I built this receiver as a companion to my 6L6 classic. The design originally appeared in the February 1941 issue of QST. Below I've borrowed heavily from the orignial wording in order to retain some of the tone of the article.
Long-established custom in 1941 dictated that a beginner's receiver, if he built it himself, would be a two tube regenerative. The two tube regen was inexpensive to construct and represented perhaps the simplest practical receiver that could be built. It had definite disadvantages, even in 1941.
Making the two tube regen a two tube superhet resulted in a worthwhile improvement. Because the regenerative detector now works on a fixed low frequency it can be designed for stability rather than operation over a wide tuning range. The regeneration control is independent of tuning so it may be set to it's most sensitive spot and left alone. The low IF also means that selectivity is enhanced over a typical regenerative receiver. Because the regenerative detector is not coupled directly to the antenna, dead spots and instability because of the antenna are eliminated. This receiver approximates the ideal regenerative set simply because utilization of the superhet principle permits the thorough isolation of the regenerative detector from the antenna and allows working the detector on a fixed low frequency.
Two tubes, though, still forces the receiver to rely on a regenerative detector. Even a stable detector over loads easily on today's ham bands. A few minutes of playing with the two tube superhet on 40 meters convinced me that a BFO was needed for reasonable CW and SSB reception. The two tube superhet became a three tuber. The regenerative IF remained. It is not typically run into regeneration but just below to improve IF gain and selectivity.
The set does not give loudspeaker volume -- the input to the audio amplifier is only a little more than a hundredth of a watt, so it would be hard to find the power to drive a speaker. Headset volume is satisfactory. As a modern safety measure headphoone are RC coupled rather than directly connected into the plate circuit of V2/6C8G.
A word about images. The receiver will, of course, respond to signals either 1700 KHz lower or 1700 KHz higher than the oscillator frequency (depending on the exact IF frequency). The unwanted response, or image, is discriminated against by the tuning of the RF input circuit. By tuning the oscillator from 5.2MHz to 5.7MHz, then, both the 80 and 40 meter ham bands can be tuned by merely retuning the RF input circuit. Besides eliminating the winding of a coil, this has the added significant advantage of minimizing warm-up drift when changing between bands.
V3/6K8 serves as a HF oscillator/mixer to convert the incoming signal to a fixed IF between 1600 and 1700KHz. L1-C1 tunes the RF input to the received frequency. L2 is the antenna coupling coil. The HF oscillator tank circuit is made up of L3, C3 and C4. C4 is the band spread capacitor and C3 is the bandset capacitor. The IF output from V3 feeds half of V2/6C8G which acts as a regenerative IF. L5 together with C5 form the IF tuned circuit. The high capacitance of C5 helps stability. C2 is the regeneration control. The output of the IF stage is transformer coupled to the second half of V2 which provides a single stage of audio. An flashlight battery provides 1.5 volts of bias for both V2 and V3. This saved a few resistors and capacitors in the original design. Half of V1/6C8G is the BFO. It's tank circuit consists of L7, C12 and C11. C12 is adjusted and then left alone. A switch is provided that turns the BFO on/off.
The heater requirements of the set are 1.0 amp at 6.3 volts. Either AC or DC may be used. 90 Volts at about 15 mA is all that is required for B+.
From the front, the RF or input circuit tuning is on the left with IF regeneration below it, band spread tuning is in the middle and bandset on the right with BFO on/off below it. Three 0-100 calibrated dials/knobs are required. The band spread dial should be a vernier drive such as a National type B. The RF Input and the Bandset need just to be resettable to a few divisions out of a hundred. A couple of knobs with 0-100 calibrated flanges will work for these two positions. I used two National type BM dials.
