As its going to be something special, something unique, something I will build completely out of my head and that I will have great fun in building, and using, in essence a voyage of discovery and nostalgia.
| My 40mtr Valve Receiver Project |
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I suppose every Radio
Amateur, and I use the term loosely here, has a Home Brew Project lurking
somewhere in the back of their mind, for some it just stays there as just a
talking point over a beer at the local radio club, for others its just one of
many such projects that they build like shelling peas, but for some like me
who have built projects in the distant past, its an ambition, something to
help me drift off to sleep at night as I plan its every detail As its going to be something special, something unique, something I will build completely out of my head and that I will have great fun in building, and using, in essence a voyage of discovery and nostalgia. |
| Mostly it seems practical
amateur radio enthusiasts, and by that I mean the ones that actually own a
soldering iron and know how to use it, have early beginnings, something
usually happens to them in early life which inspires an awe of wonder that
stays with them for the rest of their life, for me it was an edition of The
Practical Wireless, where I read an article on building a Transmitter, at the
end of the article the writer went on to describe how he used the Transmitter
and some of the contacts he made whilst using it, to me its was romantic
beyond belief! to be able to Transmit messages over long distances and to
strange places to other like minded individuals seemed incredible and
something I would simply love to be able to do, and from that moment on I was
hooked, and after a lifetime in Electronics and communication I can once more
experience those first faltering steps in this project. Thinking back My first practical encounter
with practical radio was about the impressionable age of twelve, when I was
with a young friend called Alan, after mentioning to him the delights of
amateur radio and how interested I was in it, he said that there was an old
radio in his fathers shed that had short waves on it, it wasn’t long before
I had persuaded him to dig it out from under all the junk and plug it in. He wasn’t very happy about this, being
like most none technical people a bit wary of anything old and electrical,
and being younger than me a bit worried about what his Dad would say if
things went wrong, anyway to cut a long story short we plucked up courage and
switched it on hastily moving back a few paces, nothing much happened at
first then a slight hum could be heard which slowly got louder and louder, Al
became very agitated and suggested we turn it off, and just as we were
discussing this the old radio let out a howl like a banshee or come to that a
herd of Banshees, Al was horrified as at the same time a cloud of acrid smoke
belched out of the old sets innards, he leapt forward and snatched the mains
plug from the wall, then we opened the windows and wafted the smoke out
hoping against hope that no one had called the fire brigade, and such was my
first encounter with shortwave listening, it never dampened my enthusiasm,
far from it, it all seemed rather exiting and I went on to greater things,
Dear Al sad to say died a few years later on a school climbing accident But I digress, an amateur radio project
can mean a lot of things to a lot of people it can be as simple as a couple
of leads and a few connectors to a massive all singing all dancing state of
the art all band micro controlled transceiver, for me its usefulness and
nostalgia , and if its nostalgia its just got to have a few valves in it.
Well lets face it Amateur Radio has changed, communication has changed, how
many of us have heard the old cry, "my mobile phone can reach anywhere
in the world so why bother with that old pile of rubbish" so really
these days for me anyway its nostalgia, a bit like steam train enthusiasts, I
love using my old skills and still manage to extract a real sense of
achievement which is completely self generated, so having said all that I
decided at long last, having built a small low powered valve transmitter as a
trial run, to finally build my dream project, the project that had changed
over the years to finally come back to its beginnings, a tailor made all
valve CW receiver for one band, 40mtrs Now being retired at the grand old age
of 58 I have once again taken interest in morse code on 40mtrs where at the
bottom of the band real amateurs can still be found hidden away in the last
bastion of amateur radio. So how does one start a long intended
project, well like interest in radio itself I have found it needs to be
kicked off, for me it was at a rally where under a bench I found the remains
of an old Home Brew 2mtr transmitter in a nice little metal cabinet, full of
useful spares, it seemed just right to house my project, and just what are
the criteria for a project? well again everything to everyone, for me it’s
a combination of nostalgia, cheapness, usefulness and interest, I wanted a CW
receiver for 40mtrs that was valved (nostalgia) Made from bits from the junk
box (cheapness) Excellent perfomance (usefulness) and completely designed and
built by me from scratch (interest) At last a chance to build a receiver that
had everything I found useful in other receivers I had owned or used over the
years. What becomes clear as soon as you start a valve project is the metal
work involved!! and of course the valves and valve type components that need
to be found, so I shall list here the things I found that without them the
task is almost impossible, first without doubt is a set of valve hole cutters
suitable for the type of valves you intend to use, next is a good set of
metal working tools, files drills snips and so on, and of course valvey type
bits!! nuts bolts drive shafts earthing tags and tag strips, large resisters
and so on, the easiest way by far to obtain these parts are radio boot fairs
