High-Performance Regenerative Receivers

Discussion in 'Homebrew and Kit Projects' started by G3EDM, Sep 27, 2021.

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  1. KL7AJ

    KL7AJ Ham Member QRZ Page

    I've been past the point of no return for about 45 years. :)
     
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  2. G3EDM

    G3EDM Ham Member QRZ Page

    My subminiature-tube regenerative receiver has been "dressed up" with the addition of a new front panel for the RF deck. The work will be completed soon, probably today.

    I've also done a lot of general "stiffening" by adding metal brackets, to increase mechanical rigidity and therefore, stability. I also cleaned up some of the RF wiring, which was originally done with the later re-use of components in mind, so the leads were way too long. Finally, I firmly anchored the shielded Detector and 1st AF tubes to metal or wooden supports so that they were not free-standing and prone to vibration.

    Internally, the thing still looks like a mess -- but it is functional. If I were building it from scratch today, as a settled design rather than a wild breadboard experiment, it would be much neater and more compact. But that's not really worth doing: the effort would be better spent building a superhet. I am however still interested in improving this set, so ideas are welcome. It is quite a good little performer!

    Because all of the above modification is not electronic (the circuit is unchanged), I will not post any of it here in detail. Instead, when the front panel is finished, I will post a longer account (and photos) in the Ham Radio Discussions forum, in the thread called "My First-Ever QSO -- 50 Years After Passing the Ham Test".

    73 de Martin, G3EDM
     
  3. G3EDM

    G3EDM Ham Member QRZ Page

    For @W9BRD: The current round of modifications to the regenerative receiver is done.

    The only circuit modification was in the plate load of the detector. In the reworked set, there is a front-panel switch to choose between: 180K resistive, and a 60h choke.

    Anyway, now that a direct A/B switch exists for comparison: I prefer the choke load. By comparison the resistive load sounds like listening through cotton wool.

    73 de Martin, G3EDM
     
  4. W9BRD

    W9BRD Ham Member QRZ Page

    The irony of the well-built regen is that in some respects when used "at signal" it can beat the oft-invoked "simple superhet"; and when used at IF -- which is to say, "at a low IF -- 3ish MHz or preferably even lower -- it can handily beat a simple super.

    "At signal" (frequency) we have no images to contend with. Built high-C and with a low-Z input amenable to use of one's transmitting antenna for receiving; equipped with variable attenuation that allows reduction of the antenna-system noise floor to just above the noise floor of the detector (and more, when receiving very strong signals); equipped with AF low-pass or bandpass filtering; and built such that its detector is electronically isolated from its antenna, it actually will "hear anything a more expensive receiver will hear" because getting down to the antenna-system noise floor at MF and the lower HFs is not a challenge for any well-implemented means of heterodyne detection in the same pass-bandwidth. And it won't suffer from "hand capacity" or "tunable hum" or dead spots or any of the other ills of yesterday's built-like-junk-by-those-who-expected-their-receivers-to-behave-like junk regens as a result of the electronic buffering of detector from antenna -- and because we of course build any receiver, regen or superhet, that we want not to act like junk into a metal shielding box...)

    With a detector operating "at IF" we add the superhet's propensity for image response, but that's not serious with a reasonable front end. And at IF our regenerative detector is even more isolated, and isolatable, from the "at signal" environment. Especially "at IF", but also to some degree with "at signal" detectors, we also enjoy the selectivity-of-gain effect contributed by filtering the receiver's input-signal diet through the passband of the detector's oscillatory loop. With a well-implemented detector at 3.5 MHz or lower, we can easily hear a high-C detector cutting highs from good-quality SSB with lighter coupling to its antenna-buffer stage. The AF output from even a well-built at-signal or at-IF direct-conversion receiver will (relatively speaking) be screaming with AF highs under the same conditions because its receiving bandwidth is not gain-limited by the oscillatory loop of its local oscillator. And the d-c receiver will require prodigious AF gain that we don't have to implement with a regen as a result of the high gain at RF or IF of its detector's oscillatory loop. Ditto a "simple superhet" until one makes it unsimpler with the addition of signal-width crystal or mechanical filtering; even a stage or two of IF amplification at 455 kHz with no crystal or mechanical filtering and built with the usual interstage IF transformers will be much wider than a high-C, lightly loaded oscillating regenerative detector at 3.6 MHz and below.

    One of the things one gains from optimizing regenerative-receiver behavior to task is an understanding of the difference between progress and fashion. Regens generally went out of fashion even for casual ham CW operation because they were ill-understood and expected to behave, and therefore were built, like junk. Once one is firmly uncommitted to delving into how radio circuitry actually works and can be optimized, there's much more success to be had, and sooner, with superhets -- and of course for AM reception (for which regens are actually quite poor), for all-singing, all-dancing mode/band coverage that one is willing to slap hundreds to thousands of dollars down for, the superhet -- for transmitting and receiving -- is and has long been the technology (although well-implemented direct-conversion -- er, pardon my unfashionable language, zero-IF -- designs, especially those leveraging DSP and DDS, can give good superhets a go).

    I don't like to use the phrase high performance of a regenerative receiver, as doing so predictably invites scorn. I'm just glad that my regens work much better than anything any of us has read about in any hammy publication from the regen's heyday, when many hams nonetheless made hay with regens despite building them and using them and treating and writing about them as only-until-I-can-buy-a-real-receiver, beginner-class junk.
     
    Last edited: Oct 12, 2021
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  5. W9BRD

    W9BRD Ham Member QRZ Page

    Please post the latest circuit; I'll be getting to building mine soon and want to start where you've gotten to. :)
     
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  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    Regens, or TRF receivers, give a lot of "bang for buck".
    They were used for a very long time on the maritime MF bands, and especially as watch receivers for the 500 kHz distress frequency.

