Can real DX Be Heard on a Regenerative Receiver

In an earlier episode,  I told about the somewhat unplanned project to build a regenerative receiver.
It was done out of a desire to see how such a radio would have performed if I had had one when I was a young kid.

The project had a regenerative detector and two stages of audio and included bandspread tuning and was expected to tune about 6-18 mhz.

The first night it was finished,  I tuned through the dial just to make sure it was working and to get some idea what frequency range it actually tuned.

It ended up being a couple of days before I was able to get back to the little radio,  with things coming up at work and household projects that had to be taken care of.

The first test run had shown that the radio worked and a couple maritime cw stations along with a broadcast station or two and time signal station WWV were heard.

A couple days later,  I sat down in front of the rig with it actually set up on the desk as it would be used,  rather than turned on its side with test leads connected and tools and such scattered all over the place.

The first step  taken was to get a better idea of what was where on the dial,  which was only calibrated 0-100.  I had already found where 10 MHz was because of WWV and where 9420 was from finding the very well known Greek broadcast station. It was decided to use my ham transceiver to get a few more calibration points.

I put my Icom 720 on a dummy load and had the drive turned all the way down to provide the test signals.  Then calibration points were found for the bottom and top of the 40, 30 and 20 meter ham bands.  Tuning up from the mark for the top of 20 meters had me running into the 15 MHz WWV which gave another calibration point.

Going down the other way, the radio did not tune down far enough to reach the 5 MHz WWV.  In fact,  the bottom of the 40 meter band just did make it within the tuning range of the radio,  and the 49 meter shortwave broadcast band didn't quite make it at all at first.  By turning the tuning slug all the way in on the input coil,  only a portion of it could be reached.

I spent a little time getting the feel of the controls tuning through the 20 meter cw band.  It seemed that the most stable point for copying cw without overloading the radio was with the regeneration control set just barely past the point of oscillation.

It being early summer,  twenty meters was still open with lots of signals even at just before sundown.
It was obvious that the selectivity was not going to be as good as the Icom or my FT-101 with its 200 Hz cw filter.  It might prove difficult to pull out weak signals from DX stations in a crowd.

But then,  it must be possible,  because in the early days of hamming,  regenerative receivers were all many ops had....and they did work DX.  But were the bands as crowded as today? No way to know.

So it was down to tuning. Starting high in the cw portion of the band,  I immediately ran across a strong station that could only be one thing from the content....it was an ARRL bulletin.  It seemed somewhat appropriate that one of the first ham stations in the log from the little receiver would be the ARRL Club Station, W1AW !!!

Going on down the band netted a number of US stations,  but there was still hope for some DX.  The little rig did not disappoint.  It took some very careful juggling of regeneration and bandspread controls,  but soon I had confirmed an ID for OA4CWA from Peru!

I was pretty happy with that,  but there would be a few more.  Within the next half hour I had logged OM3KFF from the Slovak Republic,  IK3HZK from Italy  and F1MCC from France.  Not rare DX by any stretch of the imagination in the eyes of big time DX-ers,  but I felt it was pretty good for the little regen rig.

A quick run down to 40 meters did not yield quite as spectacular results.  Stateside signals were much stronger and it was early in the evening.  I did identify K4DMT, W0RFN and KA9KWR. I was anxious to try a run through the maritime cw band at 8 MHz,  but did stop on one last station that could be considered DX I suppose,  KP4/W8HNI from Puerto Rico.

The maritime cw band was a real bit of an adventure,  but in some ways was a little easier,  first from the standpoint that the stations generally ran more power than hams,  and also because they were not piled on top of each other helter skelter and were easier to separate.  In about 45 minutes I was able to put several of them in the log. Those pulled out included the following:
PJC Netherlands Antilles
FLBA ( being a four letter call apparently a ship as opposed to a shore station)
NMC
WNU Slidell, Louisiana
FUF  Guadalupe Island
WNU-33 Slidell
WCC Chatham, Mass.  ( probably the best known of the shore stations of the day)
ZSC Capetown, South Africa
VCS  Halifax
FFL St Lys Radio,  France
DAN  Norddeich Radio, Germany
OST4  Ostend Radio, Belgium
HPP  Panama
CLA  Cuba

Not bad stuff for a junk box regen radio.

