The output transformer spans the outputs of the two SRPP circuits and makes no connection to ground. The signal current flowing through the transformer is what is given to the load. If the transformer sees an anti-phase signal at each end of its primary winding, then that signal will be transferred to the output. If the transformer sees a in-phase signal at each end of its primary winding, then that signal will not be transferred to the output. Thus, no matter how much power supply noise appears at the output of these two SRPP circuits relative to ground, relative to the transformer's primary, the noise is in phase and cannot be transferred to the output. (Actually, this very rosy scenario only applies at near idle conditions: as the output signal nears full output, the power supply noise reappears, but at least it is masked by the greater signal. Nonetheless, a high quality power supply is still required to allow the best sound from this amplifier.)

   The coupling capacitor keeps the transformer safe from DC currents and its value is set by:
    C (in µF) = 159155/F/R,
where F = the lowest frequency that must be passed and R = the output transformer's primary impedance. If the output tubes were perfectly matched and were incapable of drifting with time, then the coupling capacitor could be eliminated. An alternative choice is to incorporate a DC servo to force the two top triode's cathodes at the same DC voltage.