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dayleedwards
Mar 17, 2023
In Tube Radios
I have not seen this before, maybe unique, thort to pass it on. Developed this to use in my Deltahet. Any triode will work, The .01 anode cap is to isolate the HT from the resonator, they hate voltage gradients. One caveat, the resonator will oscillate about 5kC above the nominal, use a 450 kC to get 455, a 455 will resonate at 460. Small freq changes , as in very small can be coaxed by fiddling either resistor values or capacitor values. I developed this to use the triode section of the ECH81 in the product detector of the Deltahet.
Simple and stable triode BFO content media
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dayleedwards
Feb 11, 2023
In Solid State Radios
After successfully acheiving very satisfying results with the Deltahet triple conversion RX using vacuum tubes, I thought to implement a similar design using silicon. I figured many functions can be integrated into modern "do it all" RF chips. The tuneable IF and detector functions can be wrapped up by using an AM radio chip, some even incorporate the Audio stages. The actual loop itself could use an FM radio chip, these use a front end mixer with integral oscillator VHF capable, the usual 10.7 meg IF IF stages "should" function reasonably well at 37/38 megs, the oscillator easily handling the 40 to 70 meg oscillator frequency. This leaves the signal front end. Again, another dedicated FM radio chip can utilised here, using the Rf amplifier and mixer section. With just 3 ICs, it may just be possible. Of course, we need a SSB detector, this too can be done with a second AM radio chip, the oscillator becoming the BFO, the mixer being the product detector..... so thats just 4 ICs. Finally we need the 1 meg harmonic comb generator, one transistor will do this. That leaves the tuned circuits, the L/C headache. Reading JAGs post about filters and SMD coils, it seems possible to use off the shelf inductors and simple RF chokes for the required filters without complicated winding instructions.... seems noone likes coil winding and are actually frightened of them...... I have no experience with store bought colls and chokes, always being a wind my own type delinquent. So, we need a 1 meg wide 40 meg bandpass filter, a 37.5 bandpass filter around 200kc wide , the tuneable IF coils tuning 2 to 3 megs, and anything to give us a 455kc 3rd IF for selectivity, this can be addressed with cheap ceramic filters, or possibly go down to a 100 kc last IF which will give very good selectivity with just L/C stages.... even a tad of regeneration. The big issue is tuning the front end VFO, the front end BPF, and the tuneable IF. Varicap diodes are no of use, the only alternative are tuning capacitors, the only tuning capacitors reasonably available are those horrid polyvaricons. Although better than Varicap diodes. these are temperature unstable and will require good robust reduction drives to work efficiently. This creates the biggest logistics problem with designing this radio. The next issue is frequency readout, these days one microchip will do the deed. It will require the first two MSD of the 40 to 70 mhz VFO to be read, then subtract 40, the first IF, display that on the left two digits, then read the tuneable IF VFO, subtract the third IF and display that on the remaining right hand digits, this is much simpler than creating a mechanical display. There will still be a requirement for some indication of preselect frequency on the front panel. When thinking about this, I went off on a small tangent. If we take the first 40 to 70 Mhz VFO and divide this by 10, that signal of 4 to 7 mHz can be inputted to a simple phase detector, the CMOS 74HCT4046 springs to mind, these are cheap, and can easily handle these low frequencies.. The other port sees a 100Kc xtal oscillator . As the 40mHz VFO is tuned, the phase detector will "lock" on each 1 meg boundary using the harmonics of the 100kc oscillator, This gives us 30 fixed oscillator signals 1 meg apart, offest by the first IF frequency of 40 megs. The PLL section sees a divided by 10 version of this. Doing it this way, we get a lock indicator, we get a 4 to 7 meg signal easily processed in a very basic digital frequency readout, the 100kC master oscillator doubles as the BFO in the 100kC final IF. A frequency readout eliminates any tuning scales and mechanical effort. Because we now have an oscillator frequency feedback loop, we can now use varicap diodes for the 40 to 70 mHz section without too many problems. This negates the whole of the Wadley loop section, as no drift cancellation is required, now being handled by the rudimentary PLL, so simplifying the design further. Polyvaricons are fine for the relatively course tuning of the front end preselector, this now only leaves the tuneable IF where the ultimate stability of the receiver depends. Here we need a GOOD tuning capacitor