Patrick Turner wrote:


Its possible that by removing many turns off existing 455 kHz IFTs,
the 2MHz could be achieved.

The oscilator would operate at the BCB frequencies + 2 MHZ.
So the oscillator coils and circuit would need revision, but then that'd
be easy,
since the coils do not differ much from the usual low end short wave
types.



A good set of ~2.8 MC IF transformers can be had in the 6-9.1 MC ARC-5
receivers.


Jeff Goldsmith

Phil B wrote:

Jon,

Wow! Long wish list. You can boil your list down to two requirements:
1. Very low distortion introduced by your ideal tuner.
2. No audio rolloff up to 5kHz.

There have been a number of threads in this group concerning distortion
introduced by the receiver detector stage. Do a Google groups search to
find them.

AM broadcast stations are required to cut off their high audio frequency
abruptly at 5kHz to prevent interference to adjacent channels spaced +
or - 10kHz. You won't find high fidelity among the AM stations no matter
how good your tuner. The best you can hope for is a tuner that doesn't
add it's own frequency response limitations below 5kHz. Do you really
want more than 5KHz response to listen to Rush?

I was about to argue here until I realized you were talking about the "right
wing wacko", as Tom Leykis would say, and not the progressive rock band

As for the tuner... try the TRF which is in some RCA tube manuals... it uses
a 12AU7 and some rectifier, I believe. There's also the Heathkit AM tuner
which was made in the 50s, and may be worth copying. Some people have also
had excellent luck with a "crystal" radio - I believe AES sells a kit which
is intended for hi-fi AM use, and again, one was sold in the 50s.

(I think a high
frequency limit of, say, 20 Hz would be more appropriate for his show).

Otherwise, I like your Yahoo groups idea. It's a great idea to provide a
forum for discussion of your ideas. It will serve to educate all
participants.

Phil B

"Jon Noring" wrote in message
...
[New Yahoo Group started: "AM Tube Tuners". See end of this message
for more info.]


In the last couple of years I've posted various inquiries to this and
related newsgroups regarding high-performance, tube-based AM (MW/BCB)
tuners, both "classic" and modern.

I'm very interested in building such a tuner to match with
audiophile-grade tube amplifiers and pre-amplifiers now being built by
hobbyists (as well as those sold by commercial vendors.) There are
quite a few nice kits now being marketed for audiophile quality tube
amps/pre-amps, such as those made by diytube (http://www.diytube.com/
-- there are many others like diytube.) So why not similar kits (or
workable designs) for a tube-based AM tuner?

(Obviously, a stereo FM tube tuner will be of even more interest to
the tube-o-philes, but there is also a market for an AM tube tuner.
Some may prefer an integrated AM/FM tube tuner, and that's fine, too,
but my focus here is on MW/BCB -- it certainly has special needs
requiring dedicated design even if it is incorporated into an AM/FM
tuner.)

What sort of specs should this AM tuner have? Well, that is certainly
a very open-ended question, with no right answer. However, I believe
the following preliminary list of qualitative specs and requirements
essentially outlines the likely preferred parameter space for the
typical expectations of those who will build and use this AM tube
tuner. Undoubtedly this list is very preliminary, and will be improved
as the experts weigh in (I am NOT an expert on AM tuners), hopefully
even adding real numbers to the resultant specs and requirements.

1) Excellent audio quality at the line-out, effectively reproducing,
with acceptably low distortion, the full fidelity of the broadcast.

(The tuner itself, unlike the radios of yesteryear, will not have
a final audio amplifier stage -- it is assumed the line out will
connect to an audiophile-grade sound system. Low noise is important
since the audiophile system will certainly resolve any noise
present.)

2) Sensitivity, selectivity, etc., will also be quite good, so with an
appropriate antenna, the tuner will be usable for casual MW DXing.

(Obviously it will not, and should not, compete with high-end gear
used for serious MW DXing, such as the Drake R8B and a modded ICOM
R75, to name a couple. But on the other hand, the design should be
"fun" to listen to when the AM band happens to be active at night
-- it should at least be comparable to my venerable RS DX-399 with
RS 15-1853 AM Loop.)

