. The tube is not widely used in commercial audio products, but its sonic qualities — particularly its low noise floor and freedom from microphonics — have earned it a loyal following among those who have discovered its potential in audio circuits. Online forums dedicated to valve audio frequently feature build threads and discussions about subminiature pentodes including the EF70, with builders sharing circuit designs, operating point recommendations, and listening impressions.