( And another good reason for keeping logs.  Then and now I log virtually everything I hear each time I hear it and have kept logs going back to 1957)

Over the next few days,  I played around with some things trying to overcome the overload and pulling that occurred with some stations.  I ended up putting a 100 pF variable capacitor in series with the hot lead of the antenna to provide a little decoupling at times.

It may seem perhaps sacrilege but at one point I brought out a little MFJ tuned preselector and put in front of the input.  It was not so much as to provide gain for weak signals,  as there was really no need for that.  It did seem to provide a little " cushion " between the antenna and the detector and let it see a more stable load on the grid circuit.  Most of the time the gain control on the preamp was kept very low.  The additional selectivity did seem to help.

It is unfortunate that other things got in the way of spending much more time on the little rig.  Field Day was coming up and a local group was planning to operate at a nearby park and attention got diverted from it.

After Field Day,  pieces for a 10-meter beam were picked up and the design and construction of the yagi got my attention,  working out the details of maximum gain,  optimum wide spacing of the elements,  then getting it on top of the 40 feet of Rohn 45G that had grown outside the shack at my folks house ( my dad,  who is now a silent key and I had a joint shack for a time when I lived in an apartment and before I moved to my current location across town with my own real yard and future antenna  farm)

After that it was more projects,  overtime at work, and other things and the little radio got put on the shelf.  Now almost thirty years later, it sits dust covered in my back yard shop.  Perhaps its time to dust it off, re cap the power supply and bring it into the indoor shack and give it a chance to pull in some more DX.

Thinking about it still brings back the says of yearning for a Knight Ocean Hopper or Span Master.  It  would be nice to find one and spend some time twirling the dials,  but unfortunately they are not great in number any more and the collectors seem to snap them up and put them on a shelf,  not leaving many to be found by us old geezers who would just like to spend some time with them finding adventure.

A Trip Back in Time

I think we may have all done this at one time or another. We've sat in front of our quasi-modern rig and thought back to days before we had such nice gear,  wondering if knowing what we know no we could have landed more DX with older,  simpler gear than we have now.

There is also the desire perhaps to have a chance to get the receiver or transmitter that we wanted then.  It might not be a matter of thinking the gear would be better than what we have now,  as much as a curiosity of finding out if it was as good as we thought it might be.

From experience at picking up some of these rigs at hamfests,  I have come to learn that sometimes this is true and sometimes not.  The gear might be better than we had  " back then" but sometimes we learn that perhaps it was more a matter of learning how to use radios better that has lead to more DX.

I have often longed for one of the old Knight regenerative receivers that I wanted so bad as a kid.  I never got one and ended up with what was probably a better receiver for a first SWL rig anyway ( a kit from a radio-tv correspondence course my dad had taken) That radio was not without its own frustrations, including very broad selectivity, almost no image rejection, and a level of stability that almost required keeping a hand on the tuning knob while listening to a station for more than five minutes.

But the urge to find out just what might have been struck rather hard one day in the early nineties as I set in my quasi-modern ( i.e. less than twenty year old) rig  during a stormy afternoon.  The static was so bad that listening was like having a little man inside my headphones delivering blows from a little sledge hammer directly onto my eardrums.

Thoughts turned toward a construction project,  perhaps a regenerative receiver knowing what I know now and seeing if any real DX could be heard with it.

At the time I had a pretty well equipped junk box ( ok, more like a junk closet) full of various parts, transformers,  salvaged chassis and other goodies,  along with a huge stock of tubes,  I began the search. I already knew what I would start with.  I had picked up a chassis and front panel that had been somebody's home brew project for something at a hamfest.  It had a two-gang variable capacitor of the broadcast type along with a small transformer power supply.  It looked like it might have been some kind of signal generator.  The capacitor had a gear reduction drive on it and there were three octal tube sockets behind the panel and some other holes,  along with a really nice panel light.
The dial on the front was calibrated 0-100,  not unlike what my first SWL receiver had had.

The first step was to strip out everything including the wiring for the power supply. I already had plans for that which included building a choke input filter that would give somewhat less than the 180 volts that came out of it as it stood,  but would also give a little bit better voltage regulation.

The it was decided that I would use a 6SN7 for the detector.  This tube is a dual triode and I figured that I could at least have an audio stage to give a bit more audio to the headphones.

