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