 |
 |
 |
|
|
|
|
|
|
||||||||||||||||||
|
|
|||||||||||||||||||
|
|
|||||||||||||||||||
|
|||||
|
|
![Previous (earlier) Story](/images/sty-down.gif "NTIA Administrator Lauds FCC for "Broadband Over Power Line" Inquiry"){kind=small} Previous: NTIA Administrator Lauds FCC for "Broadband Over Power Line" Inquiry |  Next: Columbia Panel Issues Preliminary Findings in Shuttle Tragedy |
By Michael
Heim, KD0AR
May 8, 2003
Grab a piece of the Roaring '20s by assembling a classic, handsome regenerative receiving set
 The front view of the radio, with a custom-made voltage meter face. [Author photos] |
Several years ago I learned of a growing group of amateurs who were re-living the beginnings of Amateur Radio through the building and restoration of the early CW sets used during the late 1920s. I leapt on that bandwagon with the construction of a simple two-tube regenerative receiver. Throughout my amateur career, I have been fascinated with the simplicity of the regenerative circuit and I am still amazed by its performance. The receiver I will describe here is my latest project. This receiver is a culmination of about a year of very extensive research into the early days of Amateur Radio. A 1924 article by Schiller Kruse, 1OA, the ARRL Technical Editor at that time (S. Kruse, "Low Loss Tuners," QST, Feb 1924, pp 8-13) inspired me to build this radio and I have adopted it for use on the 80 meter band, where most of this type of equipment is used today. (See also H. P. Corwin, 2BRC, and E. C. Homer, 9UN, "A Well Designed Tuner," QST, Dec 1924, pp 52-59.)
A Battery of Batteries
My receiver uses as many 1920s era parts as I could locate. It contains only two modern components, the tuning capacitor and the filament rheostat. Everything else electrical in this tuner is of 1920s vintage. The set runs from batteries. In this model I used five 9 V batteries in series for the plate supply and a 6 V, 4 Ah gel-cell for the filaments. Regeneratives always seem to run better from a battery supply and I strongly suggest using batteries to power this radio, especially if you build it using vintage parts. Tubes of the era have directly heated filaments and using ac on these tubes will introduce a very loud hum in the headphones. The slightest ripple in the plate supply will cause loud hum as well. The plus side of using batteries for this set is that the plate battery will last a very long time, with five alkaline batteries capable of about a year of casual operating, as the set only draws a couple of milliamperes. The filaments can be run from four alkaline D cells in series, which should operate the filaments for many hours. The C battery containing three AAA cells should last indefinitely, and the smallest of cells are sufficient as no current is drawn from them. This radio can be built using old style components or using easier-to-find modern parts. If you try the receiver using modern parts, I suggest that you still obtain a pair of No. 201A triodes. Used 201s cost about $10 to $15 each from several tube vendors on the Internet, and sometimes can be found for less at hamfest flea markets.
 |
Cabinet Considerations
I began building on a piece of stained and lightly varnished poplar board. Builders of this era used wood exclusively because it is easy to work with. Regenerative receivers with wooden cabinets also perform better because there is very little metal in the fields of the coils that can spoil the Q of the circuits. Your wood chassis needs to be about 14 by 8 inches. You can go larger if you choose, but if you go much smaller you will crowd the components and it's important to keep the coils in the clear as much as possible. If you wish to keep the batteries inside the set, it would be good to go with a 16 inch width; I use an external battery pack with mine. The front panel is a piece of jet black acrylic sheet cut 1 inch shorter than the base. The sides and back panels are 1/2 inch stained poplar like the bottom chassis. Varnish the wood before assembling the cabinet and use small finishing nails and tiny wood screws to assemble everything. I will leave much of the woodworking up to the individual builder, as some builders are more skilled than others in this regard. No doubt with careful work, the average woodworker will come up with a very good-looking radio. If the black acrylic is unavailable, a thin piece of plywood, stained like the rest of the wood will produce a very attractive set. I have built a real nice wood-faced radio similar to this one.
Mounting the Parts and Winding the Coils
Locate your panel mounted parts, the two variable capacitors, rheostat and headphone jack. A filament voltmeter is optional, although it is nice to have. Drill the front panel for the controls. I put the regeneration capacitor 3-1/2 inches from the left side and the tuning capacitor in the center. That leaves the right side for the rheostat, headphone jack and meter, if you use one. Mount those parts on the panel.