The junk box yielded a collection of Miller factory made coils that I had bought in a closeout at a local electronics store that was closing up. There were coils for the broadcast band, 1.6 to 6 MHz and 6 to 18 MHz designed to be used as input coils tuned by a 365 picofarad variable capacitor.  Since I had had a broadcast band regen set as a kid,  I decided I wanted short wave,  and a range that would give at least a chance at hearing some DX. So the 6-18 MHz coil was chosen.

The only thing was,  the coil had just two windings: a coupling coil meant to go to the antenna and ground,  and a slug tuned main winding meant to go to the input of the first stage.  There needed to be a tickler coil for feedback for regeneration.  I took care of that by winding a few turns of enamel covered magnet wire from the junk box.

I was beginning to get into the deal by this point,  so rather than just scramble winding it over the other windings and taping it down,  I actually used a little shellac to hold it permanently in place. There just happened to be a nice 3/8's inch hole right next to the octal socket nearest the variable on the chassis that would make a good home for the coil,  with the active windings somewhat shielded from hand capacity effects under the chassis.

While the shellac was drying,  I got to work on the power supply.  It was pretty quick.  I pulled a 5Y3 out of the tube stash figuring it would have plenty of current capability for what this little thing would pull. There were  both 6.3 and 5 volt filament windings on the transformer along with the tapped high voltage secondary.  Both filament winders were center tapped,  so this was going to work out nicely.  I had a filter choke salvaged from a scrapped out Silvertone radio that went in place and one of the extra holes in the chassis allowed a for-real can-type filter capacitor to be used, a dual section 30 microfarad job.

Noticing a nice hole the right size for another  tube socket was right next to the filter cap hole,  I got to thinking that while I was part-way to having a nice, stable power supply,  why not go whole hog and regulate it....so a VR 150 went into the spot.  The project was quickly taking on a life of its own. I figured I would send the regulated voltage to the half of the 6SN7 used for the detector and let the other half run off the unregulated part of the supply.

That left one other tube socket empty over near the right rear part of the chassis.  By now the project had grown from an quick afternoon throw-together thing to a four day after work thing.  How about another audio stage so this monster could drive a speaker instead of just headphones.  A quick dig in the tube stock turned up a 6V6 and a plate-to-voice coil transformer that just barely fit on the chassis. ( ok so I cheated a bit,  the mounting tab on one side of the transformer was too close to the edge and it would be held in place by just one screw...don't tell anybody)

All of this set me to wiring up the audio part first.  A quick trip to the RCA Receiving Tube Manual front section led to information on resistance coupling audio stages to match up the second half of the 6SN7 to the grid of the 6V6.  There would not be room for another audio transformer so this would be necessary. 

An aside here,  the RCA and Sylvania tube manuals are excellent sources of design information and info on theory of tube circuits.  They are becoming rare so snagging one at a ham fest is a good idea if you do anything much in the line of building tube-based projects or repairing boat anchors. RCA printed two versions,  one for receiving tubes and one for transmitting tubes. Even without building anything,  they make fascinating reads and there is a ton of true geek material to look through regarding the characteristics of the tubes.  I used these as study material for my ham licenses and for getting ready for the test for First Class Radiotelephone License ( no mere GROL--the real thing) back in the sixties.

There were four holes in the lower front of the chassis and panel that I was using,  so a potentiometer for a volume control went into the right-most hole.  I did not use the AC power switch on the back of the pot because I wanted to keep the AC voltage well away from the detector wiring,  knowing from experience that regenerative circuits can sometimes be susceptible to hum anyway.  This led to one rather unorthodox thing...the power switch was a toggle switch mounted on the left side of the chassis near the rear next to a fuse holder.

Those were the only holes I had to drill in the chassis and kind of went against the original idea of using things as they were.  I really did not want to build an unfused power supply ( NEVER be tempted to do this!!) and did not want to use one of those in-the-power-cord things.

Wiring up the audio stages went pretty quickly.  The volume control was put in the grid circuit of the second half of the 6SN7. That stage was then resistance coupled to the input of the 6V6 stage.
Another trip to the tube manual helped determine the value of a cathode resister for the 6V6 for self bias with a bypass capacitor across it to ground.

Just like in the old magazine construction project articles,  it was decided to test the audio stages before the detector was wired up,  mainly so if things didn't work there would only be one place at a time to trouble shoot.  The power supply was already tested and had about 170 volts unregulated and a nice 150 volts regulated.  A six volt bulb was put in the little pilot light socket already in the panel and the smoke test begun.

