• 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.

Gain stage and phase

Group Delay might have meant something totally different to him.
Instead of the 'together timed arrival' of different frequencies that started out at the same time; versus 'separate timed arrival' of different frequencies that started out at the same time.

Did he formerly work for the military or a 3 letter US agency?
I remember the guy who heard some kind of sound on the radio, that caused a flashback, and he suddenly jumped under the pool table.