"Mystery Tube" Superhet

Here's the schematic diagram of the 20m superheterodyne receiver that I recently built using three of my sub-miniature "mystery tubes." 

Component List

T1: 1.2 to 2.1uH slug-tuned with 50 Ohm coupling link
T2: 3.5 to 6uH slug-tuned; 16t primary, 4t secondary
T3: AF step-down transformer, 11H unloaded primary inductance, 22-1 turns ratio
CV1: 15 to 30pF air-variable
C1: 550pF
C2: 75pF
C3, C11: 100pF
C6, C9, C15, C16: 100pF @ 100V
C4, C13: 22nF @ 50V
C5, C14: 100nF @ 100V
C7, C10: 1000pF @ 50V
C8: 33pF
C12: 100nF
C17: 4.7uF @ 50V electrolytic
C18, C19: 39pF @ 50V
C20: 3.9nF @ 100V
C21: 47uF @ 100V
R1, R4, R8: 1Meg
R2, R6: 47k
R3, R7: 1.5k
R9: 560k
X1, X2: 11.0592MHz quartz crystal
L1, L3, L4: 10.7MHz slug-tuned IF "can" with internal capacitor removed, ~2.3uH
L2, L7: 3.5 to 6uH slug-tuned
L5: 2.2mH RFC
L6: ~10uH slug-tuned
HP: 600 Ohm headphones
V1, V2, V3: Raytheon subminiature pentodes; type QF721 (February 1953 date-code)

Circuit Description 

The first converter stage generates a VFO signal in resonator CV1/C1/T2 via link-coupled feedback from V1's anode. The VFO tunes from 2.9465 to 3.0165MHz in order to cover a receive frequency range of 14.0 to 14.07MHz. The 20M input signal from the antenna is injected via resonator T1/C2. Converters such as these can be plagued by interaction between the input and VFO resonators, however the problem may be alleviated to a large extent by operating the VFO at a frequency well-removed from the input signal resonator. In the above receiver, adjustments made to the 14MHz input tank circuit, or changes in input loading, pull the receive signal beat-note only very slightly. The low VFO operating frequency also aids the receive frequency stability.

The quartz crystal bandpass filter configuration dates from the 1930's; I believe it was first known as the "Telefunken Filter." DJ2KY appears to have further popularized this filter amongst European hams with a pair of articles titled, "Ein Amateur-Kleinsuper mit Quarzfilter." These appeared in the July 1956 and May 1957 editions of Funktechnik magazine. In the above circuit, L2 forms a parallel resonant circuit at the IF frequency in conjunction with the quartz crystal's parallel capacitance (holder capacitance). However, given this small capacitance (~5pF or less) would require a rather large resonating inductance, I've added an external capacitor, C8, in parallel. Using an 11.0592MHz "microprocessor" crystal taken from my junkbox I measured the bandpass filter center frequency at 11.0535MHz. The opposite sideband signal suppression approaches 30dB...a modest performance by modern standards, but it feels like luxury having exclusively used simple regenerative and direct-conversion receivers these past four years! 

I chose to use this filter primarily out of historical interest. No doubt the more common, one-crystal, half-wave bridge configuration would have worked as well; or better yet, a three or four pole ladder crystal bandpass filter.     
 
As explained in my April 22, 2012 blog post, I had originally hoped to build a 20m superheterodyne receiver using only two sub-miniature pentodes. However, the quartz crystal filter had too much interaction with the crystal-controlled oscillator frequency in the second converter stage. I found it necessary to include an IF amplifer/buffer stage between the first and second frequency converters.

The second converter stage is somewhat unusual inasmuch as the screen grid (G2) is used as the control grid of a quartz crystal-controlled Miller oscillator that serves as my BFO. I chose the (parallel resonant) Miller configuration given its suitability for use as a VXO. Using an adjustable inductor in series with the 11.0592MHz crystal I had no trouble pulling the BFO frequency down to my 11.0535MHz bandpass filter frequency plus or minus ~800Hz. 