No problems.  The whole thing was almost too simple for anything to go wrong,  but you never know... There was only a very faint hiss and no hum ( yay!) coming from the speaker with the volume control all the way up ( grid of the 6SN7 highest above ground) A little finger touch to the center lug of the volume control brought a satisfying growl from the speaker so it appeared all was well.

Time to get to the meat of the project.  I had decided that even with the gear reduction drive on the main tuning capacitor,  it might be nice to have a little bandspread tuning.  A small variable with only three rotor plates taken from some old rig that had been junked for parts was used.  It went in one of the lower front panel holes.

That left the question of regeneration control.  I had decided against using resistance controlling so as not to give too much of a changing load to the plate of the detector while making adjustments to the feedback.  The plan was to have the ticker feedback winding high side go to the plate of the detector through a 100 picofarad capacitor ( plate voltage on the cap through the winding not a good idea with
 fingers near it)with the " cold end of the winding going to the stator places of the small capacitor that would control the feedback.  Since the rotor side of most variables is common to the frame,  that side was going to ground. The shaft of the capacitor was cut short and an insulated coupling and extension used to go to the knob to try to reduce hand capacity effects.

From there,  it was just running the mica capacitor paralleled with a resistor from the grid of the 6SN7 half used for the detector to the top end of the tuning coil,  and the other side of the coil to ground.  The coupling winding went through a short piece of shielded audio cable to a chassis mount SO-239 fitting on the back right side of the chassis.

Then there was a capacitor to couple the plate of the first side of the  6SN7 to the other,  a plate resistor for the detector once again chosen with the help of the tube manual, an RF choke between the low end of that and the power supply to keep from having any RF getting in there and a bypass capacitor to ground for good measure.

Time now for " smoke test number 2." Power came up, volume came up and a little louder hiss came from the speaker as the volume was brought about a third way up. Then the regeneration control capacitor was rotated to more messed and there was soon the expected "plop" sound in the speaker with a somewhat louder rushing sound as the stage went into oscillation. That meant that the 50-50 chance of getting the polarity of the tickler coil connected right was successful.

Now was the time to see if something could be heard. I set the main and bandspread tuning capacitors to mid range and brought a jumper lead to my 80-meter windom antenna over and plugged it into the SO-239,  met with a satisfying crackle and increase in noise.  The raw lead from the antenna was fed into the radio,   not through my antenna tuner as I usually did for the ham rig,  the idea being one less thing to tune to get things to the point of hearing something.

Of course,  I had no idea where I was frequency wise. As I swept the main tuning back and forth,  I ran into some cw signals,  easily identifyable as maritime shore stations ( this was back in the 90's when many were still on the air) The stability was not too bad and callsigns for WLO and KFS were heard.  Perhaps the 8 MHz marine band?  Tuning toward less capacity and thus higher frequency,  I soon ran into several broadcast stations,  marked by whistles as the rig was still in oscillation. Backing off the regeneration control took away the whistle and let the audio of the signals come through. I hit one with rather recognizable music style and rested there a minute and soon was able to confirm that I must have been on 9420 as it was indeed the Voice of Greece,  or as it was known then Foni ti Helladis ( if I remember right ) Tuning up through several stations,  then hitting a stretch of blank space, there was  then the unmistakable WWV.  So the point on the dial corresponding to "70" was obviously 10 mhz.  I was well on my way of finding my way around the dial.

Tuning up a bit further,  I hit as expected an SSB signal.  Tuning these with a regenerative receiver is a real trick.  You have to be in feedback but not too much,  and you have to tune carefully.  Using the bandspread control with a little touch and release as there was still some hand capacity frequency pulling affects,  it was determined that the station was transmitting aviation weather. New York Aviation Weather! There was still significant drift,  but I would never have expected full communication grade stability from this thing.

Tuning up higher,  I ran into the ( still to this day for me) unidentified  radio teletype signal just inside the 30 meter ham band and a few cw signals.

Obviously the thing was working and I had traveled back in time to my childhood...sort of.  There would be more experimentation and some real DX tuning another time,  but it was late and dinner had been missed,  and there are priorities.  I will share more of the DX adventures with the rig dubbed Little Howler II at another time.