EF183 (=6EH7) Pentode

EF183 / 6EH7 variable-mu pentode for studio and tube-audio DIY

The EF183, also designated 6EH7, is a high-transconductance remote-cutoff pentode on a Noval B9A base. Its gain can be reduced progressively by applying a more negative control-grid voltage. This characteristic makes it an interesting valve for purpose-designed variable-mu control stages, tube compressors and other dynamic audio circuits.

Originally developed for wideband IF amplification, the EF183 is not a universal audio preamp valve. Hi-fi, studio and guitar-amplifier applications require a circuit designed around its pentode characteristics, screen supply and variable transconductance. It is not a plug-in replacement for an EF86, ECC83 or another standard audio valve.

Suitable applications

• Experimental variable-mu gain-control stages in studio and tube compressors
• Dynamic gain control in purpose-designed valve-audio circuits
• Controllable small-signal stages for studio, hi-fi and guitar-amp DIY
• Laboratory, measurement and signal-generator projects
• Repair of equipment specifically designed for the EF183 or 6EH7
• Legacy wideband intermediate-frequency amplifier stages

Technical specifications

Valve type

Variable-transconductance pentode, also described as a remote-cutoff or variable-mu pentode

Equivalent designation

6EH7

Base

Noval B9A, 9-pin

Heater

6.3 V at 300 mA, indirectly heated, suitable for AC or DC and parallel or series operation

Dimensions

Maximum diameter 22 mm, maximum bulb height 54 mm, maximum overall height 61 mm

Typical data-sheet operating point

Va 200 V, Vg2 90 V, Vg1 −2 V, Ia 12 mA and Ig2 4.5 mA

Transconductance

12.5 mA/V at the typical data-sheet operating point

Internal resistance

500 kΩ at the typical data-sheet operating point

Example gain-control characteristic

With Va at 200 V and a 200 V screen supply through 27 kΩ, transconductance falls from 12.5 mA/V at Vg1 −2 V to 0.125 mA/V at Vg1 −19.5 V

Maximum anode voltage

250 V

Maximum anode dissipation

2.5 W

Maximum screen-grid voltage

250 V

Maximum screen-grid dissipation

0.65 W

Maximum cathode current

20 mA

B9A base connections – bottom view

Pin 1

Cathode

Pin 2

Control grid g1

Pin 3

Cathode

Pins 4 and 5

Heater

Pin 6

Internal shield

Pin 7

Anode

Pin 8

Screen grid g2

Pin 9

Suppressor grid g3

Notes for audio and studio builders

The remote-cutoff transfer characteristic is the key difference between the EF183 and a conventional sharp-cutoff pentode. As the control grid becomes more negative, transconductance and stage gain decrease progressively. A suitable circuit can therefore use the EF183 as a voltage-controlled gain element in a tube compressor or another dynamic control stage.

Distortion, control law, noise and available headroom depend on the selected operating point, load, screen supply and circuit topology. The valve does not have one universal audio sound signature. Balanced or stereo gain-control circuits may benefit from similarly performing valves when their circuit design specifically requires matched control elements.

Because the EF183 has high transconductance and was designed for wide bandwidth, an audio build should use short signal paths, effective supply decoupling and suitable stability checks beyond the audible frequency range. It is not an output valve and is not intended to drive a loudspeaker directly.

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 assembled and tested with appropriate knowledge and safety precautions.

BTB has specialised in electronic valves and components for amplifier construction, professional service and workshop use since 1946.

Questions about selection and audio use

Is the EF183 suitable for a variable-mu tube compressor?

Yes. The variable transconductance of the EF183 allows gain to be controlled by the control-grid voltage, making it suitable as the gain-control element in a purpose-designed compressor circuit. It is not a complete compressor module or an unchecked substitute for other variable-mu valves. A selected pair may be useful in balanced or stereo circuits when the schematic requires two closely performing valves.

Are the EF183 and 6EH7 the same, and can an EF184 be substituted?

EF183 and 6EH7 are the corresponding European and American designations. The EF184 uses the same B9A pin arrangement and heater rating, but it is a sharp-cutoff pentode and does not provide the same gain-control characteristic. It is therefore not an equivalent replacement in compressor or variable-gain circuits.

Can the EF183 be used in a hi-fi preamp or guitar amplifier?

Yes, but only in a pentode or gain-control stage designed specifically for it. The operating point, anode load, screen supply, feedback and stability measures must suit the EF183. It is not a plug-in replacement for an EF86 or ECC83 and should only be fitted to an existing amplifier when the original circuit specifies it or has been properly redesigned.