Re: OPT equalising circuit

"Patrick Turner" <info@[EMAIL PROTECTED]
> wrote in message
news:481C8DCE.3448A31C@[EMAIL PROTECTED]
> > > I repair, rewire, or seriously modify at least 5 AM radios per year.
> > > I've done 3 this year and there are 3 waiting to be done, and its
only
> > > May.
> > >
> > > I built a complete radio in 1999, 10kHz of AF bandwidth, variable
> > > selectivity, linear IF amp, linear detector, linear AF amp with
> > > EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from
1953
> > > with a dome tweeter from 1972, because the 12" speaker is fairly
flat
to
> > > 5kHz.
> > >
> > > I have never heard any other AM sets that come close except
> > > the one in the AM-FM tuner at my website, which I designed, and of
> > > course the
> > > Quad AM tuner all tubed, that isn't bad at all.
> >
> > Could you please give a link to your website? Is there a schematic of
that
> > radio, mentioned above, which you designed? Is it TRF?
> > Regards,
> > Alex
> >
> > > Patrick Turner.
>
> Try
>
> http://www.turneraudio.com.au/am-fm-radio-tuner-multiplex-decoder.html
>
> Patrick Turner.
Interesting.
My comments:
1. I am not that fanatical to build a stereo-decoder around LC tanks which
require precise tuning.
2. It was funny to see a reactive tube for AFC instead of a tuning diode
(or
any diode like 1N4148).
3. As far as the AM detector is concerned, I think it is not optimum.
Voltage on the detector appears to be about +35V. Forward bias current is
about 90uA. Differential impedance of the diode is about 250ohm (no
signal),
220pF capacitor impedance is about 1.5K at 455KHz. Thus at no signal the
CF
is loaded by about 1.5K. The CF, running at 1mA DC current would have
about
0.5mA/V transconductance, therefore output impedance is about 2K. Thus the
gain of the CF is only 0.4 or so.

For the detector to begin detecting, obviously the AC current through the
diode shall exceed the DC component (90uA). It will require about 150mV on
the input to the detector or conversely 400mV on the grid of the CF.
Sensitivity of such a detector is rather poor (as you know, sensitivity of
a
pure p-n diode is about 25mV.)

It is easy to improve the result 10 times -- just reduce the bias. Connect
R17 not to the ground, but to point "R" (+31V). After that the diode will
be
biased only by the self bias of the tube, which is about 4V. Sensitivity
of
the detector will increase 10-fold.

Regards,
Alex