3) The kit/design should be relatively easy (for those experienced
with building audiophile tube amps/pre-amps), and not require a lot
of effort, expertise and new knowledge to construct, align and
adjust, nor require constant adjusting to keep it tuned once built.
The number of tubes in the AM tuner probably should be kept low
(4-6 tubes are preferable by my lay reckoning -- it does help that
there is no final stage audio amplifier.)

(I envision that with the right design, ready-made PCB boards can
be built, like what diytube makes for its amplifiers, for the AM
tube tuner -- to make the design reasonably "fool proof". Obviously
issues not seen in audio amplifiers, such as RF/IF interference,
have to be specially dealt with -- multiple, shielded boards?
Clearly a high-quality AM tuner is a step above audio amplifiers in
complexity and potential problems, but those already skilled in
building tube amps should be able to move to the next level to
assemble the AM tuner and get it working.)

4) The design should specify parts which can be bought new today at
reasonable prices. That means: NO SCROUNGING NEEDED for parts (such
as from old radios on eBay.) Many who will build the AM tuner will
not be old radio collectors, and thus prefer all new, modern parts.
The tubes should be commonly available.

(For example, it appears that multigang tuning capacitors are still
manufactured today by several manufacturers. The components which
require special construction are RF and IF coils. Maybe with a good
design, someone may be able to have a bunch of them made to specs
for use in the kits?)


Strategy and Issues as I see them now:

As noted above, I am clearly not an expert on AM tuners, although I've
been studying whatever resources are available on the Internet,
learning about the designs of yesteryear and those who are trying to
push the envelope with today's better components. Thus, I hope that
the experts here, who have actually built radio tuners and know their
stuff, will take an interest in this. Obviously the first step is to
better state (and later quantify) the requirements and specifications
as attempted above.

However, I can certainly suggest some things which appear important to
discuss (and this list is not prioritized, nor exhaustive), such as:

1) Should we simply find a suitable radio/tuner from yesteryear and
"modernize" it? From the late 30's through the 50's, there are
certainly many worthy candidates to choose from.

Of course, let's begin suggesting candidates!

2) Basic type of receiver. For example, should we consider TRF, or
stick with superheterodyne? TRF, especially using modern components
and modern design, is actually intriguing after reading many of
the messages by John Byrns and others. It potentially can have very
high fidelity audio (from an audiophile sense it is a "purer"
architecture), and does not generate IF interference which again
may turn off audiophiles worried about that. The downsides are
well-known (mainly with selectivity, requiring several carefully
tuned stages to have acceptable selectivity), but there are
workarounds. Superheterodyne is the tried and true receiver type,
with a seemingly endless number of good commercial designs to
choose from. And since simplicity of circuit design is preferred,
would a "supercharged and modernized" AA5 circuit meet the specs?

3) Variable bandwidth control. It appears that a user-adjustable
bandwidth control is called for, especially for switching between
local high-power stations, and weaker distant stations.

4) Antenna input, and antenna gain control? I envision the tuner to
be flexible in the kind of antenna types it will be able to handle.
The types of antennas I've seen used for MW include a ferrite rod,
a simple wire (both can be augmented with, for example, a Radio
Shack AM Loop antenna 15-1853), and more fancy antennas such as
the active loop antennas by Wellbrook (see
http://www.wellbrook.uk.com/products.html#ALA1530 ). I would
assume that an antenna gain control will be needed, but then maybe
not.

5) One problem with building a tuner to cover the MW band is that it
must cover over a 3x span, from about 500khz to 1800khz. This seems
to negatively impact on some receiver designs. Interestingly, has
anyone considered breaking up the BCB band into multiple bands, for
example three bands (500-800, 800-1200, and 1200-1800khz)? Would
doing this confer benefits for some receiver types?

6) Another interesting possibility is that the tuner will almost
exclusively be used to receive commercial broadcasting. In most
of the world, and especially in North America and Europe,
broadcasting is done in very specific frequencies (every 10khz
in the U.S., every 9khz in Europe). So one can envision that
instead of using a multigang tuning capacitor or inductor, to
prewire each channel, specifically tuned for a specific broadcast
frequency -- then have a switch to switch between the channels.
This is especially intriguing for multi-stage TRF designs. Of
course, for the U.S. this would mean over 120 such channels, and I
assume more for Europe. Could get to be unwieldy and calibration
may be an issue -- but then the cost and space of multigang
variable capacitors is significant.