I found that adjusting L7 to produce the maximum BFO signal amplitude at the V3 screen (G2) overdrove the second mixer and reduced its conversion efficiency. The situation was greatly improved by de-tuning L7 such that the BFO signal amplitude measured ~500mVpp on the V3 screen (using a high-impedance probe). This seems reasonable given that my early 1970's edition of the ARRL's  Amateur Radio Handbook recommends that the sum of the signal and oscillator voltages impressed upon a pentode mixer grid should not exceed the grid bias voltage.

The receiver audio output signal level well suits me, although I admit that I generally prefer receivers with a considerably lower than normal audio output level. I think most folks would add at least one audio amplifier stage to this design. 

Speaking of which, I did go on to build a two-tube version of this receiver. Only, I used a sub-miniature 1V6 pentode/triode in place of the 2nd converter stage shown above. The improved BFO isolation provided by the 1V6 allowed for the elimination of the IF amplifier/buffer stage. However, having become accustomed to listening to signals with the above receiver for several weeks, the minimized QF721/1V6 design sounded a bit too quiet for my liking! I ended up adding one-stage of audio amplification using a black, top-hat style, germanium transistor; type 4JD2A6, dating from 1958. The collector supply potential was provided by my 1.2Vdc filament supply voltage. 

Here are two close-up photos of the QF721/1V6 receiver. The first converter and crystal filter are shown in the first photo (notice that I'm not canceling the quartz crystal parallel capacitance in this photo). The 1V6-based 2nd converter and transistor audio frequency amplifier appear in the second photo.

                             

Both receivers appear to work equally well aside from the fact that the 1V6 is considerably more microphonic than the QF721.  
  

Birdsong

 

The second episode of the BBC's adaptation of Sebastian Faulks' Birdsong, was no less violent or sad than the first. But it was, if anything, more powerful.

I've been a library-rat for years; reading obscure philosophical texts: thinking, writing, wondering. As a result the world is slightly less mysterious to me now than when I was fifteen years-old. But browsing through my old journals, I was struck sometime ago to see a convergence emerging in my thinking.

The thesis of the film, Birdsong, was uttered by Jack, the miner.

"All that matters is to love and be loved."

I once thought that I might be content if I could only better understand the world. I've since discovered that understanding the machinery of this world alone, is at best, secondary. I don't just want to observe and theorize about how the building blocks of the world are setup, I want to have a vested interest in how they're stacked. I now believe that it's love that converts the curious spectator of life into a full-fledged participant.

"All, everything that I understand, I understand only because I love." Leo Tolstoy

Birdsong is available gratis for viewing on the PBS website for a limited time. It's among the best films that I've seen.

 http://www.pbs.org/wgbh/masterpiece/watch/index.html

Two-Penny "Mystery" Tubes

I took a chance on a little box of QF721 "mystery" sub-miniature tubes some weeks ago; paying the 1953 equivalent of two and a half cents per tube (including postage!). 

As Raytheon used the "QF" prefix to indicate an experimental or special-order tube, the specifications for this device may never have been released to the public. However, the listing photo suggested these were pentodes in a T2x3 glass bulb. I also assumed that it used a directly-heated cathode (filament) that was so common in Raytheon's other lines of sub-miniature tubes.   

Upon the arrival of my "mystery" tubes I noticed they were stamped with one of two date-codes: "248" (November 1952) or "308" (February 1953). They arrived in the original Raytheon cardboard "egg carton," on the side of which was printed: "Job 102," "R.T. 6822," "Part# QF721," "Type 43," "Coded 308." I also noticed what appeared to be an inspector's stamp in which a letter "H" was adjoined to the letter "R". What was this "HR"?  

Playing the technological "history detective," I uncovered a single reference to the QF721. In this patent (filed November, 1952) the inventor chose a QF721 pentode for the second audio amplifier/modulator stage of a miniaturized UHF walkie-talkie. The patent drawing shows a crystal-controlled AM transmitter and superegenerative receiver, all built from Raytheon sub-miniature vacuum tubes.

