My First-Ever QSO -- 50 Years After Passing Ham Test

Discussion in 'Ham Radio Discussions' started by G3EDM, Aug 27, 2021.

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  1. G3EDM

    G3EDM Ham Member QRZ Page

    A question for the Europeans: I thought that 7040 was one of the popular CW "watering holes" but when I operate on that frequency there is almost always digital QRM, sometimes considerable. I thought this might be because of poor selectivity on my ARRL receiver but I am hearing a similar set of digital signals also on my second receiver, The Beast.

    I do hear CW traffic there, but almost always with an obbligato of digital transmissions over the top.

    73 de Martin, G3EDM
     
  2. G3EDM

    G3EDM Ham Member QRZ Page

    @W9BRD said: But I submit that the receiver doesn't need a common-cathode, RF-amp gain stage at all and would better handle strong signals without it.

    Re-reading this, I realise that I am not sure what you mean by "common cathode".

    Edited to add: Since all the tubes are directly heated, you may simply have been saying that the cathodes of all the tubes (i.e. in this case, the heaters) are all, by definition, connected together by the heater circuit.

    73 de Martin, G3EDM
     
  3. G3EDM

    G3EDM Ham Member QRZ Page

    Powering down the rig, busy day, may not operate again until tomorrow.

    73 de Martin, G3EDM
     
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    No, the term "common cathode" means that the common reference point or "RF ground potential" for both grid and anode circuit is the valve cathode, which may be both directly or indirectly heated. For a given transconductance, this circuit provides the most voltage gain.

    A "common grid" or "grounded grid" circuit uses the grid as its reference point. Such circuits
    often have better isolation and linearity properties due to RF negative feedback effects than "grounded cathode".

    Regarding the spurious broadcast responses, the autodyne detector is a very complex non-linear system when working near the self-oscillation point. Very small changes in the RF environment may reflect on the detected signal. In your case, some form of intermodulation or cross-modulation may be causing the behaviour, or some occurrence of parasitic oscillation triggered by the broadcast carrier(s).

    Compared to a superhet, the regen has a seriously limited dynamic range, which primarily is limited by the operating point of the detector, and to some extent, of the number of tuned circuits before the detector. Some form of RF gain control before the detector becomes necessary especially in today's RF environment.

    The radio amateurs of the 20s and 30s are to be commended for "working the world" with so simple gear...

    73/
    Karl-Arne
    SM0AOM
     
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  5. G8ADD

    G8ADD Ham Member QRZ Page

    I consulted the RSGB band plan for you. 7040 - 7047 is narrow band data, 7047 - 7050 is unattended narrow band data. 7050 - 7060 is shared data and SSB, the rest to 7200 is SSB.
     
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  6. G3EDM

    G3EDM Ham Member QRZ Page

    Thank you. I've just realized that my "remembered" information is probably both from the wrong region, and (perhaps??) out of date. I was previously licenced (well, still am, actually) as KB1WSY back in 2012. Then, my current call G3EDM in June of this year.

    So I moved from ITU Region 2 to Region 1 (back in 2018) and it seems there may also have been some changes made globally in 2013, the year after I was licenced in USA. Some alignment between regions seems to have taken place, among other things?

    Following your example, I have just consulted the RSGB band plan here: https://rsgb.services/public/bandplans/#10.

    This shows one drawback of my having obtained my UK licence based on passing an exam (albeit a stringent one, the RAE) fifty years ago. I do have the RSGB book for preparing for the present-day Full Licence exam. It is probably time to bring myself up to date by reading that book, and being "current" in one's knowledge of the rules and regulations is part of the duties of any ham anyway.

    Concerning 7040: Unfortunately, although I have a crystal marked 7040, it actually oscillates at 7041 (cleanly and without chirp) and that is where I have been trying to operate. I also have a good quality crystal for 7054, but there too, am running into data traffic.

    CW is of course permitted throughout the band, but I can see why operating below 7040 most of the time should be the most fruitful. Having tested my crystals for that segment, I now have very few that are usable, until and unless I can tweak the transmitter so that the currently "dubious" crystals can actually be used. This process is already under way, I am ordering some parts....

    73 de Martin, G3EDM
     
  7. W9BRD

    W9BRD Ham Member QRZ Page

    Sure they do. The filament is the cathode. We say that such tubes are "directly heated"; tubes with a heater -- proper term, not a filament -- within a cathode sleeve are "indirectly heated", and they are said to have a "unipotential cathode" (as opposed to a filamentary cathode, the voltage on/along which is not uniform relative to common).

