This one is in RDH4. I'm a bit surprised no one has referred to it,
although I've not watched the posts too carefully recently. I know
several
out there have a copy of RDH4. Please excuse if someone has already
referenced this circuit.

See it at ABSE & ABPR

Cheers, John Stewart

John Stewart wrote:

This one is in RDH4. I'm a bit surprised no one has referred to it,
although I've not watched the posts too carefully recently. I know
several
out there have a copy of RDH4. Please excuse if someone has already
referenced this circuit.

See it at ABSE & ABPR

Cheers, John Stewart

What page in RDH4 has the improved detector?
Many of us *do* have RDH4.

I can't see anything about it at ABSE or ABPR.

Patrick Turner.

Patrick Turner wrote:

John Stewart wrote:

This one is in RDH4. I'm a bit surprised no one has referred to it,
although I've not watched the posts too carefully recently. I know
several
out there have a copy of RDH4. Please excuse if someone has already
referenced this circuit.

See it at ABSE & ABPR

Cheers, John Stewart

What page in RDH4 has the improved detector?
Many of us *do* have RDH4.

I can't see anything about it at ABSE or ABPR.

Patrick Turner.

It shows up on page 1074 of my copy of RDH4.
This is an original I bought around 1956 while I worked at the
research division of Ferranti Electric. The publish date is February
1954.

Cheers, John Stewart

John Stewart wrote:

Patrick Turner wrote:

John Stewart wrote:

This one is in RDH4. I'm a bit surprised no one has referred to it,
although I've not watched the posts too carefully recently. I know
several
out there have a copy of RDH4. Please excuse if someone has already
referenced this circuit.

See it at ABSE & ABPR

Cheers, John Stewart

What page in RDH4 has the improved detector?
Many of us *do* have RDH4.

I can't see anything about it at ABSE or ABPR.

Patrick Turner.

It shows up on page 1074 of my copy of RDH4.
This is an original I bought around 1956 while I worked at the
research division of Ferranti Electric. The publish date is February
1954.

Cheers, John Stewart

Yes, I see that schematic OK in the Book.

The 6SJ7 driver tube ahead of a 6V6 output has NFB applied from
6V6 anode to 6SJ7 cathode via a 150k and 1 k divider, so the SJ7 cathode
signal
is nearly the same as the SJ7 grid signal, which comes from the
wiper on a volume control pot which is the current sinking R from the
detector caps.
The grid of the 6SJ7 is biased from the 1k cathode R, so
the input resistance into the 6SJ7 is perhaps at least several times 1M,
which thus presents a high AC coupled load.

I might add that in real circuits, the value of R2 is quite critical for
lowest
thd, and it should be estabished experimentally for lowest thd;
too high a value will have terrible cut off distortion on the positive
peaks of the audio,
and too low a value will dissallow AVC voltage to be made, allowing too
much IF amp
current, and there will be terrible distortions.

But this circuit still has the diode of the detector powered via a vigh
impedance
circuit of the secondary of the IFT,
and its still not a best possible outcome.

Better of course would be to have a CF tube to accept the IF envelope,
and the low impedance output from the CF can then power a crystal diode, or
a tube diode
in a variety of ways I have previously explained in post on the matter.


Patrick Turner.

In article , Patrick Turner
wrote:

I might add that in real circuits, the value of R2 is quite critical for
lowest
thd, and it should be estabished experimentally for lowest thd;
too high a value will have terrible cut off distortion on the positive
peaks of the audio,

"Positive peaks of the audio"? Don't you mean negative modulation peaks
of the audio?

But this circuit still has the diode of the detector powered via a vigh
impedance
circuit of the secondary of the IFT,
and its still not a best possible outcome.

Better of course would be to have a CF tube to accept the IF envelope,
and the low impedance output from the CF can then power a crystal diode, or
a tube diode
in a variety of ways I have previously explained in post on the matter.

You still haven't explained how this added cathode follower, to drive the
detector, helps matters? Many experts even make the claim that a finite
source resistance can be beneficial in reducing distortion, especially
high frequency distortion. I can see where a cathode follower could be
beneficial if we were trying to build a radio with an IF as narrow as
possible, in which case it would help keep the Q of the transformer
secondary as high as possible, but we are talking about a radio with wide
band audio, and are probably talking about adding loading resistors across
the transformers anyway, so why the cathode follower, why not just let the
load of the detector diode do the job? A cathode follower after the IFT
seems like a waste to me, better to use it after the detector, with a
negative cathode supply voltage, to buffer the detector from the AGC and
audio lines.

