Saturday, May 16, 2020
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.