IF860 Linear Audio Post Tube ähnlich EF800 aber Uf=18V

IF860 RFT long-life pentode for value-focused high-end audio DIY

The IF860 is a high-transconductance RFT pentode developed for long service in professional telecommunications equipment. The historical RFT data sheet specifies it for preamplifiers in long-distance communication systems and identifies it as corresponding technically to the EF800. Its approximately 7 mA/V transconductance and explicitly stated production tolerances make it an interesting sleeper valve for custom audio gain and driver stages.

The electrical system is closely related to the EF860 and EF800. Compared with the more familiar EF80 family, the professional EF800 concept places greater emphasis on service life and controlled electrical characteristics. The IF860 is nevertheless not a plug-in substitute because its heater requirement differs substantially from the usual 6.3 V types.

Relevant applications

• Purpose-designed tube preamplifiers and line-level stages
• Voltage-gain and driver stages in hi-fi amplifiers
• Balanced, differential and channel-matched audio circuits
• Experimental clean-gain and driver stages for guitar amplifiers
• Studio, laboratory and measurement projects
• Repair of telecommunications equipment designed for the IF860

Technical specifications

Valve type

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

Manufacturer and original quality class

RFT professional telecommunications valve for long-distance-system preamplifiers

Related valve types

The IF860 is the differently heated version of the EF860. The RFT data sheet identifies the EF860 and IF860 as corresponding to the EF800. The EF80 is technically related but is not the same long-life professional version

Base

Noval B9A, 9-pin

Heater specified by BTB

18 V at 100 mA

Heater specified by the historical RFT preliminary sheet

20 V at 95 mA, with a stated heater-current tolerance of ±6 mA

Typical preamplifier operating point

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

Transconductance and internal resistance

7.5 mA/V transconductance and 0.5 MΩ internal resistance at the 170 V operating point

Screen-to-control-grid amplification factor

µ_g2/g1 50

Specified characteristic tolerances

At Va and Vg2 of 210 V, RFT specifies Ia 10 ±1.5 mA, Ig2 2.6 ±0.5 mA and transconductance of 7.1 ±1 mA/V

End-of-life criteria

The RFT sheet defines limits including Ia below 6.7 mA, Ig2 below 1.6 mA, transconductance below 5 mA/V or control-grid current above 1 µA

Maximum anode voltage

250 V

Maximum anode dissipation

2.1 W

Screen-grid limits

Maximum Vg2 250 V and maximum screen-grid dissipation 0.55 W

Maximum cathode current

13 mA

Maximum heater-to-cathode voltage

150 V

Dimensions

Approximately 22 mm in diameter and 67 mm in overall height including the pins

B9A base connections – bottom view

Pins 1 and 3

Cathode; the two cathode connections are joined as shown in the data sheet

Pin 2

Control grid g1

Pins 4 and 5

Heater

Pin 6

Internal shield

Pin 7

Anode

Pin 8

Screen grid g2

Pin 9

Suppressor grid g3

Notes for serious tube-audio builders

The IF860 may be used in a purpose-designed pentode stage or in a correctly engineered triode-connected circuit. It is not a power-output valve and is not intended to drive a loudspeaker directly. Available gain, linearity, headroom and noise performance depend on the chosen operating point, anode load, screen supply, feedback arrangement and power supply.

Because the IF860 is a high-gm valve originally intended for wideband service, practical audio layouts should use short signal paths, effective supply decoupling, careful grounding and suitable grid or screen-stopping resistors where required. Stability should also be checked above the audible frequency range.

Heater-data notice: BTB lists the offered valve at 18 V and 100 mA, while the historical RFT preliminary sheet specifies 20 V and 95 mA. A new design should therefore be based on the marking of the supplied valve, the offered production version and the appropriate data-sheet reference. The IF860 must not be inserted into equipment designed for an EF800, EF860 or EF80 without adapting the heater supply.

For balanced, differential or two-channel designs, the IF860 BTB matched pair is available separately. BTB describes the pair version as German-made and fitted with gold pins. Matching is particularly useful when a circuit requires two valves with similar operating characteristics.

Safety notice: Tube circuits may contain lethal voltages. Construction, measurement and commissioning should only be undertaken by suitably skilled users. BTB has specialised in electronic valves and components for amplifier construction, workshop service and professional tube technology since 1946.

Questions about selection, substitution and matching

Why is the IF860 interesting for value-focused high-end tube-audio DIY?

The IF860 combines sleeper-tube pricing with a professional long-life telecommunications design. The RFT data sheet gives defined tolerances, transconductance of about 7.1 to 7.5 mA/V and a typical anode current of 10 mA. In a purpose-designed small-signal or driver stage, these characteristics provide a useful engineering basis. They do not guarantee a particular sound; the operating point, power supply, circuit, layout and component choices remain decisive.

Can the IF860 directly replace an EF800, EF860 or EF80?

No. The IF860 and EF860 use the same B9A connection sequence, and the RFT data sheet identifies them as corresponding technically to the EF800. The EF80 is closely related but is not the same professional variant. Most importantly, the IF860 requires a different heater supply: BTB lists 18 V at 100 mA, while the historical RFT sheet specifies 20 V at 95 mA. The heater supply and operating point must therefore be checked and adapted.

When is the BTB matched pair version of the IF860 useful?

A matched pair is useful when two valves should operate similarly in a balanced or channel-matched circuit, such as a differential stage, push-pull driver or stereo pair. Matching normally provides no direct benefit in a single unbalanced one-valve stage. BTB offers the IF860 separately as a matched pair.