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Author Topic: My First Superhet Build -- Planning  (Read 1026 times)

VK6HP

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Re: My First Superhet Build -- Planning
« Reply #30 on: July 01, 2021, 07:48:11 PM »

Martin,

You've received some good commentary since my earlier reference to the AA8V receiver, which I'm glad you found useful.  One thing I would say is to think carefully about what constitutes a minimum usable receiver in a modern e.m. environment, as opposed to a (very worthy) historical radio project.  You don't want every contact to be a major chore or the novelty soon wears off.  I say this as an enthusiast for classic radio and, indeed, as a fellow HR10B owner!  In fact, for me the HR10B was well above the usability threshold despite all its issues. 

By the way my own HR10B journey has interesting parallels to your own.  I also built mine at high school in the 1970s, used it quite a bit in conjunction with a homebrew 1-tube CW transmitter (being too poor to run to a matching DX60 transmitter), outgrew it, and consigned it to my mother's storage cupboard.  After protests about clutter from her a couple of years ago I brought it home, serviced it and stabilized the HT, and treated it to the matching crystal calibrator and a DX60B.  It now looks good and works as well as it ever did, which is to say not very well. But great fun, as you say.

There was one minor thing on which I meant to comment in the discussion of the single-crystal IF filter.  For a largely CW receiver adjusted to give single-signal reception, you can improve the audio on casual phone listening by using a "Stenode" tone correction filter.  The most readable reference I know relating to this is a Wireless World article by G6XN (SK) at:

https://worldradiohistory.com/UK/Wireless-World/60s/Wireless-World-1962-07.pdf  (see Page 300).

The Robinson Stenode has been good for many an argument over the years but all you really want is the suggested tone correction circuit.  It's not magic but it does help.  In my youth I used it with an ex-military AMR300 receiver, with some success.

73, Peter.
« Last Edit: July 01, 2021, 07:51:36 PM by VK6HP »
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #31 on: July 01, 2021, 10:35:31 PM »

One thing I would say is to think carefully about what constitutes a minimum usable receiver in a modern e.m. environment, as opposed to a (very worthy) historical radio project.  You don't want every contact to be a major chore or the novelty soon wears off.

I hope I'm very aware of that issue. I do not expect my initial superhet efforts to last very long in terms of usefully conducting QSOs; the initial projects will be a spur to build something better. It will be painful and frustrating to some extent. One advantage I do have geographically is being in Europe, with the possibility of QSOs with hams from many different countries even though they are not far away in North American terms. I find that idea very exciting even though many fellow hams may find it not terribly ambitious.

Even on my little regen set, I can easily monitor, in the space of an hour, hams in a half-dozen different European countries conducting CW QSOs, a few of whom have Morse skills that are barely better than mine. From my location, European hams come through particularly strongly on 40m CW (German, Dutch, Scandinavian, former-Soviet, French, Spanish, Italian).

This will not be news to any of you but I mention it because the receiver is particularly primitive. It helps to set my mind at rest concerning the "modern e.m. environment". This may be partly because I am in a rural environment, living in the heart of a village. It may also be in part because I don't have a proper antenna hoisted yet (the receiver is hooked to a few meters of "long wire") so it may turn out to be noisy once the full dipole is up -- in which case I can always use a "long wire" just for the receiver and transmit on the dipole.

To the extent that there are problems with RFI these have been linked to modern appliances inside my own home, and these have been tracked down and eliminated. Usually they have to do with some kind of switch mode PSU, or in one case, a 50Hz to 60Hz converter that I was using to run an antique American GE electric alarm clock.

Once I am fully up and running (the antenna is the final step, and under way) the other limitation will be my little two-tube 5W crystal-controlled pea-shooter transmitter. (A VFO, and a more ambitious transmitter, are also in the list of future building projects; but getting better "ears" is still top priority for now, I think.)

I enjoy the challenges. It is at the heart of the hobby for me. Otherwise I'd just be buying one of the wonderful, and historically reasonably priced (once inflation is taken into account) modern solid-state rigs. Frankly, if I look at my spending on homebrew components and tools over the past decade, it would also have been much cheaper to go with the modern rig. But for me, that holds less than zero interest. The appeal of the hobby is different for each ham.

For the time being I'm not after awards or trying to build up an impressive QSO count. I think it will be more like: build, get on the air, notice that something could be improved, tinker with rig (or build another one), rinse and repeat. I've actually been doing that on and off for the past nine years, but without actually getting on the air. (The tinkering is intimately linked with the procrastination, or vice versa. I am trying to solve the procrastination by keeping things as simple as possible. Nearly there!)