Ah...but look who the patent is assigned to: Hoffman Electronics Corp., Los Angeles, California; the sister of Hoffman Radio Corp., also of Los Angeles. That would explain the "HR" inspector's stamp. What's more, the fellow that sold me these tubes lives on the outskirts of Los Angeles.  

Hoffman Radio began making tube-type consumer radio equipment in 1941. Following the war they went on to produce televisions, tape recorders and even Geiger counters. You may recall they were an early adopter of photovoltaic cell technology. For example, they produced the "solar cells" used in our Vanguard Satellite program. In fact, they produced a solar powered transistor radio as early as 1957; the Hoffman P411.

I suspect that Raytheon made these tubes as a special order for Hoffman Radio. It may be that Hoffman used the QF721's in one or more of their military contracts, and the engineer working on the UHF walkie-talkie design pulled them from the stockroom in order to get on with his project.

Although I haven't located a data-sheet for my QF721's, at least I now have an example of it used by a (presumably) competent designer. For example, the patent drawing shows the QF721 filament in parallel with a Raytheon CK527AX (for which the specifications are available). Thus, I know the QF721 was designed for use with a 1.25v filament supply voltage. At this potential the QF721 filament current measures ~100mA. Comparing these numbers with other T2x3 bulb sub-miniature types in Raytheon's catalog (along with its established usage as an output AF amplifier stage) leads me to believe the maximum cathode current rating would be in the range of 3 to 4mA at a B+ potential of 45 to 60Vdc.

My first project using the QF721 was a 0V0 regenerative receiver for 40m CW using tickler feedback. A potentiometer in the screen supply provided the regeneration level adjustment. I filled a sheet of paper with DX calls in two pleasant evenings listening with this receiver: ZS1JX, 4X4FC, PY2VJ, YO3AAJ, US3IZ, CP4BT and many others. 

I next built a 20m superheterodyne receiver. It begins with a frequency converter made from a single QF721 having two L-C parallel resonators in series with the grid (at 14MHz and 3MHz). Positive feedback via a coupling link on the 3MHz resonator drives it into sustained oscillation in order to provide a tunable VFO. Due to the wide frequency separation, the interaction between the 14MHz input tank circuit and the VFO frequency is insignificant.

An 11MHz L-C resonator in series with the anode couples the IF output to a one-stage crystal filter; a configuration known as the "Telefunken Filter." It was famously used in the Köln E-52 receiver (you may listen to the motorized pre-set frequency gears whir here!). The filter appears to have been popularized in amateur radio circles in the early 60's (at least in Europe) by DJ2KY; thus, it's occasionally referred to as a "KY Filter." My filter uses a single 11.0592MHz "uP" type crystal taken from my junkbox. The opposite sideband rejection is ~25 to 30dB.

I originally connected a crystal-controlled product detector/converter made from a second QF721 to the output of my crystal filter. Again, this is a self-excited mixer. However in this circuit a second 11.0592MHz crystal is configured as a Miller oscillator using the tube screen (G2) as the oscillator input grid. An inductor in series with the crystal allows the frequency to be "rubbered" down to within 800Hz of the crystal filter bandpass frequency. The input IF signal arrives at the the grid (G1).  

Unfortunately, an annoying interaction between the BFO oscillator and the crystal filter made it necessary to include an IF amplifier/buffer stage between the crystal filter and the product detector. Although I'd had in mind a two-tube superhet, these three QF721's sounded so nice together that I decided to relax the minimalism constraint, at least until I had built a matching transmitter.

My transmitter begins with a Raytheon 1V6 (pentode-triode) that uses the same T2x3 glass bulb as the QF721. The triode section is configured as a Pierce, 11.2MHz crystal-controlled oscillator. The pentode section works simultaneously as a 2.8MHz VFO and mixer. This heterodyne exciter tunes from 14.0 to 14.7MHz. The exciter drives a QF721 PA, producing a maximum output of 30mW. Keying is accomplished with a PNP transistor in series with the PA anode and screen 50Vdc supply line. The exciter crystal-controlled oscillator is switched off whilst I'm receiving. It would be better to instead shift the transmit VFO frequency upwards while receiving in order to avoid several seconds of frequency shift until the 1V6 reaches its operating thermal equilibrium.    

