WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Thank you guys for all those hints.
Another low Zout preamp using 6N1P I found.
Actually this is a kind of 6N1P version of Foreplay.
All credits to Paul Joppa.
Warning: It is just a schematic, not built and tested circuit.