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My
First Amplified Radio Project
based on the Armstrong regenerative principle with detailed notes on what lead up to building it A comprehensive story containing historical, educational, technical and biographical elements & opinions by John Fuhring My introduction
to the magic of radio science
I can remember exactly when my interest
in
radio
technology began.
In 1955, when I was turning 10 years old, my wonderful aunt
Helen
sent
me a birthday present. Inside the box was this outlandish
collection
of wire & other hardware including a rock crystal, a small
"cat's whisker" mechanism,
a
wooden coil form, a Bakelite chassis for
mounting everything and a
set of rather nice headphones (that I still use). It also
contained a sheet of instructions
on
how to wind the
coil, how to mount everything in the chassis and how to wire all the
parts together.
 My first crystal radio in its Bakelite chassis. I was searching the Internet for old crystal radio kits and good lord, there was a picture of my radio just as I remembered it.
Included were instructions on how to string up an antenna wire in a tree and how to establish a ground using a steam radiator or water pipe. In addition to all that, there was an illustration of how radio waves travel from a station and how these waves are captured by the antenna as they pass by. The sheet went on to explain how these waves are turned into electrical currents by the antenna. It told me that the different currents are then separated from each other by the tuning coil. Finally, there was an explanation of how a radio current, that is impossible to hear, is turned into electrical sound waves by the crystal and cat's whisker and then how those electrical waves cause the electromagnet inside the headphone to make the headphone's iron diaphragms vibrate and it was these vibrations that made the sound. I don't know where or how I knew some of this stuff, but I had already been exposed to bits and pieces of electrical theory and electromagnetism, so putting it all together this way made sense to me. For all this explanation, how could they call this collection of wire, rocks and other junk a radio? Naturally, I was familiar with radios and like all people back then, I avidly listened to news, comedy, music, drama and adventure programs that the AM radio waves were filled with back then. But, but, but, all "real" radios had these big tubes in them that you could see glowing red hot if you looked in the back. What is more, I knew that radios worked on electricity and they had to be plugged into a wall socket or no sound would come out. Now, here in this box were the parts of something called a "crystal radio," but there were no tubes, no batteries and no power cord. Looking at all these inert parts - just wire, screws, wood, Bakelite plastic and a piece of rock - I was extremely skeptical that anything so simple could possibly tune in radio programs. My mom (born in 1909) assured me that when she was a girl, many people in her town in rural Pennsylvania made these little radios and with them they would spend their evening hours listening to the big stations in Pittsburgh and Albany. Her story was obviously true, so I put aside my skepticism and decided to build this thing. My first attempted to build the kit and get it to work was a failure because I didn't understand the importance of winding the tuning coil neatly and I got some of the interconnecting wiring wrong in my haste to put it all together. When I complained that the radio didn't work, my mother pointed out that my workmanship was really poor and of course it wouldn't work looking that sloppy. It was obvious that she was right, so I got out the plans and this time I put it together according to the instructions. I rewound the coil, but this time all the turns were regular and the antenna slider could now move smoothly from side to side. As per the instructions, I took some sand paper and carefully sanded the varnish off the top of the coil so the slider would make contact with the wires. I took the interconnecting wiring out and carefully rewired exactly the the way the instructions said it should be. When I was done, I was pleased to see that my radio now matched the instruction sheet perfectly. The lessons in craftsmanship that I gained from building this radio were good and early lessons that have served me well since. If you want something to work, you must build (or repair) it right. Confident in my workmanship, but still feeling skeptical, I placed my radio on a little wooden shelf above my bedroom's radiator. I then connected the end of the antenna wire (that I had placed up in a nearby tree per instructions) to the little "antenna" input of the Bakelite chassis. My bedroom's steam radiator had a bright chrome-plated vent valve, so I connected a wire from the radio's "ground" connection to it. I could almost hear a drum roll as I put on my headphones. Silence -- nothing in my headphones -- so I moved the slider from one side to the other, but still silence. At this point, I learned another principle that has