Listening to my receiver while working on the transmitter, the RMS Titanic commemorative station, GR100MGY, came to my attention. I learned this station would remain on the air until April 15th; exactly one-hundred years to the day since the great disaster. My desire to work GR100MGY became an incentive to press on with the transmitter.

I judged the setup to be air-worthy on 12 April. In the afternoon I came across GR100MGY working through a pileup. I joined in, and spent at least the next hour calling. At last, there was a lull in the activity and I must have finally turned up alone. The GR100MGY operator replied, "EA1TJ 599." He picked up my correct call on the next and it was over in an instant. Got 'em with 30mW!

Sliding back down the band I heard G3HGE just signing with a state-side station. I quickly sent my call. Silence. I called again, this time with the /QRPP suffix. He came right back with a 429 report. He sent, "UR CHANCING IT WID QRP CNDX NOT SO GUD." He also commented (correctly) on the start-up drift in my transmitter. Alas, another station started in with a CQ and he heard nothing more from me after that.

In my haste I hadn't re-peaked my transmitter before calling him. Without changing any of the settings I measured my output power at 16mW.

I found a nice note from Tom in my inbox the next morning. He wrote

"I was an RAF Wireless operator with Bomber Command and we were trained to listen for weak and watery signals so finding you was about normal for me, I spend a fair bit of time grubbing around in the noise floor of my rcvr looking for signals like yours."

On April 14th I worked: OK6DJ with 29mW (599/559), F8DGY with 28mW (599/559) and SP3GXH with 24mW (599/459). When F8DGY heard about my output power he asked for the details of my setup. He copied everything with no repeats required! In a follow-up email, F8DGY wrote

Hello Mike,

Tnx for this nice QSO with ur QRPp station, Vy interesting but it must take patience! So congrats agn cu next time. Here 800 w 3ele spiderbeam. IC7400. Ur signal was good not much difficulty to hear you!

73 Chris

 The complete station is shown in the upper photo. The lower photo is a close-up of the transmitter. I hope to post the schematic diagrams in the near future.
   

OZ3FD's Crystal-Clear Remembrance

Hi Henry,

I heard you the other evening on 80m CW with a galena and
cat's whisker ("Crystal") receiver. I used a BFO of course, but your signal did all the work. It's pretty neat to think that your signal travelled from Denmark to Vermont and still had enough energy remaining to wiggle the diaphrams in my headphones! You were a perfect copy here...every bit as loud as some domestic stations.

Ciao,
Mike, AA1TJ

Hi Michael,

Some actor once said, "Make my day" - I know at the time it was said it had another meaning. But never the less - Thank you very much for the SWL report recieved on a reciever that could had been made 90 yrs ago. You did not make my day, you made much more.

My thoughts was immediately back to the time where I sat in the shack listening to my father, OZ6PK, making QSO`s with his two tube AM transmitter. Me siting on the floor making the kind of reciever you heard me on.

Writing these lines brings tears in my eyes - not sad tears, but tears of good memories.

Today, as you could hear, I have a good set-up using the K3 with an old Heathkit PA aproxx 500 watt. The antenna is a vertical with 1 km. radials.

Best of 73,

Henry, OZ3FD

DL3PB's All-Tunnel Diode Parametric Triumph

My friend, DL3PB, has done something amazing with a pair of Russian-made tunnel diodes! Peter's imagination and patience are truly an inspiration. Congratulations OM! In his own words...

