[ARC5] Question on IF transformers...

[AKLDGUY .]

> 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?

At low frequency IF's, the 12SK7 will have more gain than it will at 2830
KHz
in the 6-9.1 MHz receiver. Here, the first IF transformer is not tapped in
order
(my opinion) to get best signal to noise ratio out of this first stage. The
2nd
and 3rd stages are tapped to reduce the gain at this low IF and to improve
the
bandwidth.

> In the 3.0 to 6.0 Mhz receivers, the 1st and 2nd are NOT tapped, while the
> 3rd MAY be tapped,

Again, at 1415 KHz the 12SK7 gain is lower than it was at the lower
IF's, and
we want to get the best signal to noise ratio out of the first IF stage and
max
gain from the others. The "3rd MAY be tapped" may be an indication that this
type of IF transformer is intended for use when the 12SF7 is fitted (maybe
it
has more gain than the 12SK7).

> ...and in the 6.0 to 9.1 MHz receivers, NONE are tapped.

Again, we want more gain at this even higher IF frequency. Presumably there
was no benefit in tapping for improved selectivity (gain more important
than a
modest improvement in selectivity).

It's all about gain distribution and thus stability IMO, as well as
bandwidth.
Too much gain can result in feedback (breaking into oscillation) when tubes
are changed, tolerances drift, etc.

The writer had some experience in the telecommunications industry with
distributed gain and bandwidth setting in broadband systems where, due to
multiple repeaters in an 80 mile link, the gain of each repeater had to be
flat
within 1 dB (+/- 0.5) from 60 KHz to 4.0 MHz and had to meet the target gain
at midband within 0.7 db. If you think we've got it tough, spare a thought
for the
telecomms engineers.

73 de Neil ZL1ANM