or Rallies, just look out for old valve chassis and defunct projects that can
be stripped for spares, and of test equipment without doubt a GDO (grid dip
oscillator) AVO or similar analogue meter and a general coverage receiver,
all these items I consider essential And of making a start of course, there
are more excuses for NOT starting the project than could ever be
written down, here are a few of the commonest, Not enough time, Nowhere to do
it, haven’t got the know how, haven’t got the parts, haven’t got the
information etc etc etc, some of course are valid, you must have a place
where you can leave the project and go back to it, and you must have enough
tools to make a start but once started the project gathers a momentum of its
own, the search is on for tools and parts and information, and with it comes
the fun and the skill and the knowledge. |
| So the time finally arrived
when I had run out of excuses and made a start, the next day or so I played
with an old 2inch meter removing the case and making a hole in what was to be
the front panel and fitting a slow motion epicyclical drive and shaft and
tuning capacitor, moving them about and taking measurements until I had in my
minds eye what would work and what would not, I made a temporary scale and
fitted the whole thing together and much to my amazement it looked good
worked well and I decided to carry on with it. |
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| Sometimes these ideas just
don’t work, and after a while you just have to admit to yourself it
wasn’t as good an idea as you had hoped for, better to abandon an idea than
flog on with it regardless, not wanting to admit defeat. And now for some
electronics, I tend to try and get some interesting electronics in early to
keep my enthusiasm going Building a Power supply and audio stages is a
fairly straight forward task and I wanted to hear some sounds from the
project asap. So I used an existing PSU to take off some required heater and
HT volts and quickly made a small audio amplifier from a twin triode valve
type ECC82, and fitted a volume control to the front panel, I had decided not
to bother with speaker output as I was only ever going to use headphones and
possibly this stage may be modified later, next I cut another valve holder
for the frequency changer stage, I had decided to build an absolutely bog
standard superhet to start with and modify it to my requirements as I built
and tested each part as I progressed, the first job was to get the oscillator
to oscillate at the frequency I wanted, and to track from the start and end
frequencies in the way that I wanted it to do, this is a fiddly job, the way
I tackled it was I simply wound a coil in an old IF transformer can and
fitted a small capacitor across it and used the GDO to see where the the
resonant frequency lay, and adjusted the turns of the coil to suit, I decided
to run the Local oscillator low as I thought it would be easier for tracking
and as I had decided on a 455 khz IF frequency, the frequency of the
oscillator would therefor have to be 7.00 mhz minus 455khz and track to
7.1mhz minus 455khz so from 6.545 to 6.645mhz I had decided to use an ECF82
frequency changer, simply as I had one and I wasn’t really worried about
various problems that come with combined frequency changers and low IF
frequencies, as I had ideas to overcome these problems and also I wanted to
experience these problems first hand as I went along so I wired up the triode
part of the ECF82 and tested it out, to get the oscillator to track over this
very narrow range I decided to put a small pre-set capacitor in series with
the main tuning capacitor, with a few minor adjustments I soon had the
oscillator running very well, the next stage was wiring up the mixer part of
the ECF82 and drilling the chassis to take the first IF transformer, I was
itching at this stage to hear something and so I temporary fitted a germanium
diode from one of the secondary IF winding to earth and the other end via a
small capacitor to the audio stage to act as a very simple detector, and
connected a 40mtr dipole via a small capacitor to the grid of the mixer,
amazingly it worked very well!! the very first signs of life!! |
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The simplest of superhets,
no rf stage or IF amplification and yet I could hear 40mtr morse signals
beating with each other quite plainly, using my signal generator set to
455khz I simulated a BFO by connecting it to the detector diode, and 40mtr
SSB signals were heard quite strongly, my 40mtr dipole was working quite well
as a tuned circuit for the mixer, as other parts of the band were quite
reduced when I tried the oscillator on different frequencies At this stage I
had a break from construction and had a think about what to do next, the next
stage I decided was to be a tuned circuit for the mixer input and an IF
stage, these I wired up and tested the next day,
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| and the result was a very noticeable increase in performance, the bandwidth of the received signals was reduced considerably as was also the second channel interference due to the tuned circuit on the input to the mixer, in fact the receiver at that point could have been used for communication with strong signals, I spent the next couple of days playing with the simple receiver and testing its good points and bad, in the evenings the second channel interference made it completely unusable so I decided the next stage would be an RF amplifier and shortly after that a BFO so I could use my Signal generator for other things, this I did and was surprised to find although there was an improvement in reduction of second channel interference it still was not good, so the next few days were a real learning curve for me on the Q of coils and their matching, I found that a parallel tuned circuit on the RF stage grid preceded by a series trap gave excellent results, almost eliminating the second channel interference, the