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Old 10th Oct 2006, 9:20 pm   #21
YC-156
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Default Re: simple SW reciever.

Adi, here is the main reason why it is so much harder to receiver SSB over AM on a regenerative receiver:

You will have read that for receiving AM, you tune the receiver to the signal and adjusts the gain so that the detector *doesn't* oscillate. When the detector doesn't regenerate strongly enough to generate its own internal signal, it also happens to be fairly wide in the selectivity.

So the received AM signal is easy to tune, because the receiver is wide in the selectivity curve. Also reception doesn't critically depend on precisely what the tuned circuit in the receiver is set to at any given moment. So if you have some degree of mechanical or electrical instability, it doesn't really matter. The precise tuned frequency may actually wander around a bit, say due to temperature changes, the operator moving a hand or whatever. But as long as the station of interest stays reasonably within the passband of the receiver (which is what is moving about), all is well. You can build an AM receiver for the BC band on a piece of wood if you like. Might not be the world's greatest receiver, but you should hear *something*.

With SSB all that changes. Because you need to set the regeneration such that the detector oscillates. Ie. it generates it own internal RF signal, which is needed for proper demodulation of the SSB audio signal.

Unfortunately that internally generated signal needs to be absolutely nailed to the spot to produce intelligible audio. 100Hz off to one side is often considered way too far away, yet you could probably easily get away with being 10 times that amount away for receiving AM. And that is for two receivers running on the same frequency.

Besides, getting the receiver to generate just the right amount of internal RF can sometimes be a bit tricky. Overdo the feedback and all you will hear are squeals and howls. It is much easier to control the level of oscillations if you choose your valve carefully.

But sure, they will all readily break into oscillation if that is all you need.

Frank N.

Note to Al: On paper the 6BR7 appears to be a very good choice for a regen detector, since it has a very low value for transconductance, only 1.25mA/V. The EF86 has a whopping 2mA/V by comparison. (Mental note made: 6BR7...)
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Old 10th Oct 2006, 9:23 pm   #22
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Default Re: simple SW reciever.

Quote:
Originally Posted by G8DLH View Post
Watch this space!
It is unfortunately too dark here. Otherwise I would have photographed my little experimental regen here already. The Mark II that is.

Frank N.
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Old 10th Oct 2006, 9:41 pm   #23
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Talking Re: simple SW reciever.

Yes, Frank, I certainly agree with you on your comments re: stability requirements for the reception of SSB. That's one of the reasons why I want to give this a try! The challenge!
The approach that I have in mind is a heavy & solid aluminium casting for the chassis with a heavy ali front panel, the best quality tuning capacitors that I can find in the "junk-box" and a D-I-Y plug-in coil mounted inside a metal screening can, so as to protect it from draughts / thermal changes.
ISTR that one of the biggest challenges with a TRF design was getting a really smooth reaction control - as opposed to "out" = insensitive, slight tweak "in" = howl - and nowt in between!

Al / G8DLH
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Old 10th Oct 2006, 10:23 pm   #24
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Default Re: simple SW reciever.

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The approach that I have in mind is a heavy & solid aluminium casting for the chassis with a heavy ali front panel, the best quality tuning capacitors that I can find in the "junk-box" and ...
Couldn't have said it better, Al.

I realized that I probably wouldn't have time to contribute much to this thread for the next few days, so decided to make a few snapshot of the Mark II EF86 regen on the workdesk. Sorry about the quality, it was the best I could manage at this hour.

Normally I won't show incomplete projects, but I hope this one has a solid foundation in the previous EF86 regen I mentioned in the thread last year. That one lacked a bit in the mechanical department though (*cough*), thus the need for a Mark II. It is intended to run on 6MHz and a bit as part of an experimental receiver test bed, and initial testing will be on the 7MHz amateur band. Bandspread will be 200KHz with a 1:6 tuning scale ratio, which is barely acceptable IMHO.