    We can bring a filamentary cathode above RF ground with RF chokes. Doing so will be part of my experiments with the 1AD4.
     
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  8. G3EDM

    G3EDM Ham Member QRZ Page

    Thanks, I was slowly remembering all this stuff, in fact I even posted here a little while back that the filament is the cathode....

    Speaking of RF chokes, I used to buy nearly all of mine from Hammond but they seem to have discontinued (as "obsolete") quite a few of the useful choke values. Leaving one to scrounge NOS or pulls from old equipment. I suppose one could use the new "packaged" chokes, but I've never tried them with tube equipment.

    73 de Martin, G3EDM
     
  9. G3EDM

    G3EDM Ham Member QRZ Page

    That little muting relay for The Beast was a Bad Buying Decision. Turns out the coil current is 70mA and that is too much for my taste, a significant current drain on the "C" batteries which otherwise tend to last for a long time between recharges.

    So I'm laying that SPST relay aside. It will surely come in useful in another project some day. Instead, I've ordered a SPDT reed relay with a much lower current consumption.

    (Before changing track in this way, I did try the trick of putting a capacitor across the AF gain pot. It initially worked upon testing -- applying the cap directly to the pot lugs -- but as soon as there was any wiring, even just a couple of centimetres, the audio broke through even with the cap wired in. Not sure what is going on there.)

    With the new SPDT relay I'll be able to just break a circuit somewhere for muting, hopefully in a straightforward way without causing problems with changing loads. In the ARRL 3-transistor set, the muting relay breaks the link between two of the stages, IIRC, probably between the two AF stages.

    Yet another learning experience. Unfortunately such experiences often involve spending extra money....

    73 de Martin, G3EDM
     
  10. W9BRD

    W9BRD Ham Member QRZ Page

    I'll bet de-peaking reduces gain, but very likely (unless you de-tune pretty far) not sensitivity.

    All good and valid points (although the "radiating detector" concern can seen to be overblown, if we calculate the power at which your oscillating detector operates and consider how lightly even a direct-to-antenna detector is coupled to its antenna). Maybe a ham on the same block might hear your oscillating detector strongly enough for it to matter -- in reception of quite a weak signal.

    What a regenerative detector operating at the lower HFs and below most surely does not need is gain in front of it. Sensitivity (nowadays) characterizes not how much output a set produces for a given input level -- that is, it's not a measure of gain -- but rather characterizes how weak a signal the set can render distinct from its own internal noise. At the lower HFs and below, your antenna-system noise will be much stronger than your receiver's internal noise. Additional gain in front of the detector will only make it overload more easily. (And once our antenna-system noise floor is overriding the receiver's internal noise, if we want to make signals louder in our headphones, we build in more AF, not RF, gain.) See the antenna-to-detector path in my schematics at http://dpnwritings.nfshost.com/ej/wr/? We can use the input 1AD4 as an untuned, grounded-grid, low-input-impedance RF amplifier stage; all we need to do to begin is get its cathode above RF ground with RF chokes. (Yes, purists: We will also take advantage of mismatch loss in driving its cathode from the antenna.) It's much harder to overload a tube operating at a given grid bias by driving its cathode as opposed to stepping up the input voltage with a link-coupled tuned circuit feeding the same tube's grid.

    Takeway for now: Once your antenna-system noise floor is just/sufficiently overriding the noise floor of your receiver, your receiver is as sensitive as it can be. With a transmitting antenna used for reception, at 7 MHz we'll almost certainly find that we need to add attenuation, not gain, between detector and antenna to set sensitivity where it should be.

    And then we'll go to high-C at the detector -- X of L and C 100 ohms or less -- and I think you'll be amazed at how differently the set handles with respect to strong signals as you tune the band.

    At that point we may need more AF gain. We'll work it. (But, yes: My father once told me that I was happiest with my ham station when I found a problem and fixed it. Of course -- that's rocket-propelled learning AFAIC. It's possible to look at even a simple schematic and see problems or shortcomings built in, which may translate to improvements that could be made. An Incorrigible Experimenter I am.)

    All optional, of course. You're already making contacts -- fabulous success, the success. The Paraset helped the Allies win the war. Can't beat Having Won!
     
    Last edited: Sep 14, 2021
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