bandwidth of course was pretty bad owing to having only two tuned circuits in the IF stage so I then wired up a standard IF amplifier stage and now as there were no gain controls the IF and RF stages had a tendency to go unstable, time to experiment with some sort of gain control, I had decided very early on that there would be no AGC circuits, I had never found AGC to be of any real use in a receiver that would be driven, ie. me at the controls manually selecting the gain and bandwidth and the reason for the instability was I assumed, because everything was going flat out as indeed it was, I experimented with all types of gain control and finally settled on varying the gain of both RF and IF stages by varying their screen grid voltages together, this worked out very well although discussions on air with other engineers seemed to favour a varying cathode voltage system or a manual sort of AGC ie manual or auto with negative bias, but in the end I stuck to the screen grid method as it was simple and very effective.At this point the receiver was working very well and on Par I should say with many of the simple so called communications receivers of the 60's and 70's tuned to 40mtrs. |
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Time to start adding a few
refinements, I had long favoured a product detector for my CW and never had the chance really to try one out, so that was the next stage to be wired up, and I was very pleased with the result although I had to play with the level from my BFO into the detector to get it right, what struck me straight away was the lack of noise now from this little receiver, I had at first suspected it was a bit low in gain but after comparing and some signal tests I found it was very sensitive. Now I had to do some soul searching, I had intended to limit everything to the older valve technology but I did want very good selectivity, I had hoped to get away with just using an audio filter system, similar to the popular selectoject circuits of the 50's |
| but after a lot of
experimenting I found them difficult to use and hard on the ears as they went
in and out of oscillation when tuning them in, so I decided to try an old
Mechanical 455khz filter to sharpen up the IF bandwidth and WHAT a difference
that made, making the receiver performance superb, at this point I tried it out on air for the first time working alongside the little CW QRP valve rig I had made the previous summer. And straight away worked into Germany and up into the Midlands being able to give honest 57 58 and 59 reports, and the thrill of being able to send "Rig ere all HB es all valve" was something I don’t experience very often these days, all the experimenting, rebuilding not to mention a few electric shocks on the way made it all worthwhile, but I soon realised I was starting to struggle with some of the weaker signals especially the QRP guys alongside the QRO guys, and so I thought I’d try a 455khz crystal in series with the mechanical filter putting a switch in to select it, and this proved to be icing on the cake bringing the bandwidth down to about 300htz and ringing softly as only a valve circuit seems to do, and this, so far is how it has stayed,I have since built a small external PSU which makes the receiver even in its steel case, very light and easy to move about and also fitted a voltage regulator, On test the only fault, if you can call it that, is the usual drift in frequency of all things valve, I, having to remember to switch it on an hour before using it to warm it up, most of the drift appears to be in the BFO and I am considering a crystal controlled BFO or at least the ability to switch one in if required, the last job remaining is a crackle finish of the little steel case and front panel, a smelly job that I have decided to put off until the summer, so there it is, many thanks to all the lads on the CW end of 40mtrs that helped with the testing, I have now worked all over Europe using these two little valve units and I’ve had very many happy hours designing building testing and finally using them. So, what did I learn from this receiver project, well quite a few interesting things, I am already thinking of a Mk2 version, building on what I learnt from this this one, for starters building a 455khz IF single conversion type receiver is not easy, the 2nd channel interference is very difficult to resolve though can be done with careful front end tuned circuits, but the next one will be something like a 5.5 mhz IF to overcome this, also I found just making a single band, built for the job type receiver I could overcome many of the problems with tracking and so on, which is a problem with general coverage receivers, it seems all receiver design is a compromise, also high gain is not a requirement, after testing the receiver with only one RF stage and one IF stage I came to the conclusion selectivity and stability are the real requirements of a receiver not gain, gain I found comes with noise, carrying on from that I found I simply could not get the stability I wanted from a valve VFO and BFO so the next valve receiver project will be a hybrid, I shall try and use a transistor VFO and possibly an xtal controlled BFO, this means yet another compromise on only using valves but by using the old hybrid method I shall have the stability I require and still keep the warmth and feel of old valve technology, also I would like to include a QRP transmitter into the project making a complete QRP transceiver, so there it is, I never built the audio filter into the receiver and looks like I won’t be fitting xtals into the BFO if I start the Mk2 We shall see, building a project like this is on going, but a fully valved QRP tranceiver is an exciting prospect! so back to the drawing board and junk box…….. |
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Is there anyone else out there
building there own valve receivers? if so I'd love to hear from you its a
lonely old path :))
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