served me well all my life and it is just this: "when all else fails, read the instructions." The instructions said that I had to find a "hot spot" on the crystal by moving the "cat's whisker" over it until I could hear sound. Well OK, so I made a connection between the tip of the whisker and the surface of the crystal and then tried moving the slider again. I was absolutely stunned when I could hear sound in my headphones and the sound got louder as I moved the cat's whisker and found a better hot spot on the crystal. How could this be possible?? Here I was, hearing voices and music coming out of mere wires with no tubes, batteries or house current to supply electrical power, but that was impossible. I mean, where was the power to create these sounds coming from? I was extremely impressed and although it seemed like a miracle, I knew it wasn't supernatural, but rather it was a "miracle of science" and so I wanted to know all the mysteries behind how this stuff worked. Because the radio was made up of ordinary things, I felt confident that I would eventually understand how all these parts did what seemed to be magical things. The tuning coil I made up myself out of ordinary copper wire and it was wound on an ordinary piece of wood, the crystal and cat's whisker were ordinary pieces of metal and a shiny rock and when I unscrewed the cap on the headphone, I could see the iron diaphragm and the coil of the electromagnet inside. Although it seemed like magic, it wasn't magic and it wasn't beyond my ability to understand how it worked. It was immensely pleasing to see (and hear) how I could take ordinary things and make them do something so wonderful. This was the beginning of a life long fascination with radio and electronics that has continued (off and on) to this day. I made several improvements to my original crystal set including replacing the "cats whisker" and rock crystal with a fixed germanium crystal that actually worked a whole lot better than even the best cat's whisker. Looking back, I wonder that I was able to keep my interested in radios alive during this period because neither my parents or teachers encouraged me in this and none of my friends had the slightest idea how radios worked, or shared my interest in anything so "square" (or "nerdy," as we'd say today). My parents and teachers considered all these wires and batteries as a pernicious distraction from my otherwise neglected academic work and prejudicial to my social development. Perhaps they were right, but a nerd will be a nerd, a Dilbert will be a Dilbert, there is no other way we can be, and, as it turned out, my hobbies did more for me in my later life than any of the "book learning" that my betters forced on me. So, I continued to want to know more about radios, wires and gadgets and to teach myself what I could about all that stuff. I'd like to take a brief digression here and I hope you don't mind. I found out later that making and listening to these simple radios was a craze back when my mother was a girl and their wide-spread use was largely responsible for getting early broadcasting started. I don't think it is a bit of an exaggeration to say that these were the I-pods of my mom's generation and it was these simple devices that even poor people could afford that started engineers and scientists looking for better and better ways of receiving signals. This untiring search has resulted in the world of electronic communication and entertainment we have today. More
crystal radios
By 1957 we were living on the hospital side of Camp
Pendleton Marine Base and I was in the 7th grade. By this time,
the crystal radio bug had really hit me and I experimented with
improved crystal detectors. I discovered that the
little glass 1N34 diodes worked better than any cat's whisker and the
Marine Corps hobby shop sold them. While playing
and experimenting with crystal sets and diodes, I heard about
these little things called transistors that were just becoming
available. My folks knew what interested me, so they bought me
another little crystal set for Christmas, but this one had a one
transistor audio amplifier in it and it was so cool looking. This
radio
worked really well and it was the center piece of a radio corner that I
set up in a
little "fort" that some of us kids built out of wooden crates and
large boxes that we
salvaged from the Marine Corps' military dump. Unfortunately,
there
were no trees around, so my very poor antenna system consisted
of military field telephone wire laid along the ground. It so
happened
that the tuning of this little radio was so broad, I could hear
some of the military shortwave broadcasts the Marines made while
on maneuvers on other parts of Camp Pendleton.
I just loved it. My Remco crystal radio with a transistor amplifier. Well, in 1957,
transistors were still mostly curiosities and very, very expensive.
Transistors were also tiny and their workings were hard to
understand. At that time in history, the next step for a boy
learning
about electronics would naturally involve electron tubes (sometimes
known as vacuum tubes, valves or simply as tubes).