and reduction drive for ease of tuning, already good, because each band is just 1 meg wide and the tuning rate is the same from 1 to 30 megs. With the final VFO tunes 2.1 to 3.1 megs, so drift should be tolerable using precision voltage regulators and quality multiturn pots.... as far as I am aware 3 gang polyvaricons are unavailable. The added advantage of varicaps here is three or even four gang TUNED second IF stages are easily possible, improving image rejection given the low 100kC third IF. By further dividing the 100kC reference oscillator, say to 100HZ, its easily possible to use another PLL to serve as the main tuning, although adding another two chips...... Now, IF we are using varicaps, why not eliminate the switches on the front end BPFs and use diode switching here, there will be room in the MPU to easily do this, all it needs to do is compare the left hand frequency digits, we already have these, to a lookup table to activate the correct BPF as JAG explains in his design. If we have more spare room in the MPU, this could be extended to create a course tuning voltage for auto tuning of the front end preselector as well, eliminating the polyvaricon and using varicaps once again. Other than cross modulation issues, this is a reasonably non critical area for the varicap diode. I think I count around 6 cheap and common ICs, a few transistors and diodes..... So, thats where I am with this ...... to be continued
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dayleedwards
Feb 10, 2023
In Tube Radios
This one followed me home. Its a COMPLETE and unassembled hardware kit for the Electronics Research Laboratories , 3 tube radio, Chigago Just how this one ended up here in New Zealand is a mystery The box is a bit tattered, but everything is there, valves apparently were not supplied. Here is a video clip of the assembled kit here is what I have I think with this one it may be far more collectable left as a kitset, rather than assembled. It really should be in a museum......
1925 ERLA super reflex kit content media
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dayleedwards
Jan 22, 2023
In Tube Radios
In the 1940s, Frank Jones was a prolific writer on anything radio related and published many books and designs. One was the Supergainer, and later, the Ultragainer Years ago I had a crack at making one, and was suprised at its lively performance, especially considering its low parts count. Using modern high slope TV valves in my case made for a very compact and nice performing radio. The original did have some quirks, I think caused by the choice of components back in the 40s. The Ultragainer used not only the regenerative IF, but also a regenerative mixer stage, I would say a nightmare to tune and to remain in tune. The regenerative IF was also dodgy by design, the feedback obtained by "lifting" the cathode from RF ground and relying on Anode to Grid, and Grid to Cathode spurious valve capacitances to make an artificial tap on the IF coil. This is VERY poor design practice, usually capacitances are added to swamp these random and variable unknowns. I do wonder how many of the original designs that were built actually worked correctly in the day. Just changing the valve, or using a valve from a different manufacturer would likely have failed the project. A quick video on my version supergainer in action, and compared to the JRC, same aerial..... the difference is..... um.... quite suprising I thought. Using an ECH84 gated synch separator valve for the HF Oscillator and mixer, and the ECF80 , the pentode section being the regenerated IF/detector, and the triode section being the audio output stage. It works very well on SW AM, but leaves much to be desired on CW/SSB. After some thought and experimenting, the answer seems to be CIO injection via the screen grid of the detector. The screen grid is used as the regeneration control, the DC voltage here is varied from around 20 to 30 volts to increase/decrease the pentode gain. By injecting an external oscillator signal at the 1700kc IF frequency,and superimposing it on the DC voltage, the detector doubles as a regenerative product detector. By doing this, the detector can remain in its most sensitive spot, giving maximum selectivity, not being required to go into oscillation. Also, the detector locks on to the stronger BFO signal, and if this is crystal controlled, will keep the receiver exactly on frequency. Why this is so is a little unclear, there must be some slight interaction between the detector and first oscillator, even with the BFO offset frequency required for SSB reception. Although I hope to use the audio output triode to double as a crystal controlled BFO to save adding another valve, I dont fancy my chances of this working correctly, being in the same envelope as the high gain IF. Changing from a ECF80, or 6BL8 to a ECF82/6U8 may help, I think that one has an internal shield? Anyways, thats my next move.
