On Jul 1, 1:58*am, John Byrns wrote:
In article ,
*Patrick Turner wrote:
snip for brevity.
John Byrns
Surf my web pages at, *http://fmamradios.com/
You grasped the idea perfectly well John. This frequency compensation is
useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio
amp found in those boring AA5 style radios.
I agree with everything Alex says below except for a few things.
In most old radios with DC flow across the volume control pot track
the adjustment of the volume is often very noisy after the pot has
aged a few years.
So instead of the conventional arrangements put forward by RDH and
most others to avoid parts costs I will have the last IFT coil biased
at say +50V at one end, and the live end goes to a triode grid of 1/2
12AU7 CF buffer to remove any loading effect of diode detection on the
last IFT LC.
What is the purpose and or advantage of using a cathode follower between the IFT
and detector diode, especially if you are going to add 100k resistors to widen
the IF response as you discuss below?
The last IFT coil is a high impedance tuned circuit signal source. If
one is going to load it slightly to slightly reduce the Q thus
widening the pass band and AF response then using a pure resistance is
benign. The CF converts the high Z source to low Z source and all the
crapological behaviour of the Ge diode dissappears.
One could still use a 6AL5 if one wanted to.
Then I use Ge diode feeding RC circuit, and this can
directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
any a volume control and while employing time passive poles to give
say -3dB at 30Hz before any power amp which has NFB. I often add in
another 12AU7 gain stage for tone control to boost/cut treble; bass in
AM is usually OK.
To slightly widen AF response the Q of all IFTs may be reduced by
strapping 100k across each coil. It doesn't work in all sets, but may
be tried.
The coupling between the two coils in an IFT often needs to be increased
somewhat when adding resistors, that may explain why it doesn't work in all
sets, if the IFTs are over coupled adding resistors may ne counter productive.
Try things. Predjudice don't belong anywhere when you want to make old
junk meet modern expectations.
In my kichen set I have a knob on front which changes IF gain. This is
done by varying the distance between the two coils on IFT1 so they can
be slightly over coupled after the set is tuned with coils apart.
Magically, the AF bandwidth nearly doubles. Its because the slight
over coupling creates a double IF peak in response which complememts
the single peak of the IFT2, and you get a wider pass band but skirt
selectivity is still very good. I recall Halicrafters had a
communications set with all 3 IFTs having mechanically variable
selectivity. This was a boon to radio hams.
Related to this is the fact that the instruction manual for Heathkit's first AM
tuner kit recommended placing a resistor(s) across the first IFT if a narrower
bandwidth was needed, so things don't always work as expected. *
I've always found R across IFT tanks reduced Q and increased BW, and
tended to make circuits less likely to oscillate.
The reason the
resistor(s) narrowed the bandwidth in the Heathkit is because it was a wideband
tuner with an over coupled 1st IFT, so adding the resistor(s) takes the
transformers close to critical coupling, giving a narrower response. *
Most ordinary radios have lots of selectivity but 2kHz of AF BW - if
you are lucky.
I don't
remember if the Heathkit mod for reducing the bandwidth involved adding
resistors on both the primary and secondary, or on only one winding, I will have
to see if I can find the manual, IIRC they also used a resistor or resistors
during alignment to eliminate the over coupling, allowing the tuner to be
aligned by peaking the IFTs.
There is a simple method explained in RDH4 to add a few
turns of fine wire around the primary of IFT1 and switch it to being
in series with the sec of IFT1 which will give a doble peak to IFT
response which effectively increases IF channel bandwidth from a
typical 4kHz to 8 kHz. Quad used it.
QUAD's first superheterodyne AM tuner, the Acoustical AM tuner, used a different
scheme to vary the bandwidth. *Rather than using an IFT they used two separate
IF coils with low side capacitive coupling, the low side capacitive coupling
could be switched to vary the bandwidth.
There are a number of things doable to widen IF BW, but doing them is
always easier said than done.
Patrick Turner.