John Byrns wrote:

In article , Patrick Turner
wrote:

Steven Swift wrote:

If you are willing to live with about 5-10% THD, then you can use more
common circuits.

True, but onje doesn't have to live with 5-10%.
linearize the IF amp and detector, and thd plummets.

However, there are dozens of "Hi Fi" AM circuits published

by the hobby magazines, tube vendors and kit makers. Have a look at them.

The RC-19 circuit uses a 6BA6 as an RF amp, followed by a 12AU7 used as a
detector and audio amplifier.

RDH4 has the circuit for the Selsted and Smith "infinite impedance "
detector, where a 12AU7 performs as credible detector, and as a diode,
but I think I'll stick with a germanium diode fet by a 12AU7 CF.

The "Selsted and Smith" detector is not the same thing as the so called
"infinite impedance" detector. The "infinite impedance" or "reflex"
detector was designed by RCA, while the "Selsted and Smith" detector was
designed by, well "Selsted and Smith", or at least "Selsted" who is still
around, or was a year or two ago.

The "Selsted and Smith" detector differs from the "infinite impedance"
detector in that it has a diode in series with the grid, and also a diode
load resistor.

Yes, you are right, and I should have checked my RDH4.
But the S&S detector pulls much less power from the tuned circuit....

There is no peak detection capacitor across the diode
load, so the diode does not act as an ordinary diode peak detector, and
the triode doesn't act as a cathode follower. The triode is the actual
detector operating in a fashion similar to the "infinite impedance"
detector, with the diode apparently serving to linearize the "infinite
impedance" detector. The input impedance of the "Selsted and Smith"
detector is not infinite due to the presence of the diode load resistor.

True, but its higher than most other detectors.


For that matter the input impedance of the so called "infinite impedance"
detector is also not infinite, and can even have a negative resistance
component which can cause stability problems. The negative resistance
effect can occur when circuit conditions are right, similar to the
conditions that can cause oscillation in cathode and emitter follower
circuits if you aren't careful.

I am happier with direct feed of the RF/IF signal to a CF, with following
crystal diode and C, with nearly constant current discharge from the C.



But does the RC-19 have enough tuned circuits to give over 70 dB rejection of
signals which are 50 kHz away from the wanted station at any place on
the band?

The 6BA6 is a variable U tube, with a non linear
transfer curve.

There is nothing wrong with the 6BA6, it was specifically designed for
this service and has very low odd order distortion which is all that
matters since the even order distortion products can't get through the
IFT. I hope I got that the right way around, if not it is explained in
some detail in some of the old texts, I think "Radio Receiver Design" by
Sturley is one that explains it. You only get in trouble if you try to
run the tube at a very high signal level, simultaneously with a high AGC
voltage applied for a large gain reduction. This is mainly a problem in
the stage driving the diode detector, so it is best to avoid AGC on that
stage, but in a minimal radio that is of course problematic. This is one
of the many topics that the RDH4 gives short shrift. The 6BA6 is even
usable as a gain control element in audio circuits where even order
distortion does matter. IIRC the peak limiter at a radio station where I
once worked used four 6BA6s in the audio path, where they were connected
in push pull, presumably to cancel the even order nonlinearities which are
inherent in the design of the tube.

Indeed the PP connection of two 6BA6 would lead to cancelation of 2H in the thd.
Not a bad idea for an RF/IF amp either.
All distortion is bad.

Patrick Turner.



Regards,

John Byrns

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

Steven Swift wrote:

Hi Fi AM--

This was "almost" popular as a design project in the 60s. Even RCA in
their RC-19 Tube Manual have a circuit labelled "TRF AM Tuner-- for High-
Fidelity Local Broadcast Reception." (Circuit 19-8, p. 357)

John Byrns web site has the circuit diagram for the RCA design you
mention (or a related one if there's more than one of them):

http://users.rcn.com/jbyrns/pics/RC-17-8.jpg


Fidelity on an AM signal requires that most common circuits used in
radios be eliminated: [snip of good list]

All these "don't do" can be found in Terman, the Radiotron Designer's
Handbook and others.

[snip]

The RC-19 circuit uses a 6BA6 as an RF amp, followed by a 12AU7 used as a
detector and audio amplifier.

As the diagram at John Byrns site shows.


What's intriguing is how simple this design is -- it has one RF stage,
which indicates that a one RF stage TRF for local, high power stations
makes sense when audio fidelity is the overriding criterion.

Now, I wonder how much improvement in the audio quality is possible
if the channel TRF approach is used (which optimizes the bandpass for
each broadcast frequency)? Or does it not make sense by the law of
diminishing returns? Being able to use a higher order plug-in bandpass
filter (such as a constant delay/linear phase one), optimized for each
frequency, is intriguing.

Jon Noring

Nothing wrong with a channel approach. Simplifies most of the design
tradeoffs.

BTW, I measured all the AM stations at my house (Seattle). Using a
typical AM radio (a Sony SW7600GR with internal antenna), I tuned all
the stations that were easily received without too much QRN/QRM (day
time). I then checked the field strength of each using a spectrum
analyzer and a calibrated antenna with 1 meter effective electrical
length.

I received 15 (reasonably clear) stations. The strongest station,
only 2 miles away, gave -55dBuV (about 1.8mV/m), while the weakest of
the 15, gave -85dBuV (about 56uV/m).

QRN, due to skywave, increased substantially at night. This is "same
channel" interference, which we get from the fact that there are no
"clear channels" anymore. Better antennas actually make the QRN worse
on some channels-- even while helping local daytime reception.

I would design the radio to only work with strong local signals, if
fidelity is the goal.

Again, I say "good luck."

Steve.

--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA

In article , wrote:

Steven Swift wrote:

Hi Fi AM--

This was "almost" popular as a design project in the 60s. Even RCA in
their RC-19 Tube Manual have a circuit labelled "TRF AM Tuner-- for High-
Fidelity Local Broadcast Reception." (Circuit 19-8, p. 357)

John Byrns web site has the circuit diagram for the RCA design you
mention (or a related one if there's more than one of them):

The circuit is the same, with one exception, in all the editions of the
RCA tube manual in which it was contained. The one change that occurred
in later editions was a change from a 5Y3GT rectifier in the power supply,
to a rectifier with an indirectly heated cathode, I think it was a 6X4.
This circuit was only published during the 1950's, it did not appear in
editions before about 1950, and was dropped from later editions after
about 1960. I have often wondered if it was inspired by Williamson's late
1940's design for a radio feeder unit.


Regards,

John Byrns


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

In article ,
(John Byrns) wrote:

Please delete rec.radio.shortwave from the news group header.

Thanks.

--
Telamon
Ventura, California

Telamon wrote:

In article ,
(John Byrns) wrote:

Please delete rec.radio.shortwave from the news group header.

I'll say one thing about this group currently posting: They use more
complicated words than Bryant does!



Thanks.

--
Telamon
Ventura, California

"Telamon" wrote in message
...
In article ,
(John Byrns) wrote:

Please delete rec.radio.shortwave from the news group header.

Thanks.

--
Telamon
Ventura, California

What's the problem? This is a radio related discussion.

Frank Dresser