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.
I have measured mu in the range of 77 - 80 in about a dozen Siemens & Halske D3A that I tested some years ago. (Measured with a cascode CCS at 15 - mA, and similar numbers were achieved midband with a transformer)