Electron
tubes
I really began
to learn
something about radios and how amplifiers worked when, about age
13, I built the one tube regenerative
receiver I will shortly describe. As I mentioned before, I knew
about tubes, but I
wanted to know what made them work. The wonderful thing about tubes is that they are built to a macroscopic scale. When I was a kid, I would look long and deeply into tubes, let my imagination soar and see a fantastic, glittering, futuristic city inside that glass envelope. I could look inside the glass and into that vacuum filled Electropolis and identify each glittering element. I could break open old tubes to examine their contents under a magnifying glass and see that they were nothing more than thin strips of metal or wire. Even a kid like me could see and identify the 'cathode,' the 'heater' wires, the wire spiral that made up the 'grid ' and the thin metal 'plate' (or anode) that encased everything. By seeing all these parts and holding them in my hand, I could imagine electrons "boiling" off the hot negative cathode. I could imagine how they would be attracted to and picked up by the positively charged plate (anode) and how that flow could be modified by tiny voltages on the grid wires. It was an easy step imagining how tiny voltages appearing on the grid would be amplified into large plate currents that would run the powerful electromagnets of a speaker. Later on, when I started to experiment with transistors, it was impossible to see inside them and so it was much harder for me to imagine "holes" moving through a crystal lattice. The truth is, I've never had a "warm" feeling for transistors and integrated circuits, regardless how marvelous they are. No, but I've always had a kind of affection for tubes. As a first lesson, I learned that vacuum tubes operate on an almost forgotten principle called the "Thermionic Effect." Before I go on, let me say something about this "Thermionic Effect." Way back in the 1880s researchers at the Edison Laboratories in New Jersey (not Edison himself) discovered that some kind of an "electrical fluid" (later found to be electrons) "boiled off" hot filaments, but in a one-way direction (from the cathode to the anode). Later, John Ambrose Fleming used this principle to produce an improved diode tube (the famous Fleming Valve) to detect radio waves. At the time, this was the best detector there was. It was thousands of times more sensitive than the coherer detector in use at the time and it actually worked better than the crystal diode which came a little later. Of course, the crystal detector was much, much cheaper, required no battery, lasts forever, is very rugged and so they superseded the Fleming Valve for a long time.  A Fleming Valve Detector. Very similar to the crystal detector, but more sensitive. The Fleming Valve was superior to the crystal detector, but consumed expensive tubes and batteries. In 1907, Dr. Lee De Forest added a third circuit element to a modified Fleming Valve to make a (very expensive and unreliable) crude forerunner of the triode tube, called the audion. To get around Fleming's patents, De Forest left a lot of gas in his audion tubes so that they operated in a very poor vacuum. De Forest then claimed that it was positive ion migration that made his tubes work. In 1912, while still an undergraduate in college, a very young Mr. Edwin Armstrong wrote a paper that proved De Forest was completely mistaken with regard to the principle on which his tubes worked and indeed De Forest really didn't have a clue how his tubes worked. Well, although the audions worked very poorly, they were an important start and they soon evolved into the linear high vacuum triode tube through the work of other researchers.
Edwin
Armstrong's brilliant invention
After
working
out the mathematical theory of electron flow in a high vacuum
tube and showing that high vacuum triodes could be used for linear
amplification, the brilliant Mr. Armstrong went on to
invent nearly all the oscillator, heterodyne mixer and amplifier
circuits we
know
today including the regenerative receiver I built.
In
fact, the regenerative receiver was one of his first inventions and he
came up with it while
he was an electrical engineering student at Columbia University.
How I
built my Armstrong regenerative radio
I am no Edwin Armstrong and when I was
a kid, I knew almost nothing about radio circuits, so I must admit that
I didn't come up with my
radio on my own. I got the plans for my Armstrong regenerative
radio from an old book in the
library called "The Boy Electrician." Now, before the
widespread
use of another one of Armstrong's
inventions (the superhetrodyne radio) and while early hand blown tubes
and factory-made radios were still very expensive, these little
regenerative radios were very popular and many young people built them
from kits and from a series of wonderful plans published by the U.S.
Government
starting around 1920. Making one of these radios at home
could
save a person a lot of money because factory radios were so expensive.
