CONSTRUCTING A 1930's REGENERATIVE RECEIVER
The design for this receiver was first published in the January 1933 issue of QST. In an article by QST's Assistant Technical Editor, George Grammer, "Rationalizing The Autodyne", the author promised "A Three Tube Regenerative Receiver of Unusual Performance". The term 'autodyne' simply refers to the oscillating state of the regenerative receiver. When it is oscillating, it is in the 'autodyne' mode.
I've always liked the building styles seen in the '30's and having built just one regenerative receiver previously (the receiver-section of my Paraset), I found this design intriguing. With the receiver's ganged-tuning scheme and well-shielded stages, it looked like a challenging project for the summer workbench.
An examination of the schematic doesn't show anything too unusual other than the tuned RF stage in front of the regenerative detector. Closer examination of the physical layout however, reveals that the front-end RF stage tuning is mechanically tracking the oscillator-stage tuning so that once set up for a given band there really is nothing to fiddle with, other than leisurely tuning across the band ... which, because of the low-valued variable capacitor, is generously spread out across most of the large drum dial.
Before starting construction, a full-sized plan view of the receiver was drawn so that parts placement and optimum wiring paths could be planned.
The small photographs in the original article made it difficult to see exactly how things were wired, but by the time I had completed my own plan, I was pleased to see that it looked very similar to what pathways I was able to make out in the original photos.
Construction began by building the two shielded compartments. The 'scraps' table of the nearest Metal Supermarket provided the 20 gauge aluminum (still in protective covering) along with the 1/4" square brass bar stock needed for the corners. All of the holes drilled in the brass corner pieces were tapped to accept 4-40 brass screws.
The front panel was cut from a large sheet of 1/8" aluminum sheet that had been purchased several years earlier. The 1933 chassis (7 1/2" x 13 1/2" x 2") was built by Grammer and the only aluminum ones available today are slightly smaller (7" x 13" x 2") in length and width ... things would be fitting a little tighter.
The pre-made full-size plan helped a lot when it came to positioning and punching holes for the tube and coil sockets.
All of the required variable capacitors (four) were found on E-Bay, all NOS, and surprisingly inexpensive. Perhaps the gradual decline in homebrewing old radios and transmitters has led to fewer demands for many of these older components.
Most of the other components (resistors, fixed capacitors, pots etc) were found in my junk box collection but several of the capacitors were 'manufactured' by re-packing modern ones into some original era-appropriate cases.
Wiring began by working from left to right, following the schematic. Grammer had expressed the importance of making sure that all of the grounding points for the RF portions of the circuit be made at a single point ... this even included the grounded rotor sections of the variable capacitors! Rather than do this, I ran a heavy silver wire buss bar across the chassis and brought everything that needed RF grounding to the buss.
The ground buss allowed for a cleaner-looking, less 'rat's nest' appearance, than the original seemed to have. Grammer also gave the builder a choice of using 2.5V or 6.3V tubes and I chose the six-volt variety ... 78's (RF and Detector) and a 76 (Audio). As well, six-volt 6D6's can be substituted for the 78's while a 37 can be subbed for a 76.
I made just one modification to the original schematic and that was to convert the audio stage to impedance-coupling so that the high voltage (200VDC) would not appear across the headphones. The circuit for the new audio stage is shown below.
The high-value choke specified in the original plan for the A.F. stage is fairly difficult to find nowadays. I substituted a normal power supply choke of about 10H which seems to work just fine ... the audio stage puts out more than enough to drive my high-impedance Baldwin headphones and in normal operation, the volume control is barely cracked open. Other regen experimenters have found good success using just one or more windings of a common audio transformer. I found one winding of a small speaker transformer pulled from a very old car radio, worked just great in the audio stage of my Paraset. If you're building a regen, just try what you have and don't worry too much about locating the exact specified part.
The coils were wound to exact specifications and on original Hammarlund forms. I have a number of these in the junk box, most with many holes drilled decades ago. I've found that the holes can be easily filled with two-part wood dough and then color-matched to the original form's shade using wash coats of acrylic paint ... the old holes become almost unnoticed once the forms have been waxed, buffed and wound once again.
Building Grammer's autodyne was just one of my 'summer projects' ... most of them being outdoor maintenance jobs, including the re-shingling of the garden-shed and the wood-shed. Because of the outside jobs, construction progressed slowly and was spread out over a ten-week period but ... eventually it was finished!
I usually have a 'gut-feeling' of whether a project is going to work immediately or if there is going to be a lengthy period of trouble-shooting ahead. For this project, it was the latter, as there were just too many reasons why things might go wrong. From the hand-wound coils to many of the old components (would they still be up to the job?) as well as the old tubes ... one of my junk box 78's had a big red question mark painted across the glass, not exactly encouraging.
With much trepidation, I held my breath and ... connected the filament voltage and then B+ ...
Very much to my surprise and delight, the receiver immediately came to life! I was delighted that a long trouble-shooting phase would not be needed. It took some time for me to get the feel of the controls and what levels were best ... in fact my first impression was that the receiver, although working, was not working as it should. I found that the RF stage would generate noise if the gain was set too high. This was soon cured by swapping the questionable 78 out of the RF stage and into the detector. Finding the best balance between the RF gain and regeneration was eventually figured out and I can say that the receiver is working extremely well.
It is difficult to know just what constituted 'good' performance in 1933 as these older designs cannot be compared with anything modern ... there is just no comparison to today's radios. The only other regenerative I could compare it with is the National SW-3 (made in the early 30's as well) and the regenerative receiver in my Paraset.
Comparing it with my SW-3, possibly one of the best regenerative receivers of its day, selectivity and smoothness of regeneration are pretty much equal. Both receivers are equally sensitive, at least on 40m. Audio output levels of both receivers are the same, with lots of audio from the new receiver. Compared to my Paraset, which uses a much newer and hotter tube (6SK7), both the Paraset and the SW-3 are about equal ... so it would appear that the new receiver is performing very well. The one place that Grammer's receiver beats out the other two is in oscillator stability. His design is very stable and SSB signals are easily tuned and stay in tune, when testing on 40m. Because of the tuned RF stage, there is no bleedthrough of high-powered shortwave broadcast signals just up the band ... a common problem of simpler regens. As well, once the regeneration level is properly set and the RF stage is peaked, no further adjustments are needed when tuning across the entire band ... the receiver is a real delight to use and I can see why it was so popular in the early 30's.
Overall I have been very pleased with the outcome of this project. It is very faithful to the original plan and I believe the receiver is performing as well as Grammer originally intended. I think that most amateurs in the early 30's would have been pleased to have had this receiver on their operating table ... it is easy to imagine a smile on their face as they tuned across the band at night.
**** Click here for a RECORDINGusing the new regen ... made on a noisy August evening on 40m CW ****