Next wind the coils. There are three coils and they are easy to wind. To keep the look of the era, I used basket weaved wound coils, which are really easy to wind. On a scrap piece of wood, use a compass and draw a circle 3 inches in diameter. Along the perimeter, pound in 11 finishing nails spaced equally apart. The nails should be at least 2 inches long. Wind 12 turns of No. 22 enameled wire--or if it's available, the cotton-covered wire that was used in that era--alternating inside and outside every other nail. When the coil has been wound, take some green thread (rob the wife's sewing kit for this) and thread it up and down through the openings near the nails. Go all the way around and tie the two ends together. Make sure the thread is as tight as you can make it. Once the coil has been tied, it can be carefully slipped off the form. Lay this coil aside; it will be the tank coil.
 A close-up of the coil assembly. Note the antenna coil is mounted on a turnable dowel. |
The tickler is made in the same way, using the same coil form and with the same procedure as the tank coil. Wind four turns on this coil and tie it. Remove this and set it aside. The antenna coil has 2 turns and can be wound by making a 3 inch loop and the second turn "twisted" around the loop to hold it together. That completes the most difficult part of the wiring. One important note concerning the coils: don't use any kind of glue to hold the wire. Tie it with thread. Glue can spoil the Q of the coils and make the tuning of your radio broader than it needs to be.
To mount the coils, cut two 1/2 inch diameter wood dowels about an inch long. Take two Popsicle sticks or thin wood strips and drill two holes in them about 3 inches apart. Mount one stick to the two dowels with small wood screws and drill the chassis in the left rear corner for screws to mount the dowels vertically on the chassis. It is important to keep this assembly in the clear, but keep in mind that there will be a large capacitor mounted in front of this. Once the dowels are mounted to the chassis, remove the screws in the wood strip and take the coils and mount them in place The tank coil will be on the right, next to the dowel, the tickler about ¼ inch from the tank. The antenna coil works best if it is on a swinging link not fastened to this assembly, but can be placed at the far left end of this assembly if necessary. The coils are held by the two Popsicle sticks. Tighten them tightly and don't worry if it squeezes some. It's okay as long as the coils are tightly mounted. If you choose to use the swinging link option, you can use a wood shaft going thru the back of the set and you can use an adhesive to mount this coil to the shaft. This will not have to be adjusted much and it's how I made mine. I don't adjust mine much, but you might want to alter your front-panel layout from mine if you want to be able to control coupling from the front. Finally, fit the front panel in place, making sure you have space between the coils and the capacitor.
Mount the tube sockets next, with the audio interstage transformer between the two tubes. Mount them towards the rear of the board. You can use the more modern 4-pin sockets if you can't locate old style ones, but the old bayonet sockets add a nice touch if you can find them. I put the grid leak and grid capacitor in front of the detector tube. Wire the parts together as shown in the schematic using fairly stiff wire. The only lead length you need to worry about is between the tank coil and tuning capacitor. Make these direct runs.
The filament rheostat I used was a L-pad potentiometer that is used to control a PA speaker's volume. These are available at RadioShack and the value is about 25 Ω. It's listed as an 8 Ω L-Pad.
Construction Hints
The batteries in my receiver are connected with Fahnstock clips. I use a 1/4 inch phone jack for the headphones. The headphones are of the old style high-impedance magnetic type. Crystal earphones will not work here. If you wish to use modern low-impedance headphones, you can wire an output matching transformer with its high-impedance winding in place of the headphones. The impedance can be anywhere between about 2 kΩ and 10 kΩ. The secondary, 8 Ω winding of this transformer can then be wired to the headphone jack.
 View of the tuning capacitors with aluminum foil attached. |
An on/off switch can be wired in series with the 6 V filament supply. I was able to modify my rheostat by removing some of the resistance wire on the end of the rheostat so that when the rheostat is turned all the way down it shuts off the current flow to the filaments. With no filament supply, there will be no plate current, so that battery can be left in circuit. If you decide to use a filament voltmeter, it can be wired in parallel with the tube filaments.
