Mark Oppat wrote:

Hey, why do all this work when you can get a killer EH Scott 800B
chassis, nice chrome and all, with power supply/audio chassis for a
reasonable amount like $250???

Many have called attention to the fact that tube-based AM radio
circuit design probably hit its "zenith" (pun not intended) back in
the late 1930's through the 1950's.

Since my interest is in building a tube-based AM tuner using modern
components, adapting the circuit design of a highly reputed older
radio is certainly intriguing. Just remove the final audio amplifier
section (and tweak the line-out as necessary to conform it with
today's standards), and maybe replace a few other "autonomous"
sections with modern solid-state equivalents if there is any advantage
to do so (e.g., the DC power supplies.)

Of course, it continues to be suggested to just find an older radio
and cannibalize it to my heart's content. In my case, by the time I've
cannibalized it and rebuilt the chassis, the mechanicals, replaced
all wiring, resistors, caps, etc., etc., it probably would have been
better to just build the damn thing from scratch per a schematic and
general layout. Thus the idea is to come up with a general schematic
and layout design, and build from that.

And once a group of enthusiasts have come up with a working design
with schematics (a Mark "I" model), then lots of people who like to
build their own "homebrew" audio components can then just build to the
schematic, order the parts, follow some guidelines, and away they go.
No need to scrounge around looking for 60 year old tuning capacitors
from model such-and-such, or to use odd-ball tubes which are difficult
to find. The design would, by-and-large, use new, "off-the-shelf"
parts. Maybe only one or two parts will require custom building, but
then they could be specially built in lots of 100 or so if the
enthusiasts band together to make this happen.

Also, the point brought up by Randy Guttery is valid: it is difficult
to design an AM tuner which perfectly and simultaneously meets all
requirements under the sun. There are always compromises to be made.
It may be possible to design the tuner to have the performance of a
Collins R-390A (which Randy mentioned) but then the complexity of the
circuitry and mechanicals elevates it up to the stratosphere -- only
large companies could afford the tool-up costs to build the thing,
or the enthusiast would need access to a machine shop, and lots and
lots of $$$$$, to custom build the various mechanicals and certain
electronic parts for it (a *major* undertaking even for the very
experienced, and not a two-weekend project for the average tube audio
"homebrewer".)

The key is to be reasonable. I think the key is to shoot for the
80-90% level on all requirements:

1) that 80-90% of *all* MW DXers would find its performance to be
quite acceptable and they would consider using it for their
DXpeditions (using their own antenna systems of course -- the
antenna is the key to performance anyway -- lots of MW DXers use
quite "simple" radios but with good antennas get acceptable DX
results),

2) that 80-90% of all tube audio enthusiasts would find its line-out
audio quality to be acceptable for listening pleasure (given a
strong local station of good broadcast audio quality -- most tube
audio enthusiasts will *know* not to expect CD-audio quality, but
they'd like low distortion and low noise -- a good and pleasing
sound from what they expect from "AM"),

3) that 80-90% of those who love the aesthetics of the old tube radios
will enjoy this design, and a few may actually wish to build it and
place it into a 1930's-inspired cabinet for it (or if they have an
old cabinet sans chassis, to use this in place of the original
chassis), and

4) that 80-90% of those who routinely build "homebrew" tube audio
equipment would be able to build this AM tuner design from scratch
or from a "kit". (This does suggest a fairly simple and stable
circuit, not requiring much in the way of adjustment or tweaking,
and no elaborate mechanicals -- for example, a single multi-gang
tuning cap would be all the mechanicals they need to worry about.)

(The next step would be to translate the above general requirements
into a list of more specific requirements, such as Randy recommended
in his message.)

So, rather than re-inventing the wheel, based on the general
requirements given above, what tube radio/tuner of yesteryear (from
the 1930's to 1960's) would make a good candidate as the starting-point
for the design of this new "Mark I Tube AM Tuner"?

I look forward to the list of candidates. :^)

Jon