How To Use a EF86
The Svetlana EF86 is a version of a small pentode which was popular in Europe for use in hi-fi equipment from 1952 through the 1960s.
Unlike the majority of pentodes of this type, the EF86 possesses low distortion, low hum induction and low ionization noise. Further, it has a built-in electrostatic shield, making an external tube shield unnecessary. No other tube in production today has all these features. In addition, the EF86 is an excellent replacement for any EF86/6267 tube previously made.
This bulletin will show a few typical circuit designs which apply the EF86 and make the best use of its unique capabilities.
Figure 1 is the classic connection for an EF86 as a maximum-gain audio preamp, and is derived from numerous technical sources, such as old professional-audio designs and the Mullard "Valve Circuits For Audio Amplifiers" textbook. With appropriate changes to the grid resistor, this circuit may be used anywhere a low-noise low-level preamp stage is needed. For example, making Rg=10 megohms allows the use of this stage in a condenser microphone. The output may be attached to a suitable plate-to-600-ohm transformer for driving a standard balanced line, or to a cathode follower for driving low-impedance unbalanced loads. If a preamp for use with a moving-magnet cartridge is needed, make Rg=47k ohms. If this circuit will be used as a pre-preamp with a moving-coil cartridge, Rg should be chosen to match the load required by the cartridge's manufacturer. This resistance is typically in the range of 10 to 1000 ohms. And for direct attachment to an unbalanced 600-ohm dynamic microphone, Rg=620 ohms. DC heater power is recommended for best noise performance. The internal shield (pins 2 or 7) may be connected either to the cathode or to signal ground.
Figure 2 is a suggestion for a very simple voltage-controlled amplifier for use in electronic-music equipment. Although distortion is greater than with conventional VCAs, this circuit gives a distinctive sound which has musical value. The plate voltage changes slightly with the screen voltage, so the capacitor Cf may be required to help smooth out the resulting "click" or "thump". Cf should be selected for the desired application--if the circuit is breadboarded, try a 0.1 uF capacitor. An extra feature of the EF86 is that the suppressor grid is separately accessible (pin 8), which allows its use as an additional CV input if desired, in a manner similar to the screen grid. Most small pentodes have an inaccessible suppressor, as it is internally connected to the cathode. The voltage gain of this VCA varies exponentially with the CV, making it suitable for most voltage-controlled music synthesizers with their exponential CV response.
Figure 3 shows the EF86 in triode connection. When this is done, the resulting triode is among the lowest-distortion electronic amplifying devices ever made. Typical second-harmonic distortion with 10v RMS output is on the order of 0.05%. This result is achieved without any negative feedback, so this circuit is an excellent choice for high-end audio design. The voltage gain will be about 25 in this connection, making it suitable for line stages.