Jon Noring wrote:

Steven Dinius wrote:
Brian wrote:

10 kHz is the high-frequency limit in the USA.

I have several tube radios and the AM sounds nice enough with clean
higher frequencies and a couple with nice bass. As to why the OP wants
a tuner escapes me. To me the point of an AA-5 or AA-6 is that you can
get a decent audio amp and good power with those same tubes, put them
in a nice cabinet with a good speaker and REALLY ENJOY it. What's the
point? I thought that I buy all these different radios for the reason
that they have UNIQUE qualities and personalities. For the most part,
I have not had to modify a tube set other than the speaker for my
floor console, as they all have been satisfactory unlike some of the
SS stuff I have.

Well, being the "OP", I want a high-audio performance, modern design
AM tuner to integrate into my audio system -- and I believe a lot of
tube-o-philes likewise want that -- but not everyone obviously. There
are several reasons why most higher-grade audio systems use separate
components, the reasons of which are obvious to most everyone. The AM
tuner is no different than other audio components in this regard.

Even though it may seem strange to die-hard old radio collectors the
desire to have a modern-design and built AM tube tuner ("there are so
many old ones out there, why build one from scratch?"), it is equally
strange to those who want such an AM tuner (to integrate into their
audio system) to be told they should quit wanting what they want and
just find some old radio and restore it for their listening purposes,
and forget about integrating it into their audio system.

Both viewpoints are neither strange nor wrong -- both are looking at
things from different perspectives and goals: radio collectors are
more interested in old radios as "works of art" (where art is both in
design and electronic function), while those who want an AM tube tuner
are more interested in actual listening to contemporary broadcasts
(such as they are!), and still want to have the "tube sound": the
aesthetics and allure of the tube.

(In some regards, the tube-o-philes are not much different than old
radio collectors. The former love the aesthetics of the "tube" for
audio listening purposes; the latter love the aesthetics of the old
radios in and of themselves -- as works of the technical and visual
arts. Both are valid ways of looking at the big world of radio. Of
course, there is a third group who are totally utilitarian and prefer
to focus on building the high-performance AM tuner using completely
DSP/solid state design, and to hell with tubes and wood/bakelite
cabinets. That's also a valid perspective -- and probably makes for
the ultimate design.)

Anyway, if someone so chooses, they can take the modern AM tube tuner
design and connect a preamp/amp to it and integrate that into a single
cabinet with a speaker, to make a stand-alone radio. The cabinet could
either come from some old 1930's cabinet (where the chassis is missing
or unrestorable), or be designed from scratch to be reminiscent of a
classic late 1930's radio (maybe by combining the best features of
several highly regarded radio cabinets of the period). The latter
could make for a fun project for those so inclined and skilled. I've
even sketched out in my mind the design for such a cabinet -- I'm not
sure if I'll ever build it, but it looms in my mind ... a combination
of the old with the new. Of course, most radio collectors are now
probably aghast at my heretical thoughts. smile /

*****

On a different subtopic, Bob Casey sent me a recording made from an
AM radio broadcast (a local ABC station) using his home-built TRF
design receiver. He may have announced this recording to one or more
of these newsgroups in the past -- I don't know -- a check on Google
did not bring anything up. It has amazing sound, very FM-like (very
clean with low distortion), and it is obvious the broadcaster (I think
it is WABC) used a rolloff much higher than the "typical" 3.5khz being
bandied about here.

From my studying of what I can find on Google web sites and newsgroup
postings on AM receiver design, I believe the prime candidate for the
high-performance, modern-design AM tube tuner is a TRF design of some
sort, not a superheterodyne. The major issue appears to be how to get
the multiple stages (three stages appear necessary to attain
appropriate sensitivity and selectivity) all in "synch" (term used
loosely) for a given radio frequency.

Of course, thinking outside the box as I am wont of doing, I first of
all notice that the AM tuner will be used for commercial broadcasts
which are on strictly assigned frequencies: every 10khz in the U.S.
and every 9khz in Europe and elsewhere. If so, could we not dispense
with infinitely variable tuning and take a channel approach (like tv
receivers of old)? For TRF designs, this may be what is needed to
"perfectly synch" the three or more multiple stages, and do so in ways
not possible with the "1-D" restriction of a multiganged air tuning
capacitor (or variable inductor.) This may allow varying more
components for each channel to get the "perfect" fit between the
stages.

