Double primary windings in SE output transformers

[獅子口]

Is it possible to prevent pre saturation of a SE output transformer by

adding an identical primary winding which has a steady DC flowing

in opposite with the original primary?

What is the disadvantages of it other than higher cost?

Any successful project made?

[Phil Allison]

"獅子口"

> Is it possible to prevent pre saturation of a SE output transformer by
> adding an identical primary winding which has a steady DC flowing
> in opposite with the original primary?
> What is the disadvantages of it other than higher cost?
>

** How do you intend DC current be passed through the extra winding ??

From a PSU or a battery ??

That would place a hard short on the winding and since all windings on the
same core are magnetically coupled that shorts out the tranny.

If you did it with a tube set up as a current source - then PP operation
is just a tiny step away.

Maybe it could be done with a large, suitably gapped, iron core inductor in
series with the winding - bit like another SE tranny - but why bother
?

.......... Phil

[Gregg]

Here's a forum thread on the exact topic.

http://geek.scorpiorising.ca/cgi-bin/yabb/YaBB.pl?board=projects;action=display;num=1089659861

--
Gregg t3h g33k
"Ratings are for transistors....tubes have guidelines"
http://geek.scorpiorising.ca

[Patrick Turner]

"獅子口" wrote:

> Is it possible to prevent pre saturation of a SE output transformer by
>
> adding an identical primary winding which has a steady DC flowing
>
> in opposite with the original primary?

The extra winding will need to be equal turns to the P winding,
and take up a lot of room, and it will need a CCS feed which is capable of

standing the B+ voltage and the 200 vrms signal swings.

But if the anode current is 70mA, and P to S OPT turn ratio was 25:1,
then 25 x 0.07A = 1.75 amps could be sent through the secondary from a
CCS using SS devices.
But since the sec has some DCR of say 0.2ohms, then 0.35DV appears at the
sec,
which makes a DC current in the speakers, so cap coupling with 10,000 uF
is needed
to the speakers.

What a hassle!

After going to all that trouble, there would be only a marginal
thd improvment. at mainly lower F, since the tube thd dominates
the thd.

If you don't want DC in the OPT core, use a para feed choke to get the DC
to the
tube anode, using a choke at least as big as the SE OPT, and then cap
couple a
normal PP output transformer from the anode, which can be set up as an
auto transformer
with P in series with S and the S grounded.

Patrick Turner.