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.
Also do a search on 6BL8 which is the same tube.
There were certainly some 6BL8 preamp designs in the "Electronics Australia" magazine back in the 1970's. Will check my file tonight.
Cheers,
Ian