Please feel free to comment, cry or laugh. I know of at least one potential trouble spot I missed even on the second time around.

Front panel is 3mm aluminium (a 19" rack drawer front panel actually), detector is housed in a diecast aluminium enclosure attached to the front panel using 7x M4 machine screws and threaded spacers. All supply voltages stabilized and insanely decoupled etc.

Looking forward to seeing all the other projects. I will not have time to work on this for the next few days at the very least, so count me out for the immediate future as far as progress reports are concerned.

Frank N.
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Old 10th Oct 2006, 11:07 pm   #25
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Thumbs up Re: simple SW reciever.

Yes, Frank, your photos - interesting.
Mechanically, that's the approach I have in mind - with the exception of a more traditional "open" chassis - but with plenty of screening - & decoupling.
Re: your comment on a reduction drive: agreed. If I can find that box of Mecanno gears that I have somewhere, I might try making my own anti-backlash reduction gearing system. That'd be a challenge! As for the valve, it's tempting to go for something really "vintage" - like a PM1HF. It all depends what I find in the dusty "old radio valves" box somewhere in the attic!

Al.
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Old 11th Oct 2006, 12:24 am   #26
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Default Re: simple SW reciever.

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Originally Posted by adibrook View Post
I just thought that maybe if a guy can build it covertly in a concentration camp out of a canteen and a random stolen parts and 12v HT, it cant be that hard. I appreciate that i'm brobably dumber than that guy...but i DO have a good selection of parts, valves, 250v ht and no guards watchign me.
Adi, most of the men in those 1940s POW camps weren't much older than you are. Some of the NCOs and officers would have been a *bit* older, but most Brits were called up on their 18th birthday and would have gone into active service before they were 20. My dad was born in 1923, was called up in 1943, and saw active service during the occupation of Italy in 1945. He was still only 22 then! (He never shot at anybody in anger though )

Paul
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Old 11th Oct 2006, 12:53 am   #27
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Default Re: simple SW reciever.

Ok. Thanks for all your responces. I now understand that recieving SSB on a regen is hard because all the controls have to be much tighter.

I will still carry on with the project tho, just to see what i can recieve.

Last edited by Station X; 11th Oct 2006 at 10:23 am. Reason: Text re adding a BFO to a Domestic Receiver removed as Off Topic.
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Old 11th Oct 2006, 7:31 am   #28
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Default Re: simple SW reciever.

I have started a new thread here on the subject of adding a BFO to a domestic receiver:-

https://www.vintage-radio.net/forum/...3096#post83096
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Last edited by Station X; 11th Oct 2006 at 10:20 am.
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Old 11th Oct 2006, 11:27 am   #29
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Default Re: simple SW reciever.

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Originally Posted by adibrook View Post
I will still carry on with the project tho, just to see what i can recieve.

Well done....that's what I like to hear ! The spirit of adventure still lives in some of us!!

Maybe what I should have added to my own post earlier was that despite the shortcomings of my particular set, I just accepted them and carried on to bigger and better things. It's the only way to gain experience.


Rich.
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Old 11th Oct 2006, 5:03 pm   #30
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Default Re: Simple SW Receiver.

Go for it Adi.
I never had the pleasure of building a simple radio, either valve or transistor, I was to busy repairing tv's and radios for a job. If I had more time I would certainly like to put one together.
A few years ago I did repair a one valver for a mate, I think it used a DF96 valve. I was amazed at the results it gave, excellant pickup and selectivity on the short wave AM bands.
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Old 13th Oct 2006, 11:34 am   #31
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Default Re: Simple SW Receiver.

Adi,
Try a crystal set on SW with a decent bit of wire for an antenna, put about 60 turns of wire on a toilet roll tube with taps coming out every 10 turns. connect the tuning cap (about 500pF) and antenna to the tap. At night It's very lively as far as broadcast stations go.
Cheers,
Mike.
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