73 de Martin, G3EDM
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #32 on: July 01, 2021, 10:57:49 PM »

There was one minor thing on which I meant to comment in the discussion of the single-crystal IF filter.  For a largely CW receiver adjusted to give single-signal reception, you can improve the audio on casual phone listening by using a "Stenode" tone correction filter.

I will add that to my list of possible audio filter projects. I have a bunch of U.S. telephone company toroids from the 1950s/60s which some hams have used to build CW audio filters (including I think N2EY). There are also the tried and tested "Selectoject" designs.

One of these days I will deploy my Frankenstein-monster second receiver, which is a breadboard regenerative set that includes a Selectoject-style filter and various other tweaks (pictured below), built "on the fly" with a lot of input from eHam Elmers. It works surprisingly well but takes up almost the entire operating desk, because I never moved it off the breadboard and onto a proper chassis. Each section is built one a small section of wood, probably 2x4 because that's what was hanging around, then screwed onto the big wooden base plank.

All the tubes are "pencil" sub-miniatures and the whole thing is battery run, including the B+ which comes from a string of 9V batteries. (All the batteries, A, B and C, are rechargeable. They are used for filaments, grid bias, and B+).

From left: tunable RF stage with small vernier dial, regenerative detector with Jackson dual-speed dial, first audio stage for headphones, CW audio filter, second audio stage with push-pull stage for speaker. I've left some stuff out of the description and would have to dig up the completely unique schematic to remember how it all works! IIRC some of the iron just to the right of the main dial is a high-inductance load for the detector.

The set is very "quiet" apart from the regenerative rush, partly because there is no AC involved in the power supply. The big advantage of tinkering with those tiny tubes is that you don't have to build a power supply, and the voltages are low enough to be safe, for the most part.

It is monoband 40m but the coils are wound on vintage Amphenol plug-in forms so it could fairly easy be made multi-band. I have been hoarding those Amphenol forms for nearly a decade....



73 de Martin, G3EDM
« Last Edit: July 01, 2021, 11:17:59 PM by KB1WSY »
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N2EY

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Re: My First Superhet Build -- Planning
« Reply #33 on: July 03, 2021, 05:27:01 AM »

In a stroke of luck, I just obtained a couple of 1700KHz crystals. They are CR-18/U, so there is no possibility to open one of them up and grind it to 1700.5 for a two-crystal design, but never mind for the moment.

The avalanche of advice here has been most gratifying. I have tentatively decided to start small and build something like the "Simplex Super Mark II" from the 1963 ARRL Handbook. It's a 3-tube design with a single-crystal 1700KHz filter. There are no unobtainium parts and I already have a stock of the specified Illumitronic/Miniductor coil stock (and even if I hadn't, there is enough info in the book to reproduce them).

The Simplex uses a 6U8A Mixer/oscillator, another 6U8A detector/BFO and a 6CG7 audio stage. "Selectivity at the IF is obtained through the use of a single crystal. This, in conjunction with some regeneration provided by the detector, is sharp enough to provide a fair degree of single-signal CW reception." Faint praise, but it's only a three-tuber.

Three tubes, but with 6 tube circuitry.

A couple of suggestions if you go that route:

1) The receiver provides reception of WWV at 5 MHz, which was fine for US hams but not worth the parts in your application - plus you already have a receiver for 5 MHz. I suggest removing the 200 pF capacitor in the oscillator and S2A, and replacing S2B with an SPST switch to turn the BFO on and off.

2) The power supply uses solid-state (selenium!) rectifiers in a voltage doubler circuit because, when the receiver was designed, that was the least expensive way to get the needed ~250 volts at about 25 mA for the B+. You can substitute other power supply circuits, depending on available parts.

3) I would make the power supply capable of 50 mA or so. The reason for this is so you can add voltage regulation of the tunable oscillator and BFO in the future if you so desire.

4) The stand-by switch reduces the receiver gain by shorting the antenna input and removing the screen voltage from the 6U8 pentode section. For convenience in TR control, I'd make provision to do this with a small relay rather than a switch - that way, you can have single-switch TR control.


I agree with those who have said above that adding more tubes is not that big a deal. But I would prefer to start small and try to understand in depth how it all works on a component by component basis.

That's a good approach. Which leads to another suggestion:

5) Try to use expensive/hard to find parts in a way that makes them easily reusable. For example, I don't cut the leads on power supply and audio transformers or chokes. Instead the extra length is coiled or folded up. I don't pull plates from tuning capacitors.

