Re: build a flyback?

Hi Jan,
          In answer to your original question: No, an E-I core will not make 
a good flyback, for the reason that it has far too high rf losses in the iron 
core and if you could get a suitable core would take you several eons to wind 
the secondary.
It is however very possible to drive more power into your existing flyback.  
A flyback from a small colour TV will easily take 200~250W.  A large unit 
will stand an easy 500W and you may be able to drive it as high as a kW or 
so. (You can try for 2kW but I really wouldn't reccommend it.  The voltage 
destroys the diode and then the drive current blows out the primary.)
To achieve the increase in drive power you may want to either increase the 
drive voltage or put a higher gain transistor in the drive stage.  If you 
want to keep the transistors you are using at the moment then a darlington 
pair or a simple collector coupled design would work fine.

Have Fun
Nick Field