Jon Noring wrote:

[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

Building tuners for AM, ( including stereo AM )
and FM stereo tuners to suit the Zenith system used internationally
is a fine project for the diyer, except it does take a very deep working
knowledge of
coil winding for RF, 88-108MHz, 550kHz to ,1,700kHz, IF transformers,
both 455 kHz, and 10.7 kHz, and discriminator coils, and 19 kHz and 38 kHz
coils,
plus all the LC filters used for the stereo decoder.
As soon as ppl have to get off their butts and understand and wind coils
for all these F, they give up,
because its too hard, and there is simply so much to know, and it all
takes months to get anywhere.

In 10 years of being interested in such things, I have found almost zero
interest world
wide in ppl wanting to build an AM radio from start to finish, and
I have and only 3 enquiries about the workings of the FM MPX decoder
I have at my webpages in the 4 years I have had a website going.

I do have a schematic of a decoder at
http://www.turneraudio.com.au/htmlwe...mpxdecoder.htm

Anyone is welcome to try to build what I made up from parts in a Trio
receiver, which originally gave the most appalling stereo decoding when I
bought
it second hand for $100.

My design is totally different to anything by Trio.

I don't really want to spoonfeed anyone with helpful information to build
such a thing
unless they are well prepared as I was to do all their own research in
their local university archive libraries
which will maybe have much of the 1960s info about how this stuff actually
works.

I have reams of info in hard copy form which I photocopied.
I have had a second decoder on the drawing boards for 4 years, which
should give lower
mono to stereo thd conversion, and clearer audio.

There are some hi-fi AM tuners which were all solid state, the AudioLab
was one such
which had wide AF bandwidth and low thd.

Stereo AM using tubes would be quite a challenge, but why oh why?
I don't think the programmes I listen to on my home made AM radio
warrant the effort involved to get a stereo signal.

To build a tube based AM tuner for mono is a nice project, and
one can re-cycle the litz wire RF input coils and 455 kHz IFTs.

The info about increasing the pass bandwidth of IFTs is in RDH4, and it
involves a
a switched tertiary winding of a few turns on IFTno1 of an existing set
How this works out
for the diyer depends on the tenacity and discipline which is employed to
measure the IF bandwidth.

The tubed superhet I found was the best type of radio for AM.
Forget TRF, or direct conversion using tubes, ie, the synchrodyne of
homodyne receiver.

The input coils ahead of a converter tube like a 6BE6 or 6AN7
need to have a pass band at all RF frequencies of about 20 kHz each side
of the station F,
so I used two cascaded LC circuits, slightly stagger tuned to get wide BW
at the LF end of
the band where a single LC circuit has too high a Q and causes sideband
cutting, and audio attenuation
of the transmitted modulation.
Anyone not understanding what I just said should hurry off to their
library to find out;
I ain't interested in doing your home work for you.

The IFTs found in most old sets are usually 455 kHz.

To widen the poor natural BW of these tuned LC transformers,
the tertiary can be used on IFT1, as described in RDH4,
or the LC circuit can have its Q reduced by placing a 100k zcross each LC
circuit.
Experimenters will find the right value of R to reduce the Q.
But the downside is that the skirt selectivity will suffer, and stations
only
50 kHz away from the wanted signal will be heard, so add another IF stage
with R damped
LC circuits.

With luck, maybe you will squeeze a bandwidth of 14 kHz after all these
tuned circuits,
and this allows 7 kHz of audio, -3dB point.
The rate of attenuation beyond the 3 dB point is severe, and has a huge
amount of phase shift
This can be reduced a little with an RC step filter which boosts the
treble at 6 dB/octave
after a pole at 7 kHz, and perhaps you can extend the recovered audio out
to 9 kHz.

I tried fitting 9 kHz notch filters to remove the whistles heard on DX
listening, but
it does not remove very much except the carrier interference; the
modulation of a station on a nearby
F still gets through.

Stations here in Oz are 9 kHz apart.

Some are allowed to modulate their carriers with whatever is on the CD.
Others are limited, because the sidebands of stations only 9 kHz apart
will interfere if the modulation audio extends beyond 4.5 kHz for each
station.

The basic problem with distance listening of AM is that there are so many
stations.
I don't bother with DX AM, and since all the stations are networked, there
is no point
listening to rock and roll from MP3 at reduced audio BW from hundreds of
miles away
when the same trash can be heard locally, which bores me to tears anyway.
Noise ruins most DX listening, and the hums and buzzes from switchmode
power supplies
all around the local area.

I only listen to the govt owned ABC stations because their news and public
affairs info is good,
and there is no advertising, and no blue collar based lowest common
denominator redneck
low grade talkback shows spaced between adverts I cannot tolerate.

My set has a dual stagger tuned RF input, an RF amp using a triode
CF feeding a grounded grid triode amp resistively loaded with 22k, from
which the
tuned RF signals are RC fed to the 6AN7 converter grid.
The 6AN7 has cathode bias.
The basic selectivity prior to the converter prevents a powerful station
cross-modulating the wnated lower power station.

Then I have two IFTs with R damped windings, but No1 has variable distance
adjustment
between the two coils to slightly increase coupling to increase the BW.
The IF amp is 6BX6 set up with cathode bias with no AVC applied,
and the amplification is as linear as possible.
The AVC voltage is only applied to the RF amp.
After the last IF LC, I have a CF triode buffer which powers a germanium
diode detector,
with the diode biased on at all times with a small current to avoide diode
distortions at low levels of signal.
This is followed by a second CF buffer and RC step filter to boost the
audio at HF,
and a tone control to adjust treble +/- 6 dB, needed because many
programmes
need such adjustment.
The bass response is good down to 10 Hz.
The thd is very low, leaving the thd and compression and limiting effects
used by the station to
be heard in all their glorious ugliness, when they are used.

My set has a trioded EL34 and 12AX7 paralleled to make a normal feedback
audio amp
with a 1953 12" speaker in reflex box of about 60 litres,
with a 1972 dome tweeter for above 5 kHz.

Better AM reception I have not heard.

The set is able to produce many volts of audio at quite low thd,
but I have it set up to make only about 2vrms max, to keep the thd low,
but also
enough to climb above the set noise.

The antenna is a peice of wire about 3M long.

Directional antennas might be better, but the only hassle I mainly have is
with some
local ******* who has a switchmode PS which puts out a buzz ridden version
of
the stations I like to listen to. This is an increasing problem despite CE
legislation to
compel asian made crap from causing radio interferences.

I have been sitting happy at breakfast when this ******* turns on his
gear, and BZZZZZ.
It ain't my set's PS, because it has a lot of capacitance in the PS, and
it runs on a few secs after turnoff.
So when I turn it off, the hum continues, so the buzz isn't from my PS.

I earthed my set well to a water pipe just below where the radio is, but
still I get it.
Maybe the water pipe is the source of the signals, along with the mains
wires, but
fitting filters to the mains didn't help the problem and I think
its being picked up by the input coils, so whatever is causing the problem
is
perhaps modulating the stray pick up of the RF stations, and
re-transmitting it.
I had a Shimasu telephone answering machine which had a plugpack linear PS

and it managed to cause severe hum interference on any radio used on the
same
house power circuit, despite placing caps to shunt all the supply lines
and mains input
to its PS.
It went into the bin and I now have a tapeless digital answering machine,
which is more reliable, with better audio quality.

Patrick Turner.