However, by
the time I built my radio things had completely turned around.
The fact was, I could have
easily bought a complete five tube factory built AM radio for less
money than what I spent on
parts for this little radio and I wouldn't have had to spend any time
building it. Nevertheless,
this one tube radio was something I could build myself and something I
could learn from so the education I got and the satisfaction I received
was priceless. God knows that I wasn't learning much in
school, so I needed to learn something.For parts, I bought some of them new, but the headphones I reused from the original crystal radio my aunt Helen gave me. The 1H4GT triode tube, the carbon resistor, the sockets, the potentiometer and the silver-mica capacitors I bought at the Sears Warehouse that we used to have here in town and where they repaired Sears "Silvertone" radios and record players. I remember the clerk wondering what in the world a dumb kid like me wanted with one of those 1H4 tubes, which were pretty old fashioned by then and they only had one of them in stock. I also remember how amused the guys behind the counter were when I asked for "two, two hundred and fifty micro micro farad CONDENSERS." Even by then, the word 'condenser' had gone out of style. The tuning coil was another matter. The book called out "standard" plug-in coils, but nobody had them or even heard of them -- they must have been for some very, very old radios. There were plans for winding your own coils, but I couldn't get or make the coil forms so I had to wind my coils on an old toilet paper tube (which is now falling apart and I had to 'dope' back together). I could be wrong, but I'll bet I was the only kid in town who made one of these radios from scratch and without any adult encouragement or help. I didn't have the tools or the experience or the skills to put together the project the beautiful looking way it was pictured in The Boy Electrician book, but I came up with a way of building it using what skills and materials I had. I picked out a nice piece of pine panel left over from a nearby house under construction to be the main chassis. On that board I mounted the tube and other components as shown below. To make a box for the batteries under the chassis board, I sawed and nailed up some left over pieces of redwood fence boards. It was really crude and really ugly as you can see, but I was just a kid without any tools or any expert help. Here is what it looks like today:  Main tuning control in the center, regeneration control on the right.  On/off switch, antenna, ground and phone connections.  Rear showing the "battery box," Not
exactly an Atwater Kent, but
pretty good for a 13 year old kid.
Today I live in a small city, but way back in 1958 Santa Maria was just a large town. Nevertheless, just about everything could be had locally and in many respects, hardware and even electronic parts were easier to get back then. Today all the local merchants have been put out of business by huge mega corporations owned by faceless boards located god-knows-where and today my town's stores only carry those Chinese made products that are quick selling, pre-manufactured products. In 1959 we had a Sears Warehouse where they had electronic parts and several radio and TV shops where you could buy such things as wire, capacitors, tubes, sockets, resistors and soldering irons. Today we are particularly fortunate to have about the only electronic parts store between San Francisco and Los Angeles, but by no means do they carry every thing a kid would need to build stuff like this. Well, the plans for this regenerative radio called for a triode tube and one that was already very old fashioned, but, as luck would have it, the Sears Warehouse had one in stock. The tube is a 1H4 and that's it in the picture. All vacuum tubes require a hot cathode to "boil off" electrons and most more "advanced" tubes have a cathode indirectly heated by a filament. The 1H4 tube was designed for low power so its cathode is the filament and because of that it operates at a low voltage (1.5 volts) and a low current (only 15 milliamps). Because the 1H4's filament requires so little power, will operate for many hours on a single ordinary 'D' cell. In the old days, the battery for the filament was called the 'A' battery and originally I used one of those huge "telephone" 1.5 volt cells that were so common back in those days, but to get it working today I use a 1.5 volt alkaline 'D' cell. To power the plate circuit of the 1H4 tube back in 1958, I used a small (and what was then a commonly available) 45 volt portable radio battery. These batteries haven't been made in 50 years, so to make my old radio work again, I made up a 'B' battery by ganging together 9 volt batteries until I had 45 volts. The DC voltage from the 'B' battery flows through the headphones where the current varies to produce the sound, through the 2.5 mH choke coil, past the regeneration control, through the 'tickler coil' and then to the anode (or plate) of the tube where it can power the amplification process. A
technical discussion of how an Armstrong Regenerative detector works
To pick up radio signals, a piece of wire has to
be
strung up
as high and as long as practical and some kind of a ground must be
connected too.
