[HBR] IF Transformer Theory Questions

[Hopperdhh at aol.com]

BJ,
 
Very good questions!
 
I'll take a crack at your first question.
 
The difference in selectivity is due to whether the capacitive coupling aids 
or opposes the inductive coupling.  In tube type high impedance circuits, even 
1/4 to 1/2 pF of capacitance between the two coils can make a big difference 
in the response of the transformer.
 
A good reference and a very complete answer to your question can be found in 
the Meissner "How to Build" Instruction Manual (copyright 1943) reprinted by 
Lindsay Publications Inc, Manteno, IL.
 
For the second part of question 1: For economic reasons both coils are 
usually wound at the same time, therefore in the same direction.  The coupling 
coefficient would be the same if they were wound in the opposite directions.
 
73,
 
Dan Hopper K9WEK
 
 
 
In a message dated 10/4/2007 8:58:08 PM Eastern Standard Time, 
bjtatum1 at peoplepc.com writes:
Hello-
If I could bother anyone with the knowledge and experience in working with 
tube circuit IF transformers I would like some help in these areas: (455 KC 
units)
1. The subject of reversing the leads of one coil, and said coil is wound in 
opposite direction of other coil. I noticed this done on an IF transformer, in 
comparing it with a very similiar one using the same coils, same spacing, 
perm tuned, and same fixed capacitors. The former transformer was rated at 9 KC 
selectivity at 6 dB down (diode transf.), the latter was rated at 12 KC 
selectivity at 6 dB down (input / interstage transf.).
What occurs if you simply build an IF transformer with coils wound in 
opposite directions, is the coupling greatly reduced? Is this why they also swapped 
the leads around on the coil wound in opposite direction, to only slightly 
reduce the coupling and gain an improvement in selectivity?




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