On 02/23/2011 06:43 PM, Joel Koltner wrote:

1) The really big 450kHz IF transformers you see in tube sets... why did
they wind the coils in the form of "pancakes" rather than "the usual
way" (single-layer coils)? Is it just a consequence of needing lots of
turns (to get enough magnetizing inductance) but, for the coupling
coefficient desired, finding that you'd end up with, e.g,. a foot-long
tranformer if you only used a single layer?

They do this to try to keep the self-resonance of the winding up above
the operating frequency. By spreading the winding out into series
connected pancakes the stray capacitances have a harder time shunting
large inductances. You see this kind of construction on RF chokes too.

Paul Probert