Re: Output transformer HT connection.
Assuming the filter choke is rated to carry the amp's full HT current and the second filter capacitor (i.e. the first of the 'smoothing' capacitors) has a high enough value to supply any audio AC drawn from it, then you might as well connect the OP transformer centre-tap to that second filter capacitor.
If the output valves are tetrodes or pentodes, the amp is not conventionally ultralinear and the choke and/or second filter cap is a bit weedy then you might do better to connect the OP tranny's CT to the first filter cap (the 'reservoir') and use the choke and second filter cap to supply just the output valves' screen grids. If the quiescent DC currents through the two halves of the transformer primary are sufficiently well-balanced then the action of the transformer will null out hum due to the 100Hz HT ripple. It won't null out the ripple's 100Hz modulation of the output stage gain though, which will lead to +/-100Hz sidebands on the audio signal.
EDIT: Crossed with Robert, who makes some of the same points.
Cheers,
GJ
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