Passing
radio waves from a distant transmitting station electro-magnetically
couple to this wire and,
in response, a very tiny, but high frequency electrical voltage
appears on the wire. The frequency is so
high
that the signal passes through the Trimmer capacitor and
starts a alternating current flowing back and forth in the coil between
points G
(and the top of the tuning condenser) and point C (and the bottom of
the
tuning condenser). That tiny high frequency alternating
current
is then coupled through the "grid tickler" network consisting of a 2
meg resistor and the 250 mmfd capacitor (to the right of the
coil/condenser) and from the "grid tickler" the signal appears on the
grid, pin 5 of the 1H4 tube. (Note: this discussion assumes you know something about resistance, capacitance, inductance and tuned circuits. If you slept through science class, you may wish to skip this section.)
The "regenerated" signal that is magnetically coupled to the main tuning coil is coupled to the grid of the 1H4 through the grid tickler network (250 mmfd capacitor & 2 megohm resistor) and the signal is amplified again. This goes on and on and, depending on how the regeneration control is set, the overall amplification may be several hundreds or even thousands of times the original signal level. Amplifying a signal over and over again like this would be just great, but this kind of amplification (called "positive feedback") can easily be overdone and you don't want to over do it. It is of utmost importance that the amount of "positive feedback" (regeneration) be set very carefully. If the throttle is set up too hot with too much bypassing to ground, the tube will go into self-oscillation and a squeal will be heard as the original radio signal "hetrodynes" with the oscillation of the tube. On the other hand, if the throttle is not set up hot enough, the tube doesn't amplify very well. Finding that elusive "sweet spot" of just exactly the right amount of regeneration is why a smooth throttle control is necessary. If you will look at the upper left of my radio's schematic above, you will see something labeled '2.5 mH CHOKE COIL.' A choke coil blocks high frequency radio current, but allows audio frequency current to pass through. A choke coil must be used here or the bypassing will be very chaotic and the regeneration level (throttle) will be very unstable or it won't work at all. During all this amplification going on inside the tube, the parts of the original radio signal (the carrier wave and the audio sidebands) are all mixed together (or heterodyned) and audio frequency currents are produced. This audio frequency current passes through the choke coil and from there, the current flows through the electromagnets in the headphones, those currents cause changes in the magnetic field that attracts the iron diaphragm of the headphones causing them to vibrate and it's those vibrations that I hear as music and voice sounds. (* A reader named Ray has kindly submitted a much better explanation of how the audio is produced and you will find his write-up at the end of this article.) As mentioned, the audio is stripped off and appears across the 2 K Ohm headphones shown on the right (the choke coil does not block audio signals, only radio frequency currents). The
importance of smooth control of the regeneration
To control the amount of
regeneration, you
need a way to carefully increase or cut down on how
much
radio frequency current is flowing in the tickler coil. Too
much
current and all you get is a squeal, but too little
current and you don't hear anything. This is called
"varying
the amount of bypassing" and you have to have it set exactly right for
best
performance. In the real old days of radio, this regeneration
control was called "the throttle circuit" because they "throttled"
the regeneration up and down just like you controlled the speed of an
car's engine by its "throttle" control. For throttling in the old
days, they always used a variable capacitor.
 Typical Regenerative Radio of the 1920s. A variable "throttle " condenser was a common and excellent way to control the amount of bypassing and thus control the amount of regeneration.  Original Regenerative Radio Design from The Boy Electrician. The amount of regeneration was "throttled" by a rheostat across the tickler coil. This proved to be very difficult to control and so I came up with a better way on my own. Well, at the time I built my radio, variable capacitors were expensive, hard to get and hard to mount, so a variable capacitor to control regeneration was out of the question. As suggested by the book, I used a rheostat to "swamp" the tickler coil and control the regeneration that way, but soon discovered that this method worked very poorly and the right amount of regeneration was just about impossible to control. I then came up with what I thought was a very clever idea.  My
radio with its modification
as it would have
appeared in The Boy Electrician.