7) A hybrid digital/tube system may be acceptable to the audiophiles.
Any advantages here?

(But there is something to be said for using only components which
are similar to those used in classic radios -- an aesthetic issue
important to some. After all, many well-designed solid state AM
tuners are excellent performers, so restricting ourselves to tubes
is arguably an "aesthetic decision".)


If anyone is interested, I've created a YahooGroup to discuss this
further in a dedicated forum. If you already have a YahooID, you can
subscribe to it via:

http://groups.yahoo.com/group/am-tube-tuners/

If you don't have a YahooID, send a blank email to:



Hope to see you there.


I look forward to your feedback, thoughts, and, yes, candid
criticisms!

Jon Noring

"Jon Noring" wrote in message
...
[New Yahoo Group started: "AM Tube Tuners". See end of this message
for more info.]


In the last couple of years I've posted various inquiries to this and
related newsgroups regarding high-performance, tube-based AM (MW/BCB)
tuners, both "classic" and modern.

[snip]

Does anybody broadcast true hi-fi AM anymore? The FCC limits the total
bandwidth to 20 kHz or an audio bandwidth of 10 kHz but I think most
broadcasters don't even go that far. More than that, nearly all the
broadcasters now seem to be pre-emphisising the trebles, and AM usually
sounds shrill on a wide bandwidth radio. I assume they do this to somewhat
compensate for the normal IF roll off in a typical radio. Add in the other
audio processing that broadcasters use, and AM doesn't really sound a whole
lot better on a wide band radio.

I did hear some classical music on a local ethnic station a few weeks ago
which sounded quite good. It actually had some dynamic range and the
station is one of the few which sounds like they use their full bandwidth.

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

Frank Dresser

"Phil B" wrote in message
...

AM broadcast stations are required to cut off their high audio frequency
abruptly at 5kHz to prevent interference to adjacent channels spaced +
or - 10kHz.


I've read articles which claim there was no specific limit on AM audio
bandwidth, but the FCC required the stations to limit bandwidth to limit
interference. Given the normal minimum station spacing of 30 kHz in a given
area, this would imply a maximum audio bandwidth of 15 kHz. If sideband
splatter is any indication, I know the old rocker WCFL at 1000 kHz would
splat out the lower sideband of KDKA at 1020 kHz in the Chicago area. Now,
KDKA is an easy nightthime catch.

As stations were added to markets, the FCC limited the audio bandwidth to 10
kHz.

I've read textbooks which claim a maximum allowed 5 kHz audio bandwidth but
I don't trust the textbooks, so I searched the FCC website.

I came up with:

3. Sound Broadcasting

Sound broadcasting, double-sideband..

BINFn/INF=2M, M may vary between 4000 and 10000
depending on the quality desired

This defination was among a group above the FCC's formulas:

BINFn/INF = Necessary bandwidth in hertz

So, if I'm reading this correctly, the necessary bandwidth for standard AM
will be twice the audio bandwidth, which must be between a minimum audio
bandwidth of 4000 Hz and a maximum audio bandwidth of 10,000Hz.

This is from:

http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi?TITLE=47&PART=2&SECTION=202&YEAR=2001&TYPE =TEXT

Frank Dresser

Patrick Turner wrote in message ...
Syl's Old Radioz wrote:

"Jon Noring" a écrit dans le message

...high-performance, tube-based AM (MW/BCB)

I'm very interested in building such a tuner to match with
audiophile-grade tube amplifiers and pre-amplifiers ...

Audiophile AM is an oxymoron...

Syl
Now there's talk of digital broadcast, and the phasing out of FM and AM
broadcasting.
But I don't expect it to dissappear soon, and even more channels for
people's attention seem
to spring up daily to consume the leisure time of the masses,
and TV gets the main share.
Digital recievers need to be costed below the existing radio receiver
costs before
folks will buy them as an add on for their TV watching.
People's expectations about home entertainment are far beyond
just sitting down listening to music.
Most AM is listened to in cars, if at all, but usually while folks are
doing something else.
There will always be broadcasting of some sort, because its possible,
and the spectrum exists, but the programme quality decline continues.
As fewer listeners tune in, there are less advertisers willing to pay
the stations,
and its not worthwhile building a super dooper radio to listen to them.