This receiver is built on a 7" x 9" x 2" chassis. From the top/back, the shielded V3/6K8G converter tube is centered on the chassis, the input coil L1-L2 is on the right and the oscillator coil L3-L4 is on the left. The detector/AF amp V2/6C8G is on the right next to the input coil and the BFO V1/6C8G is next to the oscillator coil. BFO tuning is via a set-and-forget adjustment on the left side of the chassis. The three binding posts are for power. Centered on the chassis rear are two pin-tip connectors for the audio output. These can be either transformer coupled to headphones or to an extra stage of audio and a speaker.
Underneath the chassis the IF coil L5-L6 is half way between the 6K8 and IF/AF 6C8G sockets. The antenna-ground terminals are on the left. The BFO coil L7-L8 is on the right fastened to the side of the chassis.
To eliminate hand capacity effects some sort of shield must be provided between the controls and the bandset/bandspread capacitors. A metal front panel or sub panel is ideal. I found the internal metal construction of the National verniers to be sufficient. A larger chassis would also be helpful. Ordinarily the BFO tube should not be right next to the oscillator coil. (For me the BFO tube was a last minute addition and that corner of the chassis was the only place available for it.) Warm-up drift would be minimized if a larger chassis is used that allows the BFO tube and the oscillator coil to be separated.
The IF frequency can be anywhere between 1600 KHz and 1700 KHz. The main thing to avoid is the frequencies of any local broadcast stations and any sub-harmonics of the ham bands. You'll hear a harmonic of the BFO otherwise. The IF frequency is fixed and the BFO frequency adjusted to match it.
The BFO circuit is optional. The receiver picks up AM foreign broadcast stations fine without it. As mentioned already, copying CW and SSB is much easier with the BFO. Stray coupling provides enough injection into the IF stage.
|C1, C2, C3||100 uufd. variable (Hammarlund SM-100)|
|C4||15 uufd. varialbe (Hammarlund SM-15)|
|C5||250 uufd. silvered mica (Dubilier Type 5-R)|
|C6, C13||0.01 ufd. paper|
|C7||0.005 ufd. mica|
|C8, C9||100 uufd. mica|
|R1||50,000 ohms, 1/2 watt|
|R2||1 megaohm, 1/2 watt|
|R4||33 Kohm, 1/2 watt|
|RFC||2.5 mh. r.f. choke|
|T1||Audio transformer, interstage type, 3:1 ratio (Thordarson T13A34)|
|L1-L4||See coil table|
|L5||55 turns No. 30 d.s.c., close-wound on 3/4 inch diameter form (National PRF-2); inductance 40 microhenrys|
|L6||18 turns No. 30 d.s.c., close-wound on 3/4 inch diameter form (National PRF-2); inductance 40 microhenrys|
|S||S.p.s.t. toggle switch|
|C10||100 pf. mica|
|C11||200 pf. silvered mica (Dubilier Type 5-R)|
|C12||10-90pf. (approximately) trimmer/variable|
|R3||47K ohms, 1/2 watt|
|L7||55 turns No. 30 d.s.c., close-wound on 3/4 inch diameter form (National PRF-2)|
|L8||18 turns No. 30 d.s.c., close-wound on same form as L7|
Most of these parts or reasonable substitutes (except for the Hammarlund caps and the vernier dials) are available from Antique Electronic Supply, Tempe, AZ. AES even carries a suitable power supply kit (K101A) and a chassis.
Both coils on each form should be wound in the same direction. If the connections to the circuit are made as shown there should be no trouble in obtaining the necessary oscillation. If the IF won't approach or go into oscillation or the HF or BFO circuits won't oscillate, reverse the plate winding / tickler leads on the suspect coil. L6 of the IF coil and L8 of the BFO coil are mounted towards the chassis.
All plug-in coils are wound on 1 1/2 inch diameter forms (Hammarlund SWF-4 or similar). Grid windings on coils B through E are spaced to occupy a length of 1 1/2 inches. The grid winding on coil A is close wound. Antenna/tickler coils all close-wound, spaced 1/8 inch from bottom of the grid winding.
