"Joel Koltner" wrote

What stymies me about the double-tuned transformer is this: If you look at
Hagen's exaplanation for how they can be modeled -- I've stuck a scan
he http://koltner.com/Hagen.png -- he's modeling it as the primary's
parallel capacitor is resonating with the magnetizing inductance of the
IFT and the secondary's parallel capacitor is resonating with the leakage
inductance... and this is then made more obvious if you use the high-Q
approximation and transform a parallel RC circuit into a series RC
circuit. But if that's the case... doesn't it seem as though the most
straightforward way to use an IFT would be to have a parallel capacitor on
the primary and a *series* capacitor on the secondary?

I realize that you can move impedances from one side of an IFT to another
and change the equivalent circuit model and so on and present numerous
different "views" that all end up with the same correct mathematical
behavior to model what are really just two coupled inductors, but still...
does anyone using a series resonating capacitor on their secondaries?

If you use a capacitor in series with the winding to create series resonant
circuit, then the impedance at resonance will be low.

For a tube IF, you really want high impedances at resonance.

In the plate circuit this is to minimise current draw.

The grid input of the preceeding stage is high impedance and is voltage
driven, therefore a parallel tuned circuit is ideal.

So hence the use of dual parallel tuned circuits.