Use big knobs if you can. A vernier knob for the tuning is a help, but not totally necessary if you use a large knob. My radio has a vintage 4-inch diameter knob for the tuning and a 3-inch knob for the regeneration. The filament knob can be any size as this is not a critical adjustment.
To eliminate most of the hand capacitance effects that will show up, tape a piece of aluminum foil to the back of the front panel. Put a 3/8 inch hole in the center and sandwich it between the front panel and tuning control. This will ground the foil.
A word about operating on different bands: I designed my radio to operate on 80 meters. It should function well on 40 as well as 160 by changing the grid coil. Doubling the number of turns should double the wavelength and put you in the ballpark of 160. Halving the number of turns should put you on 40 meters. If you choose to operate this radio on 160, you may have to increase the tickler a few turns. A plug-in coil affair is typically used in regenerative sets, and there isn't anything wrong with going that route, but I wanted to optimize my receiver for 80 meters where one finds most of this kind of operation. This radio can also be designed for the various shortwave broadcast bands by tailoring the coil to resonate on them. In my experience however, regeneratives become somewhat finicky above 10 MHz. It takes careful circuit engineering to make them work well on the upper HF bands.
Firing it Up
To put the receiver into operation, connect the batteries and slowly bring up the filaments to between 4 and 5 V. Do not run the filaments above 5 V, under ANY circumstances. Mine runs nicely around 4 volts, and the lower the filaments, the longer the batteries and the tubes will last. Start with the tuning control at about mid-range and the regeneration control fully open (minimum capacitance). Slowly mesh the regeneration control until a click is heard in the headphones. If the capacitor is fully meshed and no click is heard, reverse the tickler connections and try again. The point where the click is heard is the radio's most sensitive and selective point. The click indicates the point where the receiver breaks into oscillation. Connect the antenna and tune the tuning control while the set is oscillating.
 The placement of parts of the RF section. |
The 80-meter band will be quite active in the evening, with many SSB signals present. If the 80-meter band tunes too low on the dial where you cannot get the entire band, place some additional capacitance--a "padder" in 1920s terms--in parallel with the main tuning capacitor. The dial can be calibrated with your main station receiver. Simply adjust the regeneration so the set goes into oscillation. The regenerative can now be tuned in with the station receiver. A tuning chart can be made by taking readings along several points across the dial and logged on a graph or directly on a homebrew dial.
When the receiver is operating in the oscillating mode, SSB and CW can be received. If the signal you are listening to appears to overload the set, you can advance the regeneration control and that will make the receiver less sensitive. If the signal is a real strapper, you will have to uncouple the antenna link slightly by moving the coil toward a 90 degree angle from the other coils. This is where a variable link comes in handy. Use the receiver in the non-oscillating mode to listen in on the "AM Window" from 3880-3885 kHz. On an AM signal, you will get to see just how selective this simple set can be. Tune in an AM station of average strength. Get a feel for where the oscillation point is (there will be a loud squeal in the headphones when the set is oscillating). Now, with the set in the non-oscillating mode, advance the regeneration control to the point just before it starts oscillating. Listen to the AM signal. Does it sound real bassy? The receiver at this point is so narrow that it's actually cutting sidebands. To restore the fidelity, back off the regeneration a bit.
I successfully used a similar receiver during the Antique Wireless Association's Bruce Kelley Memorial QSO Party in December 2001. My best DX worked using the predecessor to this receiver and a No. 10 Hartley transmitter was with K6TQ in Los Angeles, a distance of about 2500 miles from my location. Bill was running about 20 W. It was rough going, but we did make contact. If you build this set, I am sure you will be as amazed by its performance as I am. Give a simple regenerative a try and enjoy the nostalgia that these old time radios summon.
Licensed
since September of 1980 and holding an Amateur Extra ticket, Michael Heim,
KD0AR, works as a technical engineer for Clear Channel Communications in
Youngstown, Ohio. His interests in Amateur Radio run to the technical side and
he remains an avid equipment builder. Heim can be contacted by e-mail at mike@kd0ar.homeip.net or by postal mail at 2329 Mt
Vernon Ave, Youngstown, OH, 44502.
| Previous: NTIA Administrator Lauds FCC for "Broadband Over Power Line" Inquiry | Next: Columbia Panel Issues Preliminary Findings in Shuttle Tragedy |
