Hmm.  Saying there is a difference in output isnt what I meant...   Sorry.  Its the fact that say 50vac across 1 primary gives us 100vac out of the sec, and why wouldnt there be a difference in output of the sec when we parallel the second primary with the other? Just seems to me that adding the second primary with the first primary at 50vac in would have more turns of primary with the same voltage across each turn for each case, 1 primary at 50vac vs 2 primaries in parallel with 50vac in. Why isnt there a difference in sec voltage from 1 case to the other.

Ok. Just rethinking this...   There is something about this that is driving me a lil batty.  lol And it has to do with the bifi coil.  Im beat.  Will try to put what Im thinking together tomorrow.

2 50 turn primaries and 100 turn sec.   

50vac across 1 primary winding of 50 turns. 1v across each turn. Sec 100turns, 100vac, 1v across each turn.
Now parallel the 2 50 turn primaries. Now its 100 individual turns of 1v each turn, but still 100vac out on the sec, 1v each sec turn.
If we had 10 primary windings in parallel of 50 turns each and 50vac in, that would be 500 turns of 1 vac on each turn, still only 100vac on the sec.  Doesnt it seem odd?
I dunno. maybe im just  at the moment.

mags

Ok. Final for tonight...
So in any transformer, every 'single turn', primary and secondary are the same vac.  Basics, but I just have something on the tip of my mind with the bifi on this thats bugging me. Ok, so now im going to apply that to my bifi project and see what happens.
Mags

Hi Mags,

I assume in your example the L inductance of primary coil 1 equals to that of primary 2 within an accuracy of a few %, say  +/-    1-2%. If you are thinking of making the two primaries bifilarly, then the L inductances will be quasi identical if measured separately.

Now if you mean the two primaries will be paralleled by connecting the two start wires together and also connecting the two end wires also together and feed the now single start wire and the now single end wire with the 50 VAC, then you will surely have 100 VAC out of the secondary again: what you have is still a 1:2 step up transformer which has a primary coil wound with a thicker wire versus the case when you fed only one of the primaries with 50 VAC and also had 100 VAC across the secondary. You simply reduced the copper loss in the primary coil (less heat will develop in the transformer). You may also say that a little bit more current would be used for the AmperTurns to create the magnetic flux: the overall efficiency of the thransformer improves a little.  Does this make sense for you?
Gyula

Just one question (again) Tito : can this be made in low voltage electronic circuit as a proof example or it works only in hv ?