I would assume that maximum selectivity occurs when the 85 kHz IF trasformers
actuators are fully pulled up (maximum distance between primary and secondary
windings).

Can any one please confirm or deny?

73

Tony I0JX

"Antonio Vernucci" wrote in message
...
I would assume that maximum selectivity occurs when the 85 kHz IF
trasformers actuators are fully pulled up (maximum distance between primary
and secondary windings).

Can any one please confirm or deny?

73

Tony I0JX

Yes. As the coils are more closely coupled the bandwidth increases.

Tony:
http://site298.webhost4life.com/barr...ccessories.pdf
would seem to confirm your theory.

www.qsl.net/k5bcq/HK/HK.html says: (you may have to move the figures
to the correct columns)
IF Transformer Center Freq BW -6dB BW -20dB BW -40dB
BC-453 IF (#12012) 85Khz 3.0Khz 10.0Khz 26.0Khz Notes (4)(6)
BC-453 IF (#12012) 85Khz 5.4Khz 14.0Khz --- Notes(5)(6)
two BC-453 IFs (#12012) 85Khz 1.8Khz 4.3Khz 11.0Khz Note ( 8)
Notes:
4.With the Micarda rod "OUT". BC 453 IF coils have a "Q" of about 50.
5. With the Micarda rod "IN" (over coupled)
6. One of the BC-453 IF trimmer caps was modified so both trimmers are
across the fixed padder capacitors.
With both Micarda rods "OUT" (not overcoupled) ....very sharp for SSB

************************************************** ****************
Antonio Vernucci wrote:
I would assume that maximum selectivity occurs when the 85 kHz IF
trasformers actuators are fully pulled up (maximum distance between
primary and secondary windings).

Can any one please confirm or deny?

73

Tony I0JX

"Antonio Vernucci" wrote in message
...
I would assume that maximum selectivity occurs when the 85 kHz IF
trasformers actuators are fully pulled up (maximum distance between primary
and secondary windings).

Can any one please confirm or deny?

73

Tony I0JX

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

"Lynn" wrote in message
. ..

"Antonio Vernucci" wrote in message
...
I would assume that maximum selectivity occurs when the 85
kHz IF trasformers actuators are fully pulled up (maximum
distance between primary and secondary windings).

Can any one please confirm or deny?

73

Tony I0JX

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
Los Angeles, CA, USA

Thanks to all for your useful answers.

Yony I0JX

"Richard Knoppow" wrote in message
...
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
Los Angeles, CA, USA



Some Silvertone 1930s vintage consumer radios have the same
arrangement.

Pete

"Uncle Peter" wrote in message
...

"Richard Knoppow" wrote in
message ...
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
Los Angeles, CA, USA



Some Silvertone 1930s vintage consumer radios have the
same
arrangement.

Pete

Hammarlund used to offer adjustable IF cans as stock
items. I have somewhere a circuit for a high fidelity AM
receiver using a couple of them. I didn't know any home type
sets were made with them. The advantage of the Hammarlund
arrangment over the tapped terciary coil system used by
Hallicrafter's and some others is that the pass band stays
symmetrical about the center frequency. In some other
arrangements the bandwidth expands in only one direction.
The SP-600 uses a tapped coil arrangement which does not
have this problem and also expands symmetrically.


--
---
Richard Knoppow
Los Angeles, CA, USA

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

"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.


--
---
Richard Knoppow
Los Angeles, CA, USA