[ARC5] Question on IF transformers...

[Dennis Monticelli]

Ken,

Hard to tell without getting into the designer's head.

The tap is a "free variable" for the designer.  It costs nothing to take
advantage of it...or not.  The designer can use it to achieve max power
transfer if his IF gain is on the low side or he can throw away gain if he
wants to promote IF strip stability in the face of component/tube
variations or if he wants to achieve unloaded Q for max selectivity.  Exact
positioning of the taps is often one of the last choices made in the
overall design optimization and it was probably not a critical decision
with some of the stages in some of the receivers.  In other words, we may
be overthinking this a bit.

Dennis AE6C
On Sat, Oct 18, 2014 at 8:56 AM, Ian Wilson <ianmwilson73 at gmail.com> wrote:

> Hi Ken,
>
> I thought the answer to this would be because the plate resistance of
> a typical mixer of the era was lower than that of the IF amplifier
> pentodes.
>
> However, looking at the characteristics, the difference doesn't look
> enough (e.g. 400k vs 600k) to warrant a different IF amplifier connection.
>
> The answer may have to do with what a mixer load has to look like at
> frequencies other than the (desired) IF. You want the impedance at the
> plate to be very low for the input and oscillator frequencies, if possible.
>
> Looking forward to some illumination....
>
> 73, ian K3IMW
>
>
> On Sat, Oct 18, 2014 at 8:27 AM, Kenneth G. Gordon <
> kgordon2006 at frontier.com
> > wrote:
>
> > OK, Gang, this question is for those smarter than me (which shouldn't be
> > all
> > that hard):
> >
> > Why are connections to the 1st IF transformers in the receivers up
> through
> > the 1.5 - 3.0 MHz receivers NOT tapped down on the IF coil, while the 2nd
> > and 3rd IF transformers ARE tapped?
> >
> > In the 3.0 to 6.0 Mhz receivers, the 1st and 2nd are NOT tapped, while
> the
> > 3rd MAY be tapped, and in the 6.0 to 9.1 MHz receivers, NONE are tapped.
> >
> > Yet one can substitute a tapped-coil IF can for the un-tapped ones and
> > there
> > appears to be no difference in operation.
> >
> > In my (somewhat limited) experience, one taps down on a coil in order to
> > match impedances better AND to "unload" the coil, making the circuit a
> bit
> > more selective.
> >
> > If that is the case here, why would the ARC engineers want the 1st IF
> stage
> > to be less selective than the other two, or is there some other reason
> for
> > this
> > difference?
> >
> > Will someone please explain this apparent anomally?
> >
> > This is really bugging me....but then again, sometimes I "bug" easily.
> >
> > Ken W7EKB
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