Anyway, I'll leave it to the experts to mull over the channel
approach to an AM tube tuner, to see if it will actually confer any
real advantages (especially for TRF circuits), and if so, how to
implement it in a practical sense (I have ideas), how to deal with
interference issues, etc.

Jon Noring

To get enough selectivity for local reception of AM stations,
and to get wide audio bandwidth, the Q of each LC circuit in a TRF needs to
be low.
But you still need at least 60 dB attenuation at 40 kHz away from the wanted
station.

I leave it up to the experts as to how one could have 10 kHz of AF bw
after 6 tuned circuits, even at the LF end of the band, where the Q at 550
kHz for just one LC circuit
would have to be as low as 27.5 to get 20 kHz of bandpass, -3 dB.

I don't think TRF would be a good idea.

Superhet operation with 2MHz IFTs would be far better.

Why don't you study the books to understand all the engineering involved
with the
status quo for tubed AM tuners?

Patrick Turner.

On Tue, 08 Jun 2004 16:11:02 GMT, "Frank Dresser"
wrote:


Getting wideband IF transformers will be a real problem. I don't know of
any NOS sources for them.

You can easily reduce the frequency selectivity of IF transformers by
adding resistors in parallel, though this will reduce sensitivity.

Best regards, Paul
--
Paul Sherwin Consulting http://paulsherwin.co.uk

On Tue, 08 Jun 2004 14:45:41 GMT, Jon Noring wrote:

On FM, especially among alternative FM stations, one often finds very
unusual musical programs being broadcast of music which the listener
does not have in their collection (it helps them to expand their
horizons and maybe go out and purchase said music on CD/vinyl.) In
addition, there are sometimes live broadcasts of concerts which will
never appear on CD/vinyl. (In Salt Lake City, the alternative FM
station I am thinking of is KRCL, http://www.krcl.org/ . Really a fun
station to listen to, especially the late Sunday night program
broadcasting 1920's to 1940's era recordings.)

It's very true that the level of audio postprocessing varies a great
deal around the world. In the UK all FM commercial broadcasters use
very high levels of compression (including Classic FM, a national
classical music station) because they like to sound 'loud'. Only the
BBC's classical station uses reasonable levels of compression and
limiting. This heavy compression is also used on digital feeds, where
it is completely unnecessary.

On AM there are certainly broadcasts which interest different people
for different reasons at different times. Live sports events not found
elsewhere, news, of course the venerable talk radio, and for some of
us, we like to spin the dial at night and see what distant stations we
can pull in.

Yes Jon, but that's not audiophile listening, it's using radio as it's
always been used for 80 years. You would do just as well to plug a
1970s Grundig Yacht Boy into your system (which is what I do :-) )

Well, maybe in the U.S. most stations cutoff at 3.5khz. Then that's
where they cutoff. However, the AM tuner design is intended for the
world, and as Patrick Turner noted, in Australia many broadcasters
have a much higher rolloff because of the "open highway" they have
on the BCB -- fewer stations spread farther apart.

Modern AM transmitters have a very sharp rolloff above a certain
frequency. Broadcasting above this would just waste transmitter power,
since (almost all) radios wouldn't be able to receive it because of
their IF selectivity characteristics. The 9kHz or 10kHz AM channel
width is just a convention, but once it has been adopted there's no
point in trying to receive a wider bandwidth - you'll just get
interference from adjacent stations.

There's nothing wrong with building your own high quality AM tuner,
either solid state or tube, but no matter how many gold lettered
Telefunken ECC83s you use it won't sound very good.

Agreed in principle. The AM tuner must deliver the highest possible
fidelity as broadcast, that's all. It must have very low distortion.

One question to ask is in various areas of the world (including the
U.S.) what is the distribution of HF cutoff among the many broadcast
stations? I doubt in the U.S. every broadcaster rolls off HF at
3.5khz, but maybe most do -- are there any AM stations in the U.S.
which have a much higher HF rolloff than 3.5khz? Note again Patrick's
comment on Australian AM broadcasters.

