"Lynn" wrote in message
. ..
Yes, rod pulled out for maximim selectivity (highest
"Q")............ the reciever's front end needs a pretty
stable local oscillator to keep a signal inside the new,
85kc IF bandpass.
Old Chief Lynn, W7LTQ
It isn't the Q but rather the mutual coupling between
primary and secondary windings. Hammarlund had a patent
on this kind of transformer which it used in the
Super-Pro series of receivers. In the Super-Pro two of
the IF transformers have movable coils which are
controlled via cams operated by the selectivity knob on
the front.
Richard Knoppow
Well, to be sure, the scheme (old as you and others
remember) sure
does change the mutual coupling between the primary and
secondary windings, but mutual coupling as a factor by
itself does not affect
bandpass. The mutual coupling adjustment, in this
instance, by moving one coil
into or out of another's magnetic field does affect the
"Q" or "quality" factor of
the tuned circuit's inductor. This, of course affects the
resonant bandpass shape,
Changing the coil's mutual coupling mechanically
(pulling the rod) also
affects the tuning of each coil slightly, and it was
sometimes recommended to
"repeak" the tuning of the '453's cans after changing the
coupling. I found that
I could not tell any difference by "ear", and in those
days (late 1940's) I had no sweep generator
or oscilloscope to "see" what was happening.
Old Chief Lynn, W7LTQ


I tried to find an illustration of the effect I am
talking about on the web but could not. It would make things
simpler.
Q is a measure of the ratio of inductive reactance of
an inductor to resistance. the higher the value of Q the
better the inductor but there are circumstances where the Q
may be delibrately limited. The bandwidth of a resonant
circuit at resonance is affected by Q, in fact, the
definition of Q is the ratio of the half-power bandwidth to
the resonant frequency. Varying the Q of a resonant circuit
also varies the amplitude, the lower the Q the greater the
losses and the lowe the amplitude.
Varying bandwidth by varying the mutual inductance of a
transformer behaves in a different way. Up to a value of
coupling known and critical coupling the bandwidth of the
transmission curve does not change significantly but does
increase in amplitude. If coupling is increased beyond
critical the transmission curve becomes double peaked. Where
there is no other coupling than magnetic the two peaks are
symmetrical around the center frequency. Their deviation
from the center frequency increases as coupling is increased
but the amplitude does not decrease until very large values
of mutual inductance are reached. The Q of neither side of
the transformer is affected.
There are many variations on the idea of providing for
variation of mutual inductance. The Hammarlund method, using
a physically moving coupling coil, allows the coupling to be
varied without introducing variations in capacitance. Other
methods, such as the one used in the well known
Hallicrafters SX-28, vary both mutual inductance and
capacitive coupling so that the two peaks gotten with more
than critical coupling are not symmetrical about the center
frequency. In fact, one tends to stay about at the center
frequency while the other moves.
It is possible to get symmetrical variation without
using a moving element and this is done in some later
variable coupling IF tranformers. Again, there is no effect
on the Q of either circuit.
Now, the bandwidth of an IF or RF transformer at
critical coupling _is_ affected by the Q of the component
coils which also affect the efficiency of the transformer.
However, the variation of this Q is not generally used to
vary the bandwidth of the transformer.
All of this stuff is covered in many books on receiver
design and basic circuit theory. The trick is finding one
which is not overly mathematical.


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Richard Knoppow
Los Angeles, CA, USA