EF860 (=EF800) Post Pentode

EF860 / EF800 RFT long-life pentode for low-distortion audio drivers

The EF860 is a high-transconductance RFT pentode developed for long service in professional telecommunications preamplifiers. It belongs to the EF800 family and combines controlled production tolerances with a useful 10 mA reference operating current. Today, these characteristics make it a compelling sleeper valve for value-focused hi-fi, studio and amplifier DIY.

Its driver capability has a particularly relevant practical reference: BTB successfully used the EF860 to drive four EL12N output valves in its 8-cylinder single-ended amplifier. The EF860 can therefore be considered not only for voltage gain, but also for driver stages where an ultra-low-current preamp valve may provide insufficient current reserve.

The EF860 is not a power-output valve and must not be connected directly to a loudspeaker. In a purpose-designed circuit, however, it offers an attractive basis for low-distortion preamplifiers, line-level gain stages, studio equipment and drivers for larger output valves.

Suitable tube-audio applications

• Driver stages for audio power valves
• Low-distortion voltage amplifiers for hi-fi and studio equipment
• Line-level gain stages and high-impedance preamplifier outputs
• Clean gain and driver stages in custom guitar amplifiers
• Differential or balanced circuits using selected valves
• Service of equipment specifically designed for the EF860 or EF800

Technical specifications

Valve type

High-transconductance small-signal pentode designed for long-life professional preamplifier service

Manufacturer and background

RFT version produced for professional German telecommunications applications

Valve family

The EF860 corresponds to the professional EF800 family and is closely related to the EF80

Base

Noval B9A, 9-pin

Heater

6.3 V with a permitted variation of ±5%; approximately 295 mA, commonly rounded to 0.3 A

Typical data-sheet operating point

Va 170 V, Vg2 170 V, Vg3 0 V, Rk 160 Ω, Ia 10 mA and Ig2 2.5 mA

Transconductance

7.5 ±1 mA/V at the reference operating point

Internal resistance

Approximately 0.4 MΩ at the reference operating point

Screen-to-control-grid amplification factor

µ_g2/g1 50

Maximum operating anode voltage

250 V; maximum cold anode voltage 550 V

Maximum anode dissipation

1.7 W

Screen-grid limits

Maximum Vg2 250 V and maximum screen-grid dissipation 0.45 W

Maximum cathode current

12.5 mA

Dimensions

Maximum bulb diameter 22.2 mm, nominal size 45 and approximately 7 mm pin length

Matching

BTB also offers a separate matched-pair version for stereo, differential and balanced circuits

B9A base connections – bottom view

Pin 1

Cathode, internally connected to pin 3

Pin 2

Control grid g1

Pin 3

Cathode, internally connected to pin 1

Pins 4 and 5

Heater

Pin 6

Internal shield

Pin 7

Anode

Pin 8

Screen grid g2

Pin 9

Suppressor grid g3

Why the EF860 is attractive for audio builders

The combination of approximately 10 mA anode current and 7.5 mA/V transconductance provides high voltage gain together with useful current capability for a small-signal pentode. This can be valuable when driving a following stage with significant input capacitance or when one driver must control several output valves.

Its telecommunications background and specified characteristic tolerances provide a sound engineering basis for stable long-term operation. Low distortion is not created by the valve designation alone, however. Bias, anode load, screen supply, feedback, power-supply quality and the following load all remain important parts of the design.

Because the EF860 was designed for wideband operation, practical audio layouts should use short signal paths, effective anode and screen decoupling and careful grounding. A control-grid stopper mounted directly at the socket, together with a screen stopper where appropriate, can improve stability beyond the audible frequency range.

The EF860, EF800 and EF80 share a similar base and heater environment. Existing equipment should nevertheless be checked for the correct operating point, screen circuit, anode load and dissipation limits before substitution.

Safety notice: The 6.3 V heater rating is not the required anode-supply voltage. Valve circuits may contain lethal voltages and should only be built, measured and commissioned by suitably skilled users.

Questions about selection and audio use

Is the EF860 suitable as a low-distortion driver for audio output valves?

Yes, the EF860 is a strong candidate for a purpose-designed driver stage. Its reference condition uses 10 mA anode current and 7.5 mA/V transconductance. BTB successfully employed it to drive four EL12N output valves in its own 8-cylinder single-ended amplifier. The bias, load and required voltage swing must still be calculated for the intended output stage.

Can the EF860 be used in a high-quality preamplifier or line stage?

Yes, its high transconductance, approximately 0.4 MΩ internal resistance and professionally specified characteristics support effective voltage-gain stages. A correctly designed pentode circuit can be used in hi-fi, studio and clean-audio equipment. Low-impedance loads still require a suitable buffer, transformer or other impedance-conversion stage.

Is the EF860 a worthwhile alternative to the EF800 or EF80?

Yes, particularly in a new design, the EF860 can provide an attractive long-life option within the EF800 and EF80 design family. The base and heater requirements are similar, but the operating point, screen supply and maximum ratings should still be verified. A separate BTB matched pair is also available for stereo or balanced projects.