In the US and Canada, AM stations are allocated 10kHz bandwidth,
giving a theoretical 5kHz treble cutoff. In most other place that's
9kHz/4.5kHz. Stations transmit a more restricted frequency range than
this though, for a number of technical reasons. That's where my rough
and ready 3.5kHz figure came from.

Best regards, Paul
--
Paul Sherwin Consulting http://paulsherwin.co.uk

"Paul Sherwin"

In the US and Canada, AM stations are allocated 10kHz bandwidth,
giving a theoretical 5kHz treble cutoff. In most other place that's
9kHz/4.5kHz. Stations transmit a more restricted frequency range than
this though, for a number of technical reasons. That's where my rough
and ready 3.5kHz figure came from.

Best regards, Paul


** In Australia the AM channel spacing goes in 9 kHz increments, however
the transmitted bandwidth is not affected by that fact since the authorities
have kept a wide frequency separation between transmitters serving the same
areas.

The recovered audio from many transmitters is of good quality on speech
and music with high frequencies extending to 12 kHz in some cases - the
government owned networks being the best in this regard. At night, far
distant adjacent channel signals can produce an audible 9 kHz background
whistle which a sharp notch filter deals with most effectively. I use an
Australian made valve AM tuner designed for hi-fi reception of local
broadcasts and have tried out a few SS hi-fi AM designs as well.

The secret of good AM reception is the use of a balanced loop or frame
antenna to reduce man made and static noise to insignificance.



............ Phil

"Patrick Turner" a écrit dans le message

I don't expect anyone to pay 3c for what I say, which could be seen as OT.

You just met our village idiot it seems...

There is an unspoken rule here..._Ignore_ his posts. Let him talk to
himself.

We don't get into fight with village idiot like you do on RAT...Keeps rar+p
"clean"...;o)

Syl

"Jon Noring" wrote in message
...

Well, being the "OP", I want a high-audio performance, modern design
AM tuner to integrate into my audio system -- and I believe a lot of
tube-o-philes likewise want that -- but not everyone obviously. There
are several reasons why most higher-grade audio systems use separate
components, the reasons of which are obvious to most everyone. The AM
tuner is no different than other audio components in this regard.

[snip]

But AM is different than other media. AM is processed to somewhat
compensate for the deficiencies in typical radios and listening situations.
AM sounds compressed and on a wideband radio usually sounds over treble
boosted. A perfect AM tuner would reproduce this processing perfectly.

A decompressor circuit might be worth considering.

Frank Dresser

"Paul Sherwin" wrote in message
...

[snip]

Modern AM transmitters have a very sharp rolloff above a certain
frequency. Broadcasting above this would just waste transmitter power,
since (almost all) radios wouldn't be able to receive it because of
their IF selectivity characteristics. The 9kHz or 10kHz AM channel
width is just a convention, but once it has been adopted there's no
point in trying to receive a wider bandwidth - you'll just get
interference from adjacent stations.


If the received signal is very strong, the tuner's gain will have to be very
low. This will supress the adjacent channel interference quite well.



In the US and Canada, AM stations are allocated 10kHz bandwidth,
giving a theoretical 5kHz treble cutoff. In most other place that's
9kHz/4.5kHz. Stations transmit a more restricted frequency range than
this though, for a number of technical reasons. That's where my rough
and ready 3.5kHz figure came from.

Best regards, Paul
--
Paul Sherwin Consulting http://paulsherwin.co.uk

The FCC requires US AM radio stations to have an audio bandwidth between 4
and 10 kHz or a total bandwidth from 8 to 20 kHz. Typical radios with IF
transformers, rather than crystal or ceramic IF filters, don't have very
sharp skirt selectivity. Few radios will be able to block out a strong
adjecent channel 10 kHz off channel. Many can't block out a strong adjacent
20 kHz away. Some can't even block out a strong adjacent channel 30 kHz
away.

The FCC limits interference only partly by bandwidth restrictions. Mostly,
it uses geographic seperation and power restrictions.

By ear, I think most stations go to about 7 or 8 kHz audio. Many of the AM
stations are talkers, but the ads can really sparkle. There's one I hear
which sounds like it goes to the 10 kHz audio max.

Frank Dresser

Paul Sherwin wrote:

On Tue, 08 Jun 2004 16:11:02 GMT, "Frank Dresser"
wrote:

Getting wideband IF transformers will be a real problem. I don't know of
any NOS sources for them.

