Zenon wrote:

When sizing the wire size for a power transforner sec. am I right in
assuming that for a non centre-tapped secondary using a silicon bridge
rectifier the wire size will need to be twice the x-section to that of a
centre-tapped secondary using a GZ34 tube rectifier.

Yes.

Logic leads me to
believe that a non centre-tapped sec. carries the full load where a with a
centre-tapped sec. each half carries only half the load hence need only half
the cross-section. A centre-tapped sec. has twice the number of turns and if
I use the same wire size as for a non centre-tapped sec. there isn't enough
room to fit the turns on.

Yes,

Going to a larger core size for a larger winding
window defies logic as the power requirements havn't increased. Does anyone
know?

The same size of core should be able to be used for a CT winding or
non CT winding of the same overall VA.
Let us suppose you have 560 turns of 0.5mm dia wire for C sec,
then the area occupied by such a winding will be 560 x 0.55 x 0.55 = 140 sq.mm.
This is allowing 0.05mm for enamel insulation on the copper.
For the non CT winding, 280 turns of 0.7 wire are required,
and the area of that is 280 x 0.75 x 0.75 = 156 sq.mm, very close to the former
amount.

There will be less layers to wind with the non CT winding, so less layers of
insulation between windings.

Non CT windings only ever became popular when reliable silicon rectifiers became

available, because to make a bridge using tube rectifiers meant the PS output
resistance was much higher, and the use of a much more complex set of tube
rectifiers was needed,
and the cost of the extra tubes and their heater windings was much greater than
the cost
of the extra turns on a centre tapped winding.

One can go one step further with a 1/4 of the turns of the CT winding,
and then use just two silicon rectifiers in a voltage doubler PS, and this has
better reg
than any tubed supply, and is so simple, and easy to wind.
I use this idea even in amps of 300 watts with 12 x 6550, so that
a 200vrms winding produces over +500v at 0.5 amps, and all I have is two
6 amp x 1,000v rated diodes, a quad of 470 uF caps, a 1.8H choke, and
a another 6 x 470uF caps as the anchor for the CT.

Many orther commercial amps used doubler suppies the day after
decent silicon rectifiers were invented.

Patrick Turner.