IL861 Endpentode Postqualität lineare Uf20V Version der E81L

IL861 RFT long-life output pentode for drivers and compact audio power stages

The IL861 is a high-transconductance RFT output pentode designed for long service in professional telecommunications equipment. The historical RFT data sheet specifies it for output amplifiers in long-distance communication systems and relates it directly to the E81L and 18046. BTB's NOS gold-pin version is therefore an unusual value-focused option for custom hi-fi, studio and amplifier projects.

In a purpose-designed audio circuit, the IL861 can operate as a voltage-gain stage, power-valve driver, transformer-coupled preamp output or compact single-ended output valve. The original data includes both a high-gain preamplifier condition and a 1 W output-stage condition. Its 20 V heater means that it must not be plugged directly into equipment designed for an E81L or EL861.

Relevant applications

• Driver stages for larger audio output valves
• Voltage-gain stages in hi-fi, studio and guitar amplifiers
• Transformer- or capacitor-coupled preamplifier outputs
• Compact single-ended audio output stages of approximately 1 W
• Experimental line amplifiers and active output stages
• Service of professional telecommunications equipment using the IL861

Technical specifications

Valve type

High-transconductance output pentode designed for long service life

Production version

Historical RFT NOS telecommunications valve with gold-plated pins

Related valve types

The RFT data identifies the E81L and 18046 as corresponding types; the EL861 uses the same valve system with a different heater

Base

Noval B9A, 9-pin

IL861 heater

20 V at 120 mA, with a heater-current tolerance of ±7 mA in the RFT data

E81L and EL861 heater

6.3 V at 375 mA according to the RFT correction sheet

Dimensions

Maximum bulb diameter 22.2 mm, approximately 60 mm bulb height and approximately 7 mm pin length

Weight

Approximately 14 g

Static reference condition

Va 210 V, Vg2 210 V, Rk 120 Ω, Ia 20 ±3 mA and Ig2 5.3 ±1.2 mA

Transconductance

11 ±1.5 mA/V at the static reference condition

Internal resistance

Approximately 0.3 MΩ

Equivalent noise resistance

Approximately 1.2 kΩ at the static reference condition

Preamplifier example

Va and Vg2 210 V, Ra 20 kΩ, Rk 180 Ω, Ia 15 mA, Ig2 4 mA, transconductance 10 mA/V and voltage gain approximately 175

Small output-stage example

Va and Vg2 210 V, 15 kΩ load, Rk 120 Ω, Ia 20 mA and 1 W output at 5% distortion

Maximum operating anode voltage

210 V; maximum cold anode voltage 550 V

Maximum anode dissipation

4.5 W

Screen-grid limits

Maximum operating Vg2 210 V and maximum screen dissipation 1.2 W

Maximum cathode current

30 mA

Maximum heater-to-cathode voltage

120 V

Maximum control-grid resistance

250 kΩ with fixed bias or 500 kΩ with cathode bias

B9A base connections – bottom view

Pin 1

Internal shield

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 hi-fi and studio builders

The valve's professional origin, specified production tolerances and defined end-of-life limits provide a useful engineering basis for durable audio circuits. They do not create a guaranteed sound signature. Noise, linearity, headroom and distortion remain dependent on the operating point, screen supply, load, feedback, power supply and physical layout.

The documented 1 W output condition requires an output transformer that presents an anode load of approximately 15 kΩ. The stated 5% distortion applies at full data-sheet output. A designer targeting lower distortion may choose reduced output, local feedback or a different operating condition.

For preamplifier-output duties, the IL861 is suitable for a high-impedance capacitor-coupled load or a correctly designed output transformer. A loudspeaker, low-impedance headphone or 600 Ω input must not be connected directly to the anode without suitable impedance conversion.

Its high transconductance calls for short signal paths, effective anode and screen-supply decoupling and, where required, control-grid and screen-grid stopper resistors. Stability should be checked above the audible frequency range as well as within it.

The RFT document specifically links service life to correct heater operation. A new design should therefore provide a reliable 20 V heater supply rated for 120 mA per valve.

BTB's selectable matching option is relevant when two valves are used in stereo channels, differential stages or push-pull circuits. Pair matching normally offers no direct advantage in a single-valve, single-ended stage.

Safety notice: The 20 V heater requirement is not the anode-supply voltage. Valve circuits may contain lethal voltages and should only be built, measured and commissioned by suitably skilled users employing appropriate safety precautions.

BTB has specialised in electronic valves, amplifier components and professional tube technology since 1946.

Questions about selection and audio use

Why is the IL861 considered a sleeper valve for value-focused high-end audio DIY?

The IL861 combines low sleeper-tube cost with the professional construction of a long-life telecommunications output pentode. The RFT data specifies tolerances for anode current, screen current and transconductance, together with practical preamplifier and output-stage conditions. The NOS gold-pin version is therefore an interesting basis for driver, line-level and low-power output stages. The valve itself does not guarantee a particular sound; circuit design and construction remain decisive.

Can the IL861 directly replace an E81L, EL861 or 18046?

Not without checking the heater supply. The valve system and B9A pin arrangement are closely related, but the IL861 requires 20 V at 120 mA. The E81L and EL861 use 6.3 V at 375 mA. Published heater-current data for the 18046 can also differ. The valve marking, heater circuit, operating point and equipment schematic must therefore be verified before substitution.

How can the IL861 be used as a driver, preamp output or small output valve?

For preamplifier service, the data specifies 210 V on the anode and screen, a 20 kΩ anode load and 15 mA anode current. As a compact output stage, it provides approximately 1 W at 5% distortion with 210 V, 20 mA and a 15 kΩ primary load. A suitable output transformer is required for a loudspeaker, while a line or preamp output must be designed for the intended coupling method and load impedance.