I have 3 HRO receivers in parts from which I plan to get two good ones,
I have several other radio projects to do, but alas no time, since I
have to work for
a living.

I'd like to try using a 2 MHz IF strip for my A radio, because at least
there
3 stations here worth listening to out of the total of 7.

I figure the 2 MHz IF frequency would allow
a Q of 50 for each LC circuit, and thus the BW would be 40 kHz for each,

so with 4 or 6 consecutive LC circuits the BW could be 20 kHz,
thus allowing 10 kHz of audio BW.
Perhaps single tuned IF coils are all that's needed.
The single tuned high Q IF auto tranny is pretty awful at 455 kHz, as
used in transistor
based circuits because with a Q of 100, the BW is only 4.55 kHz,
and with two such coils you have only say 3.6 kHz, so only 1.8 kHz of
audio can
pass, and many transistor radios have only 1.8 kHz of audio BW.
Some tube types only have that much. I have measured plenty
of impressive looking tube sets with RF stages, and the total number of
tuned circuits is
about 6 including 4 IF coils, and the bandwidth narrows down badly.

Communications radios sometimes used lower IF at say 100 kHz
to take advantage of the lower bandwidth for a given Q.
This allowed very good selectivity for short wave,
but was hopeless for local station AM.

Its possible that by removing many turns off existing 455 kHz IFTs,
the 2MHz could be achieved.

The oscilator would operate at the BCB frequencies + 2 MHZ.
So the oscillator coils and circuit would need revision, but then that'd
be easy,
since the coils do not differ much from the usual low end short wave
types.

The other way of doing an AM radio today is to use totally digital
techniques for converting what is coming from the antenna and pull out
the audio from
any wanted station in ways which nobody in 1935 could ever have
imagined.
I think this would be an interesting digital project for someone.

Everyone has a PC at home these days, and it sould be easy to
use it to sift out a few radio waves.

But if fidelity isn't transmitted, not even a PC can decide correctly
what to substitute for missing audio HF.


Just my 3c worth,

Patrick Turner.

I don't have 3c and I only have 2GB. WTF do I want to do that on this
POS. Why does 'radio' have to be done on a computer? Get moving and
build a dedicated device (radio) instead of using a damned computer.
This should be in a sci. group.

So, if I'm reading this correctly, the necessary bandwidth for standard AM
will be twice the audio bandwidth, which must be between a minimum audio
bandwidth of 4000 Hz and a maximum audio bandwidth of 10,000Hz.



Contrary to popular belief, AM stations broadcast with a bandwidth of
20KHz, which
makes for audio up to 10KHz. This much bandwidth in a tuner works well
for local stations,
but for DX work you'd want to cut your bandwidth in your receiver to
+-5KHz to avoid
excessive splatter. Even then, you'll still get some "monkey chatter"
from an adjacent channel
station. "Monkey chatter" is a technical term for the modulated audio
that came from station
A showing up "upside down" when you are listening to station B, 10KHz
away on the dial.
"Upside down" meaning that an audio tone of 9KHz station A transmitted
gets demodulated
by your radio when it's tuned to station B, as a 1Khz tone. 9.5Khz -
500Hz, and so on.
Human speach "inverted" this way sounds like "monkey chatter". The only
way to reduce
monkey chatter is to null out station A with the loop antenna. But if
there's another station C
10KHz on the other side of the desired station, and not in the same
direction of the first
undesired station, you're sunk.

You'll also want a sharp 10KHz notch filter (9Khz in Europe and
Australia and elsewhere).
That's to get rid of the heterodyne from the adjacent stations' carriers.

If you can find a copy of RDH4 (Radio Designer's Handbook edition 4), it has
lots of info on radio receiver design. But it assumes that you have a
working
knowledge of electrical engineering. That is, not a beginner's book.

Does anybody broadcast true hi-fi AM anymore? The FCC limits the total
bandwidth to 20 kHz or an audio bandwidth of 10 kHz but I think most
broadcasters don't even go that far. More than that, nearly all the
broadcasters now seem to be pre-emphisising the trebles, and AM usually
sounds shrill on a wide bandwidth radio. I assume they do this to somewhat
compensate for the normal IF roll off in a typical radio. Add in the other
audio processing that broadcasters use, and AM doesn't really sound a whole
lot better on a wide band radio.


snip

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


A bunch of approaches exist.