    One encounter was the 500 kHz receiver used at Härnosand Radio/SAH from about 1935 to the mid-50s. It was special in the sense that it uses three tuned RF stages before the detector, which can be switched between oscillating and non-oscillating states.

    [​IMG]

    It also has an "RF attenuator" for monitoring your own transmitter, which certainly was necessary in the pre-WW2 incarnation of the station where a 3 kW MF transmitter was co-located with the receiver site. The receiver covers 250-600 kHz and was found "in the dump" when the station building was vacated after its sale in 2009.

    Post WW-2 surplus became abundant.

    Coast radio services used a lot of the RBL receiver category, often built by Wells-Gardner.

    upload_2021-10-12_7-40-56.png

    The RBL and similar were used into the 1960s.

    A few military sets also were regens or TRF:s. One that comes to my mind is the mid-30s German Telefunken man-pack LF/MF/HF receiver "Torn E.b.". Originally a commercial product, it was repackaged for Wehrmacht use and was found in a few variants. What was special about the Torn E.b. is the mechanical design and packaging.

    [​IMG]

    It used an internal construction with a turret bandswitch made from manganese-aluminium alloys.

    upload_2021-10-12_8-0-35.png

    upload_2021-10-12_7-55-34.png
    upload_2021-10-12_7-49-28.png

    The variant most commonly encountered here was the "model 1942" covering 200 kHz to 15 MHz in 6 ranges. A few dozen were purchased together with Hellschreiber equipment for regimental and brigade staff communications. Lack of spare parts, especially the RV2P800 tubes, made its service period after WW2 rather short. It became phased-out already in the mid-50s.

    Performance was good compared to many superhets, especially when using the narrow audio filter. I have used one only once, when a specimen was donated to my Uni radio club auction in 1979. When testing it, it turned out that it was able to detect the leakage from even the most well-shielded signal generator in the lab...

    73/
    Karl-Arne
    SM0AOM
     
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  7. G3EDM

    G3EDM Ham Member QRZ Page

    @W9BRD: Here is the latest schematic. While the receiver has undergone major physical transformation, circuit changes are minor since the last schematic, and are as follows:
    • Change in value of the main tuning capacitor from 50pF to 56pF and planned change in the shunt capacitor from 100pF to 94pF (this is under way).
    • Addition of a switch in the detector to choose between a resistive load, and an inductive load.
    • Addition of an antenna attenuator pot. I am still trying different values, but it seems like only the first 5K of a series attenuator are useful.
    [​IMG]
    While I am about it I will also post the (unchanged) circuit of the push-pull AF final (which I assume you are not building, since almost any AF power amplifier circuit could be used). Gain is still insufficient to drive a speaker adequately with weak signals, and more gain would also be appreciated on the low-impedance headphones when a weak signal (or a heavily attenuated strong signal) can sound too faint. I assume this could be achieved with modifications to the push-pull AF final, or perhaps with changes to the first AF. I have not done anything to optimize the push-pull but I am aware that there are things that should be done. I think that subbing 1AG4s did make a difference to gain, but not enough of a difference.

    [​IMG]

    73 de Martin, G3EDM
     
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  8. W9BRD

    W9BRD Ham Member QRZ Page

    Thanks for the pix!

    1AG4s should work at least a bit better with AF-appropriate bypassing on their screens. Screen-grid tubes should be operated with their screens at ac common for the signals handled; I'd use 2.2- to 22-uF electrolytics, at appropriate voltage ratings, from the 1AG4 screens to ground.
     
    Last edited: Oct 12, 2021
  9. W9BRD

    W9BRD Ham Member QRZ Page

    Thanks for the great mil/pro regen pics and schematics, Karl-Arne!

    I have RAK and RAL regens that have both been subjected to basement flooding and await refurbishment; have used an RAL-5 before and of course it's quite good, if overbuilt and overlarge for the facility it provides. :) Also have an RCA Radiomarine AR-8503 that I've (so far unsuccessfully) used in two tries at hearing SAQ (17.2 kHz with an electromechanical transmitter, grasshoppers!). (Even with the AR-8503 on all-battery power, so far the ac-mains-harmonics noise floor has been too high at my NJ QTH with the active antenna tried. Another SAQ transmission is scheduled for later this month...)

    The Torn E.b. is yet another instance of superlative German engineering, particularly mechanical engineering in this case, as there's nothing special about the basic radio circuitry in any regen. My favorite factoid associated with the set is a passage in a late 1960s or early 1970s CQ magazine writeup about it, in which a passage in the Torn E.b. documentation is said to characterize the operation of AGC as "transferring fading from the signal to the background noise" -- a species of technical poetry!

    Anent that pic of the operator at his RBL and key, I'm always amazed at installations with the key right at the table edge, requiring that code be sent with the sending arm entirely unsupported. Since code can (and IMO should) be sent as much with a flexing wrist as with the elbow, that seems to me to set up premature sending fatigue. Likewise there is no reason to actually grasp the key, as it's anchored to the table and can't get away, and because its spring, and not any upward pulling on the part of the operator, returns its lever to KEY UP. I must remember to rev up my time machine to go set him straight...
     
    Last edited: Oct 12, 2021
  10. G3EDM

    G3EDM Ham Member QRZ Page

    I added bypass caps (10uF, 63V). It made an appreciable difference to gain, especially in headphone use, where I think the problem of low gain is probably now solved. It is much better on the speaker too, although still not as much as I would like. But the speaker is hardly ever used except for demonstrations. Plus, the spec sheet for that tube rates it for 35mW power output so I may be getting as much as it is capable of giving, in push-pull.

    73 de Martin, G3EDM
     

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