• 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.


dejanm :
C15 is connecting signal ground to chassis ground. In other words connects from the ciruitry common bus bar or circuit card track or whatever this amp uses to the metal chassis. C1, C2, C3 and C4 do the same thing at the input.

Others may comment, but I am not sure if you want such a connection at both the input and the output. I thought it was more common just to have one.