May
26, 2011. I
make
an important discovery after only 50 years.
.
I have a confession to
make. Yesterday I modified my first radio and now it isn't
exactly the same as when I built it so many years ago. I had
to
because facts came up against my pride and the facts won.For the last several evenings, I have been "DX'ing, " on the broadcast band to see just how well my old radio performs. I originally discovered the defects in the design of the Boy Electrician's regeneration control by spending time with it and becoming dissatisfied with its performance. By recently spending time searching out weak and distant signals, I once again became dissatisfied with my radio's performance. While my throttle control method is a vast improvement over the Boy Electrician's method, it has its own defects that I finally decided needed addressing. While trying to tune in weak stations, I noticed that the tuning would shift and weak signals would disappear as I adjusted the throttle and this became more and more dissatisfying. A few days ago I experimented with a variable capacitor arranged as a throttle. Of course, this is the very same way people in the 1920s used to do it. I soon discovered that the old timers were right after all and that a variable capacitor is clearly the superior way to control the throttle. Somewhat reluctantly I removed the potentiometer/capacitor throttle arrangement I was so proud of and put in a variable capacitor. I completed the installation yesterday and last night I was very gratified by how much better the old radio performed. The DX conditions these last few nights have been extremely poor, but I was able to bring in a Los Angles station, KGO in San Francisco and a Reno station clearly, if not loudly. The new old-fashioned throttle circuit made all the difference and it was a joy to use.  Using a variable "condenser" for a feedback throttle is still the best way to go. This diagram is the circuit configuration I think the Boy Electrician book should have presented.  My Regenerative Radio today using modern electronic symbols. Doing a little research on how these radios were originally built in the 1920s, I discovered that nobody used a potentiometer to swamp out the tickler coil and the original way using a throttle capacitor was the best way to control the regeneration. Something that is part of the Scientific Method
Well, I think there's a good
lesson here and even at my age, it is good to experience such
things. I have crowed loudly about my superior way of
throttling
the tickler coil, but now I'm forced by experimental results to admit
that my method is not the best. The lesson here is just
this: if proof is offered that there is a better way to do something,
you must abandon your old ideas no matter how clever you originally
thought they were. Anything else is a
dishonest attempt to deceive yourself or others for the sake of foolish
pride. Just because something has been around for a long time,
that doesn't mean it is worth hanging on to if scientific evidence says
that there is something better.
Some
final words on my early radios
Well, that's the story of how I got interested in
radio and the story of the first radio I ever
made using an
amplifier. I am especially fond of this ugly little radio because
it's my earliest project that has survived to this
day.
I didn't invent the radio and yes, I got the plans out of a
book,
but still,
it
did contain some of my own ideas. Beyond all that, building
the
radio and getting it to work
taught me several important things regarding how signals flow through
circuits, how those circuits work and how vacuum tubes amplify.
It was this early specialization that lead to my career in electronics
and, although now retired, I still find enjoyment is this sort of thing.By the way, I have restored one of those really excellent Heatkit CR1 crystal radios. I have also designed a tiny crystal radio kit based on the Heathkit and have been having fun experimenting with crystal radios once again. The truth is, my Armstrong radio is far, far superior to either one of those crystal radios in terms of both selectivity and sensitivity. My recent experimenting has reminded me once again why the crystal detector, after it got the Radio Age started, was abandoned for Armstrong's designs. During that period after 1918 and until they were superseded by TRF and the early superhetrodyne radios in the later 1920s, radio enthusiasts knew that even a simple regenerative radio (like mine) is clearly superior to even the most elaborate crystal radio. The thing that held these early radios back was the really awful and terribly expensive early tubes (Audions) that were the only things then available. Nevertheless, those early regenerative radios were so clearly superior to even the best crystal radios, that a lively market for those expensive and unreliable Audions developed. Now that people could see the possibility of having better and better radios to listen with, a demand for better and better tubes developed too. This market drove the infant electronics industry so that by the mid 1920s, engineers and scientists quickly developed high-tech tubes sufficiently advanced to allow the development of almost all the modern circuits that are used today (using solid-state amplifiers, of course). Early
20th Century technology using more modern devices
Today
the very same kind of radio can be made with a transistor. If
you
use a Field Effect Transistor (an FET), the circuit layout is almost
identical to the Audion layout of the early radios. The FET
circuit shown below works on exactly
the same principle as the vacuum tube models except that
there
would be no need for an ' A ' battery and the 45 volt ' B ' battery
would be
lowered to just 9 volts. Instead of connecting the "grid
tickler" to
the
grid (pin 5), it would be connected to the 'gate' of the FET and the
'drain'
would
be connected as the anode (or plate - pin 3) and the 'source' would
connect to the ground (or anode - pin 7). Of course,
since
an FET's gate can't pick up passing electrons and therefore can't self
bias, a biasing network (a good place to take off the audio) is
necessary as is a diode in the gate circuit to provide additional bias
when the transistor is oscillating.