In the 50s and 60s, several RF suppliers-I get Miller and Millen
confused,one at least-offered sets of cans to build, essentially,
crystal radios that you hooked up to your hi-fi preamp. They still had
high-gain inputs without RIAA for 78s in those days, I presume. This
was considered the best possible way to demod AM. I don't know how
well it would work with the crapped-up bands and disinclination to run
long wire antennas and real RF grounds today.

Later on, Klipsch dealers would have a similar set built on a display
board hooked to a matching xfmr which they would hook to a K-horn.
With no power supply or amplifying devices, if you were within a few
miles of a 50-kw station-or if you were in a very quiet room with a
big longwire and really good ground, even at night or in the sticks-
you had a surprisingly loud and clear audio feed. I can very clearly
remember hearing Simon and Garfunkel's "The Sound of Silence" for the
first time on this arrangement. When did that come out? Late 60s I'm
guessing.

Before that even, people in the 30s' (or so they say...) would modify
old A****er Kents into tuners by removing the output section and
providing a B+ and filament supply and furnishing an amp and speaker.
Camera repair guru Ed Romney, who has since went to the big
transmitter shack, talks of this in his radio repair book. He may have
even reprinted the article: I know I had ordered one on interlibrary
loan back in the 80s. Consumer Reports recommended this as an
alternative to the E.H. Scotts and Philharmonics of the day.

Finally, unless you really like heterodyne whistles, you will want a
10 kHz notch filter, or a rolloff.

But if fidelity isn't transmitted, not even a PC can decide correctly
what to substitute for missing audio HF.


Just my 3c worth,

Patrick Turner.

I don't have 3c and I only have 2GB. WTF do I want to do that on this
POS. Why does 'radio' have to be done on a computer? Get moving and
build a dedicated device (radio) instead of using a damned computer.
This should be in a sci. group.

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

I have already built a decent AM radio, and re-engineered an FM radio,
both to my own designs, so I feel OK about considering the alternatives
which might involve a PC.

Patrick Turner.

Robert Casey wrote:




So, if I'm reading this correctly, the necessary bandwidth for standard AM
will be twice the audio bandwidth, which must be between a minimum audio
bandwidth of 4000 Hz and a maximum audio bandwidth of 10,000Hz.



Contrary to popular belief, AM stations broadcast with a bandwidth of
20KHz, which
makes for audio up to 10KHz. This much bandwidth in a tuner works well
for local stations,
but for DX work you'd want to cut your bandwidth in your receiver to
+-5KHz to avoid
excessive splatter. Even then, you'll still get some "monkey chatter"
from an adjacent channel
station. "Monkey chatter" is a technical term for the modulated audio
that came from station
A showing up "upside down" when you are listening to station B, 10KHz
away on the dial.
"Upside down" meaning that an audio tone of 9KHz station A transmitted
gets demodulated
by your radio when it's tuned to station B, as a 1Khz tone. 9.5Khz -
500Hz, and so on.
Human speach "inverted" this way sounds like "monkey chatter". The only
way to reduce
monkey chatter is to null out station A with the loop antenna. But if
there's another station C
10KHz on the other side of the desired station, and not in the same
direction of the first
undesired station, you're sunk.

What you are saying here is true about monkey chatter,
and "inversion" of music and speach.
But usually it only applies to distance listening.

And a sharp bridged T LC notch notch filter won't reduce the monkey
chatter, just the 9/10kHz whistle from carriers interfering.



You'll also want a sharp 10KHz notch filter (9Khz in Europe and
Australia and elsewhere).
That's to get rid of the heterodyne from the adjacent stations' carriers.

If you can find a copy of RDH4 (Radio Designer's Handbook edition 4), it has
lots of info on radio receiver design. But it assumes that you have a
working
knowledge of electrical engineering. That is, not a beginner's book.

Well said, the RDH4 isn't easy to read. But each sentence is never a waste of
words,
so you must consider each word you read.

Building radios the way they did in 1955 from nothing upwards using a few tubes
takes a lot of effort, and the main thing apart from knowledge that makes 95% of
ppl
abandon projects is the building of coils and the discipline about
constructional quality.

Patrick Turner.

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