John Byrns wrote:
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.
It may not be critical where you connect the feed to the magic eye in many old
radios.
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.
It works, and its not neccesary to use a modulated carrier.
A carrier of exactly the 455 kHz can even be cap coupled via 10 pF applied to the
antenna terminal
with the set tuned as low as it will go, and the IF tuned up starting on the last
IF and
moving forward, and repeated, to generate the highest AGC.
Its also possible to shunt the AGC across the 0.047 uF cap at the tube grid
connections,
and keep the input signal low, and then tune for the highest AGC voltage
or detector voltage.
This method avoids connecting a lead via a cap to the anode of the mixer with a 455
kHz signal applied,
since such a connection upsets the C of the first IF, and mistunes it.
The shunted AGC allows the RF, mixer and IF tubes to operate with high gain
so to stay away from over load, the input signal should be low.
Using a modulated IF signal with 95% modulation with 100 Hz is a good idea,
because as the IF amp becomes overloaded either with negligible bias,
or with a lot of applied AGC, so to will the audio recovered.
I like to tune up old radios with a CRO attached and a high % of modulation,
and with AGC connected, because the set always works with it connected,
and when things are right, including the adjustment of R loading for the diode
detector,
there should be at least 5vrms of audio recovered from the detector.
This means about 28 volts peak to peak exist at the envelope
entering the diode detector, and maybe twice that
at the IF amp anode, if the IF transformer losses were 6 dB.
If the IF amp is a typical 6BA6 etc, and has idle Ia = 3 mA, and Ea = 200v
at the middle common AGC voltage, and ideal working point
for a load of say 50 kohms, then the load line analysis reveals about
300 volts peak to peak is the maximum voltage swing.
To cope with 100% modulation, the p-p modulation above and below the horizontal
axix is 150v, and peak modulation = 75v, so there is 53 vrms theoretically
available if the IF amp was fairly linear, which is won't be, ever, even if its a
sharp
cut off pentode and not the usual remote cut off type commonly used.
At such high output voltages, the IF amp becomes quite distorted, and
its clearly seen on the the audio recovered from the detector.
Many old radios produce about 3 vrms average audio signal at their detectors, and
the IF amp isn't working too hard, and the contribution to the set's thd by the IF
tube isn't too high.
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.
I have not tried that, but common sense tells me that a shorted IFT sec
will reduce the load seen by the IF amp severely, and reduce the envelope amplitute
at the IFT pri,
or the tube anode, and thus generate less AGC, despite the increase in GM of all
the tubes subject to
AGC control.
What happens depends on the number of preceeding stages and the gain
reaction to AGC.
Patrick Turner.
Regards,
John Byrns
Surf my web pages at, http://users.rcn.com/jbyrns/