While many AM receivers have been designed in a cost conscious way, there
have also been a few where no expense was spared, and parts were freely
used, and yet I have never seen a cathode follower used as you propose in
a commercial design, I would think if it were beneficial someone would
have used it commercially, anyone know of any examples? Some designs add
other relatively expensive parts to the detector circuit, one trick I have
seen whose effects might be worth looking into is replacing the second
capacitor in the peak detector & RF filter network with a series LC
network tuned to 455 kHz. I don't know how this circuit actually works,
but I assume that the idea is to improve the tradeoff of the total peak
detector capacitance vs. tangential clipping at high frequencies. This is
something I will have to look into further. There are other detector
circuit subtleties like this that may, or may not, be worth while.


Regards,

John Byrns


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

John Byrns wrote:

In article , Patrick Turner
wrote:

I might add that in real circuits, the value of R2 is quite critical for
lowest
thd, and it should be estabished experimentally for lowest thd;
too high a value will have terrible cut off distortion on the positive
peaks of the audio,

"Positive peaks of the audio"? Don't you mean negative modulation peaks
of the audio?

But this circuit still has the diode of the detector powered via a vigh
impedance
circuit of the secondary of the IFT,
and its still not a best possible outcome.

Better of course would be to have a CF tube to accept the IF envelope,
and the low impedance output from the CF can then power a crystal diode, or
a tube diode
in a variety of ways I have previously explained in post on the matter.

You still haven't explained how this added cathode follower, to drive the
detector, helps matters? Many experts even make the claim that a finite
source resistance can be beneficial in reducing distortion, especially
high frequency distortion.

IN my case there *is* a finite source resistance which is the 100k
R across each IFT winding.


I can see where a cathode follower could be
beneficial if we were trying to build a radio with an IF as narrow as
possible, in which case it would help keep the Q of the transformer
secondary as high as possible, but we are talking about a radio with wide
band audio, and are probably talking about adding loading resistors across
the transformers anyway, so why the cathode follower, why not just let the
load of the detector diode do the job? A cathode follower after the IFT
seems like a waste to me, better to use it after the detector, with a
negative cathode supply voltage, to buffer the detector from the AGC and
audio lines.

I do things to suit the desire for wide as possible AF bw, and the
R loading of the IFTs helps achieve that end.
I don't want severe selectivity and IFT gain; that only belongs
in Z grade AM radios and communications sets.

Try using a CF buffer to power a detector with a germanium diode, you'll hear the
difference!
Measurements will confirm the improvement.




While many AM receivers have been designed in a cost conscious way, there
have also been a few where no expense was spared, and parts were freely
used, and yet I have never seen a cathode follower used as you propose in
a commercial design, I would think if it were beneficial someone would
have used it commercially, anyone know of any examples?

I have NEVER seen any ancient commercially produced radio or audio product where
the sound quality was not compromised, often severely, with many lies told
by the market cowboys, after the maker had reduced the parts count to
reduce costs, to be able to compete.


Some designs add
other relatively expensive parts to the detector circuit, one trick I have
seen whose effects might be worth looking into is replacing the second
capacitor in the peak detector & RF filter network with a series LC
network tuned to 455 kHz. I don't know how this circuit actually works,
but I assume that the idea is to improve the tradeoff of the total peak
detector capacitance vs. tangential clipping at high frequencies.

A series LC tuned to 455 kHz needs to be driven by a low impedance
to get a decent Q to reject the 455 kHz ripple, but it simply
is far easier to achive in well known ways with R&C.
Usually, the CRC arrangement of 100pF, 47k, and 100pF is entirely
adequate for removing RF detector ripple voltage.


This is
something I will have to look into further. There are other detector
circuit subtleties like this that may, or may not, be worth while.

You need to use a soldering iron to find out about what I am promoting
about AM detection.
There is no other way.

Patrick Turner.


Regards,

John Byrns

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

John Stewart wrote:

This one is in RDH4.


Saw it. In a radio where I already split off the AVC function off the
audio detector,
I'm trying a fixed positive bias of 300mV on the "bottom" of the volume
control.
Simulations indicate lower distortion on many various levels of carrier,
and it seems
borne out in an actual set. But the only cap I'm working against is the
coupling cap
between the volume control and the 12AV6 grid. Which is essentially a
fixed value.

I'll give it some time and thought to be sure that this is in fact a
good thing or not....

Robert Casey wrote in message ...
John Stewart wrote:

This one is in RDH4.


Saw it. In a radio where I already split off the AVC function off the
audio detector,
I'm trying a fixed positive bias of 300mV on the "bottom" of the volume
control.
Simulations indicate lower distortion on many various levels of carrier,
and it seems
borne out in an actual set. But the only cap I'm working against is the
coupling cap
between the volume control and the 12AV6 grid. Which is essentially a
fixed value.


Tried another approach: Just jumper out the 12AV6 grid coupling cap. The
detected audio has an average negative voltage on it (what most AA5s filter
to get AVC voltage) and I use that to bias the 12AV6 grid. Stronger signals
can be handled by turning the volume control down. Just like you did with a
standard AA5.

------
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here in the NYC area...