In article , Patrick Turner
wrote:

John Byrns wrote:

There are a couple of other interesting effects that taking the AGC
voltage from the primary of the last IFT has. At first as we tune toward
the center of the pass band the voltage across the primary of the IFT
increases as expected, but then as we get close to the center the voltage
starts dropping, reaching a minimum when the carrier is centered in the
pass band. The AGC voltage follows and the effect is to sharpen the
apparent aural tuning characteristic because the decrease in AGC voltage
when center tuned increases the gain at that point.

If a tuning eye is fitted to this type of radio it is almost universally
connected to the audio detector rather than the AGC detector, which is
unfortunate from the AC loading point of view as the filter for the eye
tube often presents an even worse AC load than the AGC circuit. For the
longest time I couldn't figure out why the designers of most of these
radios made this apparently dumb mistake in connecting the eye tube, but I
eventually realized that if the eye tube was connected to the AGC line it
would have an unnatural and non intuitive visual tuning characteristic,
the dip at center tune might confuse the average consumer, although a
radio nut would probably delight in the effect as an indicator of exact
tuning.

But there is rarely a dip in the tuning character of most AM sets.

That was my point and is the reason sets with a tuning eye virtually
always connect the tuning eye to the audio detector, not to the AGC
rectifier.

But I often derive the AGC voltage separately from
a small cap&diode off the anode of the last IF amp, where more AGC voltage is
available because the envelope amplitude is greater than at the
secondary of the
IFT.

The IFTs can be aligned with a VTM attatched to the AGC voltage,
and the tuning done to generate the highest AGC.

Good luck with this approach, it is likely to result in misalignment, this
sort of set should be aligned either with a modulated oscillator and audio
VTVM on the output, or by measuring the detector voltage.

An interesting experiment on a set that takes the AGC from the IFT primary
is to short the secondary of the IFT with a clip lead, you will observe an
increase in AGC voltage when the secondary is shorted, this is the same
effect that causes the dip in primary voltage when properly tuned.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/