"I'd like to share with you a long-cherished dream, that recently came true, fourty years after I came to read about hams using tunnel diodes to make QSOs when I was aged twelve or so:

Finally I managed a first skywave QSO with my PARASAKI-transceiver, an 'all diode' rig: Christophe/F8DZY replied to my very first call on 20m band in REF-contest last weekend. I was running 2mW into a temporary vertical dipole on my balcony.
Distance between us is  918km - obviously OM Christophe has excellent ears.

Those interested in the cruel details of my circuit, please find attached a schematic and a photo of the pretty ugly setup. The circuit is designed straight-forward with exception of the parametric VXO, derived from Mike/AA1TJ's famous Paraceiver design. (see  http://fhs-consulting.com/aa1tj/paraceiver.html )

The low impedance of the high peak-current tunnel diodes make it very difficult to build a crystal controlled oscillator rather than an LC oscillator, synchronized by the crystal more or less, at least on the higher SW bands. The Parametric VXO provides a crystal-stable, chirp-free signal on expense of an output power of two milliwatts only instead of ten, but with an amazing spectral purity, no need for a low pass filter or such.

Of course it sounds pretty cool making a QSO with a 'bunch of diodes' and a parametrically excited crystal, but believe me or not, I'd preferred to bring that full ten milliwatt into the air - on the other hand that approach allowed to tune the rig a bit ( ~ 5kHz/per xtal ), which turned out to be much more valuable than a few milliwatts more while being 'rock-bound'.

The receiver in its 'gain-less' version works fine for strong signals - while listening to QRP(p) stations, the moderate gain of the audio amplifier helps a lot. A comfortable frequency shift between receive and transmit is realized by the 5µH inductor at the LO-port of the mixer, with little effect on sensitivity."

QRP...easy as 2N123

I recently put together an 80m CW transmitter using a pair of 2N123 germanium transistors, both of them bearing July, 1959 date-codes. The first stage is a Colpitts-type VXO. A 3.58MHz ceramic resonator tunes from 3501 to 3560kHz using a series-connected variable capacitor. A second 2N123 transistor is configured as a Class-C, common-base power amplifier. The RF output is ~50mW, although it falls to 45mW at the low end of the band.

The basic transmitter schematic is shown below; the transmit-receive keying relay does not appear in this drawing.

Part values

C1: 27pF silver mica (s.m.)
C2: 330pF s.m.
C3, C4: 150pF s.m.
C5, C7: 48nF
C6, C8: 430pF s.m.
CV1: 15 to 230pF air-variable
R1: 100k
R2: 1.5k
T1, T2: 3.5 to 6uH slug-tuned; 16-turn primary, 3-turn secondary
X1: 3.58MHz ceramic resonator
Q1, Q2: 2N123 germanium PNP transistors

Adjusting T2 for a peak RF output, the highest harmonic energy is -35dBc (the 2nd harmonic). However, by tuning slightly off-peak, the peak harmonics drop below -45dBc. Thus, no further filtering is required. 

I've paired this transmitter with my Cat's Meow galena and cat's whisker receiver. While receiving, the RF output signal from the continuously running transmitter is relay-switched into a 50 Ohm cermet potentiometer working as a dummy load. A sample of this signal is taken from pot's wiper to provide the receiver beat-frequency oscillator (BFO). A single, miniature DPDT relay is keyed in order to provide the required switching. A reed-relay was initially used to switch a 10pF capacitor across C4, for the purpose of providing a receive-increment-tuning (RIT) frequency-shift. However, I discovered that with careful tuning the variation in transmitter loading is alone sufficient to provide a RIT frequency shift of 750 to 1200Hz (from the bottom to the top of the band).

At G3XBM's prompting I measured the minimum detected signal (MDS) level required at the receiver. Using a factory-made germanium diode in place of the galena and cat's whisker, I could just copy a -118dBW signal. This is an absolute power of 1.5pW, or 8.75uVrms across a 50 Ohm resistance. Holding the input step-attentuator at this setting, I re-installed the galena and adjusted the cat's whisker until I could just hear the signal in my headphones. For the same signal copy that I had with the germanium diode it was necessary to switch a 3dB pad out of the circuit. Thus, the best sensitivity that I could wring out of the galena was -115dBW. This is an absolute power of 3pW or 12.4uVrms across a 50 Ohm load. 