You can easily reduce the frequency selectivity of IF transformers by
adding resistors in parallel, though this will reduce sensitivity.

The typical impedance of an undamped 455 kHz undamped
IFT is between 20k and 50k at 455 kHz.
Adding some R to both coils reduces the load seen by the tube,
hence its gain drops because pentode IF amp tubes have a high Ra,
and gain varies with load.
So the gain of the IF amp drops maybe 6 dB with R loads to both LC circuits in
IFT2,
and gain drops the same amount in IFT1, powered by the F converter tube

The nose of the selectivity broadens, ie, the Q of the circuit reduces,
ie, the bandwidth passed by the IFT is broadened out,
but 50 kHz away from resonance the attenuation rolls off at 12 bD/octave.
The roll off of a typical single tuned LC IF circuit away from the pointy nose
shape of
the the curve is only 6 dB per octave. The profiles of typical response curves
for RF and IF
coils are illustrated plentifully in all the good old radio books.

So with damping R, and two IFTs, the amount of attenuation of signals only 50
kHz away from
the wanted station is reduced by at least 12 dB.
This may perhaps be enough to allow a station 50 Khz away to be heard in the
backgound of a wanted station,
especially if its one thats putting out 5,000 watts and the wanted station is
putting out only 300 watts,
and they are both within 10 miles of the receiver.

Therefore its important to have some selectivity, although quite broad,
ahead of the converter tube.
I use two low Q LC circuits in cascade which are slightly tuned apart
at the low end of the BCB so effectively broadening the RF bandwidth, but
enabling a steeper roll off away from the pass band.
At the top end of the BCB, the two input RF LC circuits are very nearly tuned
at the same F,
and since the Q is still low, but the Fo is higher, the pass band does not
cause
side band cutting and a reduction of RF bw which would then limit the audio
after another 4 tuned circuits in the IF stage.

To use TRF to do the same thing would be almost impossible, and
I would need at least 6 tuned circuits tuned in the same way, and a six gang
tuning cap,
along with a seventh gang to tune the oscillator. There would have to be two
low gain
IF amps, which could be cascoded triodes instead of pentodes.

Its a hell of a lot easier to do it all with a superhet.

Not many NOS IFTs.

The old ones seldon suffer from spending 50 years in an old radio set,
and they are actually fairly ruggedly made, with brass tuning shafts for the
ferrite cores,
and in cans which have kept out the pollution failrly well.

The coils are often pie wound coils of litz wire.
The distance between the coils determines the amount of magnetic coupling, and
most IFTs
have just the right distance to cause critical coupling which gives the flat
topped
bandpass characteristic so you get about 10kHz of BW from a typical 455 kHz
IFT.

This allows 5 kHz of audio.

Two IFTs of the same response will give 7 kHz of BW, which allows
3.5 kHz of audio BW.

Now the minute one cuts the single tube the IFT coils are mounted on and
moves them closer together, say by 5 mm, the magnetic coupling increases,
and the response usually widens, but not greatly, but the shape of the
response
becomes twin peaked either side of Fo.
If you have a twin peaked IF response it means the audio BW will be also
peaked up at say 4 kHz, before rolling off even more sharply than it did
before when the
response was flat.
But sometimes the first IFT1 is deliberately slightly overcoupled to give the
twin peaked response,
which then is compensated back to being flat by the following normally single
peaked response of
IF2.
But tuning could be strange, with a tuning indicator having to be set to the
slight
null between two peaks.
Alignment of the IFTs becomes more difficult.

This is why I suggest that an IF of 2 MHz be used instead of 455 kHz, because
for the same Q the pass band of say 3 normally critically coupled IFTs
would be nicely flat topped, but still have an overall wider bandpass than
two 455 kHz IFTs.
The would have to be two IF amps instead of 3, but their gain need only be
low,
so cacoded triodes come to mind.
The cascoded triode has an effectively very high
Ra looking into the anode of the top tube, and a 12AT7
would have Ra' = 1 Mohm. If RL was 20k, gain would be about 60.
12AU7 would also be OK with Ra' = 200k,
and gain about = 29 with cathodes fully bypassed.
But pentodes could be used, with 6BA6 as IFamp1, with AVC applied,
and 6AU6 as IF2, with no AVC applied to keep the final IF amplification
as linear as possible.
Distortion of the IF envelope shape will all be detected as audio distortion
to
the shape of the recovered audio at the diode detector stage.