A question
remains though. Why would anybody today want to go to the
expense
and
labor to build
their own AM radio receiver? Yes, it would be as good a
learning
experience as it was when I was a kid, but today the reward of
listening to something that would bring you joy is missing.
There
is today nothing a kid would want to listen to on AM radio or would be
good for a young mind to hear.
A Personal Lament On The Fate of AM Radio (You don't have to agree with)
I consider
myself very lucky to have been doing all this at a time just before AM
radio took its fatal plunge and started to degenerate into the "Hate
Talk Radio" that we have today. Back when I was having so
much
fun doing all this experimenting AM radio still had on it things that
were well worth listening to and not poisonous to young minds as it is
today. Elvis, James Dean, Fats Domino and other popular music
stars were still on the AM dial as were all kinds of non-political news
and entertainment. I remember there was a very hateful and
hate-filled "religious" radio station in town, but I avoided that part
of the dial. Today and for a long time now, there is
absolutely
nothing on AM radio that I want to listen to or I would want a young
person, who has not developed his critical thinking yet, to hear.
Several years ago I wanted to make up a basic radio kit for my young nephew and see if he would find the same magic and fascination in electronics that I found when my aunt gave me a similar radio kit. I had been extremely impressed with my nephew when he was in elementary school and thought he'd be the perfect candidate for this sort of thing. I started to gather up the components for one of these radios, but in the meantime started listening to AM radio once again. I listened to our local stations and was shocked and disappointed at what I heard blasting all day long, every day. All that is left of local AM radio now is this retched so-called "Christian" Fundamentalism and so-called "Conservative" hate talk partisan propaganda. Certainly nothing that is good or inspiring to help a young mind develop and certainly nothing a young person should listen to before he or she has achieved enough maturity whereby the difference between indoctrination and learning can be discerned. Yes, I was shocked at what I was hearing all over my local AM band so I scotched the idea of trying to get my nephew interested in radio the way I had. It probably wouldn't have worked anyway because, as my nephew transitioned out of elementary school, he soon dropped all interest in everything but motorcycle and truck racing. Still, I feel badly that AM radio has degenerated into what it is today and that I was never able at least try to introduce my nephew to that thing that I found so rewarding all my life. It is also sad for me to observe that, with the advance of digital technology and the Internet, it is probably just a matter of time before the Broadcast Band and Short Wave radio is altogether extinct and these wonderful old tube radios will become as quaint and useless as telegraph sounders. THE
END
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Here is
Ray's detailed explanation of how audio is derived in a triode tube
circuit. I hope to add to Ray's write-up with some waveform
diagrams and arrange it in the text of the article, but for now, I'll
just give you the write-up as I received it:
Grid
leak detection in regenerative radios.
Involves
the use of grid
rectification to retrieve the audio from an AM signal. The capacitor charges to
a peak level
during the positive half of the modulated RF wave. This happen
because the grid conducts
when driven positive with respect to the cathode. During the negative half
of the wave, the
capacitor then starts to discharge through the grid leak resistor that
is across the
capacitor. All this causes the plate current to change at an audio rate. The
result is audio
recovery at the plate. |
