| Below is a typical circuit of a "Simple" 2
valve receiver, it is the most basic superhet, yet with
modifications I was sure it could be used quite reliably on 40mtrs CW!!
stability is the key to using simple valve receivers on crowded Amateur bands.
V1a is an RF amplifier/mixer with V1b the local oscillator, although this is shown
as a tuneable circuit I suggest it is made crystal controlled for 40mtr
amateur, the xtal frequency is not to critical as the IF stage is tuneable to
a degree, the IF being around 1.8 mhz so an xtal near 8.5 to 9mhz could be
pressed into service the frequency of the IF being tuned to suit, the IF V2a
is just a simple regen with V2b a simple audio amp, as the osc is now xtal
controlled and the regen is working very low frequency where it is nice and
stable you have a simple yet quite effective receiver for spot frequency
working, you notice I say used, by that I mean can actually be used in a QSO
as against just listening around, the two are entirely different, to hold a
QSO on a crowded amateur band over a period of time demands certain
requirements from a receiver, let alone one as simple as this one. Careful use of the regen control is of course a requirement as this is advanced to the point of oscillation for CW operation, and experiments with the tapping point of L6 can greatly improve this, the simplified circuit on the right I don't recommend. Example if you wanted the receiver to listen to 7.03 mhz QRP chan and you had say an Xtal of 8.6 mhz then , 8.6 minus 7.03 = then the IF regen detector would be tuned to 1.57 mhz or if you had a 9.0 mhz xtal then the IF would be tuned to 1.97 mhz and so on. T2 can be a fixed trimmer as shown but with say a 50pf panel control to give a nice band spread around the required spot frequency.
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| Actually although I built this sort of receiver many years ago, and I'm talking about 50 years ago!!!!!! how good they actually are and how easy they are to make has faded over the years, and I never actually built up the circuit above, being able to use it reliably on 40mtrs is just conjecture on my part, so seeing as the weather here is very cold and I can't get out without freezing, I thought I might put my money where my mouth is and have a go at putting the ideas above into a practical project, I have loads of old chassis from previous projects of long ago so I press ganged this old chassis into service, I decided to build the receiver bit by bit, and started with the pentode section of the regenerative detector, from the circuit above, and sad to say I could not get it to work reliably, so much for circuits from the RSGB handbook!!! but to be fair I didn't use their layout or coils so probably my own fault, so I used a different idea using a variable capacitor to provide the feedback and fixing the screen and anode voltages and with a few temporary connections it fired up perfectly, now this is as simple as it gets for valve receiver technology, yet with this one pentode I could receive 80mtr SSB at perfect working strength and good distance, CW was also very good on 80mtrs with quite respectable selectivity for ONE tuned circuit!! on 40mtrs however things were a little hairy as I imagined they would be, you only had to go near the chassis or coils and it was pretty hopeless, so time to try out the simple superhet idea :() the simple regenerative detector (receiver) circuit and layout I used is below, far left is the crystal controlled front end, an ECF80, the triode part being a 10mhz xtal oscillator, the right hand valve is the other ECF80 which is the detector at 3mhz the triode section is an audio amplifier. |
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| These simple one valve circuits can fool you completely, they look SO simple BUT getting it right is another matter, the answer is in the coil, it must be just right to get a gentle lift into regeneration and it is so full of variables, gauge of wire, spacing and amount of feedback and antenna coupling, if I had to do this again I would have used some sort of plug in coil as it would have been a lot easier to experiment with, but once you get it right the performance is quite amazing, after a lot of experimenting I decided to lift the IF frequency to about 3mhz, mainly as this little circuit works really well on 80mtrs and goes down to about 3mhz with no alterations, so I have used a 10mhz crystal as my local oscillator which after subtracting 7mhz signal gives an IF of 3 to 2.9mhz nice and simple, time now to build the mixer and do a few checks. |
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| Here is the receiver under test you can just see the mixer
wiring on the left with the 10mhz crystal under the chassis, this receiver is now
as good as it will ever be so and just requires tidying up, I can now do a
critical evaluation of it, so anyone contemplating making a receiver
like this can know what to expect. So where to start, well the first thing is just because the circuit is simple to look at, just two valves, don't let that fool you into thinking it will be a doddle to get going, it isn't, I like to think I'm a fairly competent electronics engineer of the old school, I cut my teeth on an old valve chassis, but when you start using anything "regenerative" you are into a different ball game, as compared to standard valve receiver design, I found this project quite difficult. Basically the regenerative detector is "controlled" instability anything and everything affects it, coupling anything to it such as RF input or from it such as audio affects it quite dramatically, just getting the detector to work as a 1 valve receiver is just the start as each time you add something to it you have to go back to it and get it going again, i.e. adding the coupling coil from the mixer loads the detector and you find the feedback control is either out of range or has gone harsh and unstable, adding the audio amplifier stage also causes instability and requires decoupling and experimenting to get it right, and on it goes, However when you get it right it is a remarkable receiver and now I'm sure this one could easily be used on 40mtrs CW as I originally set out to prove, but as I suspected the original circuit above is pretty useless in my opinion, looking at I can't see hardly any advantages over the simple 1 valve receiver, for a start the front end is tuneable? so you have effectively two tuneable circuits the detector and the front end which will do nothing for drift!! also the mixer output is not tuned to the IF frequency? so the mixer is very inefficient, personally I don't think it was ever built, just inserted into the manual as a possible enhancement to the one valve design, my idea overcomes all those drawbacks, the mixer oscillator is now fixed and "Rock" stable and the output of the mixer is tuned to the IF of about 3mhz by doing that I have inserted another tuned circuit so the receiver now has three in all and it shows in the performance, the receiver is sensitive and very stable on 40mtrs CW and also tunes in SSB signals easily, it isn't affected to much by strong adjacent signals if the regen control is carefully adjusted and so far with it's high frequency IF, image rejection is very good, so now for the final conclusions. Well although it works very well I can't say it is a beginners receiver,
so why build it? it would be easier to build a tried and tested superhet
with say a 455khz IF transformers AGC and a BFO, with some care taken in the
construction it will probably work first time, even in this project the
standard mixer oscillator part was the easiest that worked first time no
problem, OK more parts are required and more valves and I suppose that was
the reason in days gone by why these "Simple" circuits evolved, parts and
valves were expensive then even so getting them going well is a work of art.
If you are a beginner with valves / tubes I'd say don't bother with it, go
for a more complex superhet but with tried and tested stable circuits,
nothing "regenerative" having said that, the 1 valve circuit is fun and
fairly easy to get going if you just want a fun project and the joy of using
something vintage and simple but don't ask to
much from it. adding stages to improve it in my opinion is not really worth
the hassle. Ron Ayling 2/2/2009 The final circuit I came up with was fairly similar to the one above, the regenerative detector I changed to the circuit drawn above, the coupling from the mixer was changed by making L5 tunable by increasing the number of turns to the same as L6 and putting a trimmer capacitor across it to resonate the coil at the required IF and these now being separate coils placed about 5mm apart to inductively couple them, the Variable oscillator V1B I changed to a simple fixed crystal oscillator by not using L3 and L4 and coupling the crystal between the anode and grid, and the 50pf capacitor tuning L2 I replaced with a 500pf variable so it had a range that covered 80mtrs and 40mtr bands. Further to the above, after playing with this little receiver for a while
I changed the front end crystal to 10.7mhz, the reason being the IF
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