It's interesting to review a similar measurement that I made for my 80m Reggie transceiver (the receiver was also gain-less and I used the same pair of headphones) back in 2009. I then measured an MDS of -120dBW; which is an absolute power of 1pW, or 7uVrms across 50 Ohms. Allowing for the subjectivity involved in making these measurements, the results of both tests when using commercial diodes are very close indeed.
 
Here's a photo of the "finished" 80m station. The transmitter appears on the rear proto-board, while the "crystal receiver" is up-front. Again, the small green and black reed relay appearing on the left-hand side of the transmitter has since been removed ("Simplify, simplify..." Thoreau :-).  

In two evenings I've made eleven QSOs with this setup.

KA1ANW   579/599 (him/me)   Sharon, MA          225km
WA1AR     579/599             Wrentham, MA          255km
N2UU        579/449               Voorhees, NJ          504km
W1FL        559/339                    Stowe, VT            46km
W3NW      569/539           Lock Haven, PA          504km
KT8R         559/229                        Alto, MI        1021km
W3ZT       579/559            Skaneateles, NY          319km

W1PID      579/589           Sanbornton, NH          112km

WA3MIX   579/579           Williamsport, PA          469km
K1ESE       589/559             Waterford, ME          163km
N8ER        569/559                Parsons, WV          797km

My QSO with W1PID lasted nearly an hour. Jim began with an ICOM 7000 at 2watts, but he quickly switched to his Elecraft K2 in order to play the "QRP Limbo" game. Jim reduced his power in steps: 500mW, 200mW and finally 100mW. His 100mW signal came through my cat's whisker at a perfectly readable 529! I received a long transmission very nicely at 200mW, aside from a bit of QRM towards the end.

I particularly enjoyed my contact with W3ZT on account of his unusual transmitter. Joel was running a 1950's-vintage URT11, which was built for the CIA and commonly used to provide CW communications for our foreign embassies. We had a solid contact lasting 15 minutes.

I admit that I get a kick out of the responses when I explain my setup. For example:

N2UU sent, "TELL ME YOU ARE JOKING."

WA3MIX said, "MAN TTS ABT AS OLD SCHOOL AS U CAN GET = SOUNDS GREAT WID GUD KEYING ES VY STABLE."

K1ESE replied, "DONT THINK I EVER WRKED A GUY ON AN XTAL SET FER RX HI = PRETTY AMAZING"

I've heard a number of DX stations as well in the last two nights. ON6AB, OZ3FD and DL2WR had particularly strong signals here.

A Swedish Sputnik and More

Johnny, SM7UCZ has made a top-notch job of his new Sputnik transmitter!

The schematic diagram of his circuit layout, plus more photos may be found here.

Among the many interesting tube and transistor QRP projects to be seen on Johnny's web site is the story of a hand-transcribed copy of Frank C. Jone's, 1938 edition of, The Radio Handbook (please click here). 

OM, Sven Bernholm, SM3RN (1888 - 1963), laboriously translated this work into Swedish whilst standing radio watch in the years 1944 to 1945. Sven beautifully hand-copied over 400 pages of text, schematics, drawings, graphs and tables. I'm surprised that he didn't illuminate the pages in the fashion of medieval monks! His little side-notes are especially interesting; for example

5/5/1945: "Denmark and the Netherlands, free!"

7/5/1945: "Germany surrendered; Norway free!"

12/4/1945 "Roosvelt dead - Allies 6 miles from Berlin!"

Johnny recreated a Frank Jones-type "push-pull" transmitter similar to the one appearing in Sven's transcription (here).

There are several Parasets, a Hartley transmitter, a tube superregenerative receiver for the VHF BCB and a transmitter built to operate on 1.5Vdc (and less). The fine art of homebrew QRP is alive and well in this little corner of Sweden!