The result is that parts from a project can be reused in the future.

 
I have no formal EE training but a love of learning and experimentation.

It should be remembered that electrical engineering is a very diverse field and that Radio is only one small part. Hollow-state radio is an even smaller part.

(Full disclosure: I received my BSEE in 1976. In the four year course at the U, vacuum tubes weren't studied at all.)

Yes I know the performance will be extremely limited but that in itself will be interesting when comparing with the two other receivers I have built already in the past few years, both of them regenerative designs, one of them solid-state (3-transistor ARRL design) and the other one hollow-state (my own design with considerable help from folks here such as G3RZP).

Exactly!

When the building gets under way I will document the project in a thread here, hopefully in the next few months if I can figure out how to set up a small workshop in my current small QTH. (It's a challenge, to put it mildly.)

There was a period, right after University, when I was living in a small apartment. I built a complete CW receiver on the kitchen table, because it was the only suitable table in the place.

I had a toolbox with all the radio tools in it and another box with all the radio parts and the receiver chassis. Before each construction session, I'd cover the kitchen table with old newspapers, take out just the parts and tools needed, and proceed. At the end, the tools went back in the toolbox, the parts went back in the parts box, and the newspapers were wrapped up and put in the trash. Usually the kitchen floor got swept too.

It took some time to build the receiver but it worked well. I should restore it....

Once the Simplex is up and running it will be a pleasure to move on to more sophisticated projects.

Exactly. And the results may surprise you.

73 de Jim, N2EY
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #34 on: July 03, 2021, 07:25:56 AM »

Three tubes, but with 6 tube circuitry.

Indeed.

1) The receiver provides reception of WWV at 5 MHz, which was fine for US hams but not worth the parts in your application - plus you already have a receiver for 5 MHz. I suggest removing the 200 pF capacitor in the oscillator and S2A, and replacing S2B with an SPST switch to turn the BFO on and off.

Yes, I had already decided to leave out the WWV portion. My dark secret is that I have been using an old Sony SW-1, a fantastic little digital shortwave receiver from the 1980s about the size of a pack of cards, as my frequency marker. So, I can fire up the old hollow-state signal generator and then check the frequency on the SW-1. Sorry, a bit of a cheat....

2) The power supply uses solid-state (selenium!) rectifiers in a voltage doubler circuit because, when the receiver was designed, that was the least expensive way to get the needed ~250 volts at about 25 mA for the B+. You can substitute other power supply circuits, depending on available parts.

Thank you. Obviously I was already planning to substitute the selenium rectifiers, but I had not spotted the voltage-doubler. Nowadays, easy to find the right 250V transformer either from NOS or from Hammond.

I am also mindful of your earlier comments about using outboard PSUs. With the "vertical closet" layout of my new shack that might make sense, and could make it easier to add (for example) a voltage regulator tube.

4) The stand-by switch reduces the receiver gain by shorting the antenna input and removing the screen voltage from the 6U8 pentode section. For convenience in TR control, I'd make provision to do this with a small relay rather than a switch - that way, you can have single-switch TR control.

I'm already using the relay approach even in my primitive Novice station. The RX is muted with a reed relay, and I do have single-switch control (both for receiver relay muting and antenna switch-over) in my custom-built T/R switch.

One ergonomic factor I noticed today. Because my new shack is very "narrow" from left to right, it will be optimal to mount the tuning knob on any homebrew receiver as far left as possible, rather than in the centre of the front panel. Strangely enough, although the new shack is tiny, I can see that it will be just as practical as the old one, but only if rather more care is taken about these ergonomic details. (The same thing happened with our kitchen here in England, when we arrived from America. The first thing we did was retire several of the American-sized saucepans, frying pans and oven pans that were way too large to fit on our new cooker or in our new oven....).

Thank you for your comments both about husbanding parts, and finding workshop space in difficult environments. It should be doable. What I might try to do is find a local club or technical college where I could do the metalwork, which is the really big challenge when you don't have a "proper" workspace. I really miss my wonderful floor-mounted drill press, which I gave to my American brother in law when moving to England!

73 de Martin, G3EDM
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K3UIM

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Re: My First Superhet Build -- Planning
« Reply #35 on: July 03, 2021, 07:33:24 AM »

It does my heart good to see this thread! When I got hooked on radio/electronics in the 50's, we didn't have forums like this. (Heck, we didn't have much of anything for experimenters back in those dark ages! Computers were things that caused heat waves over Washington, Dc.) Hi.