It would be possible to perhaps simply remove turns from a 455 kHz
IF coil and halve the existing capacitors to raise the Fo to 2 Mhz.
This all has to be done carefully, so that after halving the cap size,
just the right no of turns are removed to get the IFT to tune to 2 MHz with
its tuning
slug in the middle of its travel range.
I have never done this, so perhaps its just easier to wind ones own
new IF coils, but large sized old ones with cans of 35mm dia are plentiful.
The tiny IFTs which became prevalent in radio sets in the 1960s are a PITA
to modify.

The use of 2MHz IFTs requires strict adherence to using shortest leads
from tubes to IFT connections, because the higher the F, the greater the
likelyhood
of oscillation and IF amp instability.
So the IFT and tube line up will be in a neat straight line, with small 7 pin
tubes being able
to be close as possible to the IFT cans, and perhaps with additional grounded
sheet metal shields up off the tube sockets.

Patrick Turner.












Best regards, Paul
--
Paul Sherwin Consulting http://paulsherwin.co.uk

Syl's Old Radioz wrote:

"Patrick Turner" a écrit dans le message

I don't expect anyone to pay 3c for what I say, which could be seen as OT.

You just met our village idiot it seems...

There is an unspoken rule here..._Ignore_ his posts. Let him talk to
himself.

We don't get into fight with village idiot like you do on RAT...Keeps rar+p
"clean"...;o)

Syl

Well, with all due respects to all gentlemen and possible idiots
on all groups to whom this subject thread is cross posted to,
I reserve the right to decide who I will ignore or not.

I will desperately try not step on anyone's toes as I act in well intentioned
freewill.

I won't budge from the idea that its possible to digitise the signal from the
antenna and simply apply
suitable algorithms, and get digital decoding, without all the phase shift
caused by consecutive tuned circuits.
Like on expensive CD players, a tubed output filter on the final DA converter
could be used,
and a decent sound could be had, at least in Oz, where the audio transmitted by
AM is often very
wide bandwidth, depite the fact that the networked stations send their radio
shows to air at different times,
and via satellite, before finally being broadcast by a local AM transmitter.
God knows how many links the signal goes through, afaik.

I have tried to address the problems caused by tuned circuit delays in
recommending
that 2 MHz IFTs be used.
I do think tubes are good for IF amps, certainly the last IF amp, because
of the huge dynamic range of the tubes, and far better performance
can be had compared to using j-fets and a lousy 12 volt B+ supply.
Some might argue silicon opamps would be better still.
I would have no objection to whatever they used, as long as it
achieves the goal of high quality sound, and it was a valid way of doing it, as
far as they were concerned.
But a j-fet balanced converter and first IF amp would be permissable
because the signals are so low before they get to a second and final IF amp.

To get ideal signal from an AM tubed receiver, the AF signal
from a 100% modulated AM IF carrier should be around 2vrms
at least, so the tube isn't working beyond its linear class A range.

Anyone have anything to say about this?

Patrick Turner.

Frank Dresser wrote:

"Jon Noring" wrote in message
...

Well, being the "OP", I want a high-audio performance, modern design
AM tuner to integrate into my audio system -- and I believe a lot of
tube-o-philes likewise want that -- but not everyone obviously. There
are several reasons why most higher-grade audio systems use separate
components, the reasons of which are obvious to most everyone. The AM
tuner is no different than other audio components in this regard.

[snip]

But AM is different than other media. AM is processed to somewhat
compensate for the deficiencies in typical radios and listening situations.
AM sounds compressed and on a wideband radio usually sounds over treble
boosted. A perfect AM tuner would reproduce this processing perfectly.

A decompressor circuit might be worth considering.

But how does one know how to apply an expander to exactly match
the inverse of the compressor characteristic?
I doubt two wrongs will make a right.

Anyhow, in Oz there isn't to much evidence of compression or emphasis of
audio HF on the stations worth listening to; I find the better the receiver,
the more like FM reception the AM signal becomes.

Patrick Turner.



Frank Dresser