The number of general coverage coils may be minimized by using the same coil in both the antenna and oscillator positions. It should be noted that the same oscillator coils, D and BB, are used for two frequency ranges. This is possible because the oscillator frequency is placed on the low-frequency side of the signal on the higher range.
RF and Oscillator Coil Winding
|Grid Winding (L1 and L3)||Antenna (L2) or Tickler (L4)|
|A||56 turns No. 22 enameled||10 turns No. 24 enameled|
|B||32 turns " "||8 turns " "|
|C||18 turns " "||7 turns " "|
|D||12 turns " "||7 turns " "|
|E||10 turns " "||8 turns " "|
|BB||20 turns " "||8 turns " "|
|Frequency Range||Coil at L1-L2||Coil at L3-L4|
|3500 to 4000KHz||B||BB|
|7000 to 7300KHz||C||BB|
|Frequency Range||Coil at L1-L2||Coil at L3-L4|
|1700 to 3200 KHz||A||B|
|3000 to 5700 KHz||B||C|
|5400 to 10,000 KHz||C||D|
|9500 to 14,500 KHz||E||D|
Alignment of this receiver is a lot easier if you have a well calibrated general coverage receiver (or frequency counter) and a crystal controlled oscillator in the frequency range of interest. Repeat the steps below for each band of interest.
1) Adjust the HF oscillator coil: With the 80 or 40 meter coils installed and the BFO off, first determine that the HF oscillator is working by listening for it on your general coverage receiver. Try to adjust coil BB so that the HF oscillator oscillates on about 4.5 - 5.0 MHz when the Bandset capacitor and Bandspread capacitors are near maximum capacity.
2) Determine/Adjust the IF frequency: Tightly couple your crystal controlled oscillator to the antenna connection and then tune the bandset oscillator listening for the crystal oscillator. While decreasing coupling, tune RF Input and Bandspread for maximum signal. Using your general coverage receiver or frequency counter determine the HF oscillator frequency. It should be about 1600 KHz above (for 80 meters) or below (for 40 meters) your crystal frequency. Once you've found it, take the difference between the HF oscillator frequency and the crystal frequency as the IF frequency. Adjust L5 until the IF frequency is between 1600KHz and 1700Khz.
3) Set the BFO frequency: Turn on the BFO and using your general coverage receiver or frequency counter set the BFO frequency to the IF frequency.
4) Calibrate the dials: To calibrate the receiver, first calculate the HF oscillator frequency for either 3.5 MHz or 7.0 MHz. (3.5 + IF frequency = HF oscillator frequency or 7.0 - IF frequency = HF oscillator frequency.) Set the bandspread dial to maximum capacity and then tune the Bandset dial until the HF oscillator is on the right frequency. Record the Bandset dial setting. Calculate the HF oscillator frequencies for calibration points up the band. Tune the HF oscillator using the Bandspread control and record the Bandspread dial settings for each calibration point. Once you've calibrated the Bandspread/Bandset dials, peak the RF Input circuit on noise or signals and record that RF Input dial setting.
Operation is a matter of plugging in the coils for the band of interest, setting the bandset capacitor, peaking the RF Input circuit on the proper input frequency and tuning with the Bandspread dial. The RF Input dial may be used as an RF gain control. If you included the BFO in your receiver, run the regen control at just below IF oscillation (indicated by a "plop" and a soft hiss) for maximum sensitivity and selectivity. I've found that setting the bandset dial usually requires a calibrated signal source such as a crystal calibrator or other crystal controlled oscillator. Set the Bandspread dial to the frequency of your calibrated signal source and tune in the signal using only the Bandset dial. Warm-up drift is a problem with this simple design. I found that mine drifts about 10KHz in 15 to 20 minutes before settling down.
This receiver was a fun project, it's a good match for my 6L6 transmitter and, since it is homebrew, I don't have to worry about spoiling a piece of history by fiddling with the circuit. Let's see now, what can that unused half of a 6C8G be used for....
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