I believe I have learned more since finding this forum than the 20+ years prior! Even while in the army 4 years and haunting the Radio Room, which showed me how to change tubes, etc. LOL

Thanks to all of you that ask questions and find answers here. As I've said many times, "Life is a never ending learning experience!"

Charlie
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Charlie. K3UIM
Where you are: I was!
Where I am: You will be!
So be nice to us old fogies!!

G3EDM

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Re: My First Superhet Build -- Planning
« Reply #36 on: August 09, 2021, 10:00:05 PM »

A quick update. For the first time in my strange ham career (50 years of dithering, without actually getting on the air yet), I have a "proper" antenna: a full-size dipole for 40m, albeit at a mediocre height above ground: about 12 feet, hopefully to be increased to about 15 or 16ft with some future enhancements.

The antenna, which is coax-fed, located away from the house, and properly grounded, is electrically extremely quiet compared to anything I've had before (i.e. various "long wires" strung out of the window). Because of the improved s/n ratio and greater pulling power for weak signals, my two primitive regenerative receivers have acquired a new lease of life, making the building of the future superheat a bit less urgent. The existing receivers are still as wide as barn doors, and get easily overloaded with strong signals, but with careful use of their existing signal-attenuator circuits, they will be usable for my first QSOs!

73 de Martin, G3EDM
« Last Edit: August 09, 2021, 10:02:47 PM by G3EDM »
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G4AON

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Re: My First Superhet Build -- Planning
« Reply #37 on: August 09, 2021, 11:38:58 PM »

A couple of comments, firstly Voltage regulator tubes sometimes contain a short half life radioactive “starter”, which may have decayed significantly in “NOS” tubes. One type advertised on eBay mentions Krypton 85, which has a half life of just under 11 years.

Regarding constructing a superhet receiver, the Hycas IF and AGC circuit from QST, copied on the W7ZOI web page, makes building a high performance receiver relatively easy. We are talking about a home brew RX that equals any amateur transceiver regardless of cost. The circuit is at:
http://w7zoi.net/hycas-pcb.html

The Hycas (hybrid cascode) approach originated in tube designs and each stage works in a similar manner to a dual gate mosfet.

I use the Hycas IF in my own home brew RX, which holds its own against an Elecraft K3:
https://www.qsl.net/g4aon/g4aon_rx/

73 Dave
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #38 on: January 20, 2022, 08:43:15 PM »

Greetings.

Yes, this is an old thread, with the most recent post in early August of last year. That is just before I went on the air for the first time.

Anyway, I would like to "bump" the thread and revive the discussion.

The background:

I went on the air for the first time ever in late August, nearly six months ago, after half a century of dithering. For the whole of those six months, my receivers have been regenerative.

I started out with the "Three Transistor Receiver for the Beginner" which is an ARRL design from 1968. That set turned out to be usable, but rather "deaf" so after my first few dozen QSOs, I progressed to another regenerative set of my own design, which uses subminiature tubes and proved much better at pulling in the signals. (You can see a description, and a photo, of this set in Post #32 in this thread. Since then, the set has been "prettified" and mounted in a proper chassis, with a front panel and dials and so forth, but it is exactly the same radio "under the hood".)

Neither set has anything like the selectivity that I need in the long term. I currently operate CW-only on 40 metres. The band is, of course, really busy and much of the time, the signals in my headphones just sound like a massive pileup!

I have 125 QSOs under my belt, with 21 countries. But it's time to move on. I'm going to re-read this current thread very carefully and finalize plans for "my first superhet". Very exciting!

A reminder for anyone who does not know already. I am a "fundamentalist" homebrewer. No kits! Also, for the moment I am "living in the past" in the sense of eschewing any solid-state gear. I think that will change with time, but for the moment (the next few years at least!) I want to stay in the world of hollow-state.

Also: I have no plans to operate any mode other than CW. So it would be useful to optimise this "first superhet" for CW and not worry about reception of other modes.

