[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