[ARC5] Question on IF transformers...

[Ian Wilson]

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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