73 de Martin, G3EDM
« Last Edit: January 20, 2022, 08:47:40 PM by G3EDM »
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #39 on: January 20, 2022, 11:50:16 PM »

Probably the most crucial part of this project will be the IF filter. In the months since I last posted in this thread, I've been shopping for crystal filters so let's take inventory of the junkbox:
  • 3700 kHz crystal usable as a single-crystal filter.
  • Cathodeon IF crystal filter pulled from NATO gear, CF 1750 kHz, BW 400 Hz (potential issue with the IF harmonics clashing with the bottom of some common ham bands, workaround is to design the BFO accordingly).
  • Cathodeon IF crystal filter designed to meet the "Very Narrow Requirement for HF Maritime Receivers", CF 1499 kHz, BW 250 Hz.
There is a very basic choice to be made:
  • Go for a relatively simple superhet with a single-crystal filter. This would provide a modicum of selectivity, but hardly the "single signal" outcome that we are yearning for. It has the advantage of being relatively simple, so perhaps faster to build. It should in any case be much better than the regenerative receiver I am using now.
  • Alternatively, grasp the nettle and go straight for a set that incorporates a top-quality IF filter and if so, I am rather inclined to use component (3) from the above list. It has the advantage of not being awkward harmonically, at least for the lower-frequency HF bands. Also, if I can get used to the really narrow bandwidth and does not "ring" too much, it obviously has excellent selectivity.
Surprisingly, I am now minded to go for the "complex" option and build a really good receiver now. Get it over with!

As I understand it, the issue with such a choice is that because of the inherent attenuation from a really good filter, additional stages of IF amplification are likely to be required. Also, because of the narrowness of the filter, extremely high stability in the oscillator circuit is essential.

My tendency in construction techniques is to "build it like a tank" so as long as the basic framework is solid and the layout logical, there is a chance of "getting it right".

Rather than just building something "off the shelf" i.e. from a published ARRL or other vintage design, I am minded to "pick and choose" from the best circuits. To start with it might be best just to build the front end and first IF, without the filter, and make sure we have a rock-solid design. Leave enough space on the chassis to add the "selectivity circuits": filter, and additional IF stages.

Does that make sense?

BTW according to G3RZP, that narrow 250 Hz Cathodeon filter, Model BP 4725-01 (serial number 1210), probably has an impedance of either 50 ohms or 1K ohm, in shunt with 15 pF. He is not totally sure because the "-01" model number variant is not in his data. Peter is familiar from these units from his time with (Plessey?).

The unit I bought is pristine "new old stock".

I suppose another issue is the choice of IF. My knowledge is rusty, but off the top of my head:
  • The lower the IF, the higher the selectivity; however:
  • The lower the IF, the greater will be the problem with images.
I hope I've got that the right way around!

Seems to me that 1,499 kHz isn't a bad choice for a single-conversion set. In the old days we might have worried about AM BC breakthrough, but I assume that is not as much of an issue nowadays. The Medium Wave band in this country is a shadow of its old self, in my experience.

Also, I suspect it would not be hard to find 1500 kHz IF transformers. Indeed, I think I may have at least one already in the junkbox.

Modified to add: There might be something to be said for making the receiver monoband, tuning 3.5 MHz to about 3.65 Mhz (only interested in CW). Then adding the other bands with an outboard crystal-controlled converter.

73 de Martin, G3EDM


« Last Edit: January 21, 2022, 12:08:21 AM by G3EDM »
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #40 on: January 21, 2022, 01:06:29 AM »

After a brief skim through the excellent posts contributed in this thread last year, I think my ideas will "fly" but I will go through all the posts more carefully and formulate a detailed plan.

Random question: If I have trouble finding 1500 kHz IF cans, can't I just use ones for anywhere between 1400 and 1600 and change the capacitor value? Or just retune the slugs, in some cases?

Quick thought on the construction technique. I'm thinking of using a pretty large chassis and building the first iteration of this set in the horizontal middle of it, leaving empty "skirts" on either side.

In the initial build it would be just: mixer, oscillator, bfo, some kind of AF.

Secondly: add RF stage to the left of the mixer, and IF filter and extra IF stages to the right of the existing build.

Thirdly: build a crystal-controlled converter on the left skirt, to add additional bands.

(Because it's CW only, I see no special need for AGC or for an S-meter.)

This could also be achieved by building the receiver on three separate chassis, bolted together. Let me think about that. It has the great advantage of being able to do vigorous metalwork without the danger of damaging the existing build. I suppose it also adds a layer of inter-stage shielding under the chassis, and that can't be a bad thing.

I suppose the final tube count on this receiver could be fairly large. By building it in stages, we can reduce the "intimidation factor". At this stage I have a lot of hombrewing experience, but I've never even remotely tried to build anything this complex from scratch.

