Is it common practice to wind HV secondaries with the CT tap as the two outer
layers and the outputs as adjacent layers in the middle?

wsc wrote:

Is it common practice to wind HV secondaries with the CT tap as the two outer
layers and the outputs as adjacent layers in the middle?

No.

This method would bring the ends of windings with a high potential difference
close together, where they could arc, since a 350-0-350 tranny
has 1,000 peak volts between the HV sec ends.

I wind the primary on first, then the heaters in one layer,
with one end or CT earthed to the chassis, not to the 0V,
so the heater winding is like a shield, then the B+ winding goes on,
and finally the bias and perhaps another heater which is rectified for the
DC to V1 heaters.

I use silicon diodes, and a voltage doubler so the B+ winding
is only 190v to get 500v B+.
I sometimes wind for use with a bridge.
But if you are using a tube rectifier, you gotta use the CT
full wave winding, and If I did that it would have the CT half way up
the winding.

People say doublers have poor regulation.
But they were used by McIntosh, the day after decent reliable silcon diodes
were invented.
And with large caps, the regulation is far better than any tubed full wave
rectifier.
The doubler with SS is even better regged than a choke input filter
with a swinging choke, unless it's enormous, and has a negligible DCR.

These comments need to be considered in the light that
most class AB power amps don't sustain much B+ shift
at all during hi-fi music operation, even though their supplies would droop
15% with a sine wave at clipping, and a lowish load and fixed bias.

Patrick Turner.