73 de Martin, G3EDM
« Last Edit: January 21, 2022, 01:13:10 AM by G3EDM »
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #41 on: January 21, 2022, 05:58:59 AM »

I thought I had managed to source some 1500 kHz IF transformers. They are Meissner brand, not the full-size point-to-point models (912 range) but somewhat smaller stud-mounted models, 13/16" mounting size. I assume they will do the trick.



Unfortunately the outfit selling these coils, under new ownership, has not been able to get going because of Covid. They are telling me "perhaps later this year". I have tried various other sources and I'm sure I will manage to track down these or their equivalents somewhere, but if anyone has any suggestions....

I see that such transformers are routinely described as either "input" or "output" and must admit to not knowing the difference.

(I am almost sure that I have one, but only one, 1500 kHz IF can in my junkbox. Merit brand, IIRC.)

The other possibility is to wind them myself, believe it or not. I actually have a "build your own IF transformer" kit from the 1950s and a large, British-made Pye wave-winding machine. But (1) that's an awful lot of faff, and (2) I assume I could never make coils to rival the commercially available ones?

73 de Martin, G3EDM
« Last Edit: January 21, 2022, 06:01:36 AM by G3EDM »
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SM0AOM

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Re: My First Superhet Build -- Planning
« Reply #42 on: January 21, 2022, 07:39:23 AM »

I would say that the 1749 kHz IF with the BFO at 1748.6 kHz would be a preferred design.

You then get the "band-imaging" between 80 and 40 m, and could also include 30 m by using harmonic mixing from an oscillator fundamental of about 5925 kHz, which is within reach of an oscillator normally tuning between 5250-5450 kHz.

With a -6 dB bandwidth of only 400 Hz, it takes a very gentle "touch" of the tuning mechanism and absolutely no back-lash to be enjoyable to use.

BFO harmonics are usually only a problem when wide tuning ranges are attempted, and it is a good idea to avoid exactly 1750 kHz as the BFO frequency.

A receiver primarily intended for use on the "lower bands" with full-size antennas does not need to be very sensitive. It is only in exceptional cases the antenna noise figure falls much below about 40 dB in populated areas.

If a receiver then has a 10 or 25 dB noise figure has very little importance.

A suitable design could be a mixer-oscillator with a 6U8 or similar, the filter, a regenerative detector also with 6U8, a crystal controlled BFO on 1748.6 kHz made by "pulling" a 1750 kHz crystal to the right frequency using a series inductor. This is advantageously followed by a band-limited AF amplifier.
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #43 on: January 21, 2022, 08:04:07 AM »

You then get the "band-imaging" between 80 and 40 m, and could also include 30 m by using harmonic mixing from an oscillator fundamental of about 5925 kHz, which is within reach of an oscillator normally tuning between 5250-5450 kHz.

I will definitely want to use the receiver on 20m also (eventually; that option does not have to be included in the initial build). The planned pattern of usage eventually is to operate mainly on 20m, 40m and 80m with 30m also available for times when I want to avoid contest traffic! I assume the 20m option using a crystal oscillator converter will work, and that the IF is high enough to avoid images.

For the initial build: just 80m and 40m is fine.

What about my original idea of building it as monoband 80m and doing all of the other desired bands with a converter? It just seems to me that heterodyning in this was is a much simpler way to do the band-switching ... in addition to perhaps providing better performance on the higher-frequency bands.

Concerning sensitivity, my rural location does seem to be very quiet, but the only thing I have to go on is my regenerative receiver. The locale may be noisier than I think....

With a -6 dB bandwidth of only 400 Hz, it takes a very gentle "touch" of the tuning mechanism and absolutely no back-lash to be enjoyable to use.

I have a bunch of possibilities in the junkbox including several pristine Eddystone 898s that can be paired with a low-torque capacitor, or alternatively a capacitor from an LM-series frequency meter along with its reduction mechanism onto which I could add a home-made display dial.

I have been saving this "good stuff" for the future "really good receiver" but why not make this pending receiver project a kind of movable feast, getting better and better as time goes on. To do that, the core of the set has to be really well built and use top-quality components.

Edited to add: The PSU should probably be separate, for maximum future flexibility.

73 de Martin, G3EDM
« Last Edit: January 21, 2022, 08:19:03 AM by G3EDM »
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G3RZP

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Re: My First Superhet Build -- Planning
« Reply #44 on: January 21, 2022, 09:29:49 AM »

Martin,

The 'output' transformers are generally meant for driving a diode detector where a higher C to L ratio means a lower impedance for a given Q: that makes it a bit easier to get the diode DC and AC load lines more equal. Not normally worth worrying about.
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