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dayleedwards
Dec 31, 2022
In Other Electronic Projects
https://www.youtube.com/watch?v=63j_0R8A-gY I have subscribed to this gentlemans channel, he has an interesting way to build his projects.;. It all in German but he generally will include the circuit in what he has built. He puts much effort into his videos and projects.... I think many will enjoy....
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dayleedwards
Dec 31, 2022
In Tube Radios
This wee gem was bought off a local auction site. Its a circa 1925 Silvertone knock off made by King Hinners of the US. Its a 5 valve nuetrodyne, requiring 90, 45 1,5 and 6 volts in batteries. It is in beautiful condition. Even the horn speaker looks nice. Im unsure if it goes, the original valves test good, the heaters at least. I need to build a nice wooden case and may also make a small mains power supply disguised as a battery box. Because its in such good condition, I did wonder if it was a fake, but it does have the old bright emitter valves and the Thordasen interstage transformers. Not sure with the horn speaker, the trumpet appears to be aluminium...... did they have aluminium in the early 1900s? Anyway, enjoy the pics.
King Hinners model A content media
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dayleedwards
Dec 31, 2022
In The Radio Shack
Recently I picked up this wee gadget, figured it was useful for parts. Then I discovered the power supply had died, and chances are that it would actually fire up once it was repaired. Its a 1980s spectrum analyser, 100 kc all the way to 22 gigs!! It would have cost an absolute fortune in the day. Before dismantling it, decided to have a quick fault find. First problem was the voltage regulator chips, the 5 volt at 8 amp section was faulty, as is the -18 volt. The supply is current limited via 2milliohm shunts, and overvoltage protected via SCR crowbars. These are vicious, in that when the voltage rises over a set amount, the SCRs literally will short the supply to ground, blowing all the fuses. Anyways, the supply is faulty. The 723 regulators, 4 of them are the older round can type packages with 10 leads, so nothing else will fit. I did consider building a new supply using the existing huge and heavy transformer, but its a lot of work if the thing is truly knackered. As luck would have it, Alibaba has these old chip in stock, they are on the way to me as I speak!! Thought Id post this, as I am quite excited to see if it is repairable. If anyone has any data on this thing, Its proudly made in the US, and seems to use a lot of Hewlet Packard microwave bits, a Motorola CRT display module, these were a generic thing fitted to many devices of the era. Its a real hodgepodge of different modules. Its branded as an Eaton 757, thats whats written on the service manual I found on the internet, the brand on the SA is ETN advanced electronics. Here are some pics in case it activates anyones memory. I have no use for it, but its a waste to dump it without trying.
29.5 kilos of happiness content media
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dayleedwards
Dec 22, 2022
In Tube Radios
After 4 iterations of my Wadley loop receiver, all my hard work was a disaster. So, ripped it all apart and learned from my mistakes. It now has more in common with the original Pogson Deltahet than my interpretation of it, hence the new topic. Here is a link to the very first signals received, here on 9 megs, some weird god station, always good for strong signals during a bad time of day, but no miracles were performed despite the theological BS. It was still a battle to get this far, and birdies remain a problem, especially at the 1 megacycle boundaries. This is not suprising as the harmonic comb generator is as yet unsheilded. Another issue was getting valve mixers to have enough conversion gain at these frequencies, I had long forgotten just how hard it can be. Ideally I need an extra amplifying stage in the 37.5 meg IF, I found that using a hi slope pentode with grid leak injection worked fine and saved another current hungry valve, the mains transformer is working close to its limits. ... we have 15 valves!! I tried cathode injection on the Wadley loop input, here the comb feeds into the cathode, the VFO into the grid, this too works very well. Another 300mA saving in heater current was using a ECF80 , the triode portion as the VHF oscillator, the pentode section as the third mixer. The first mixer remains problematic, this one converts the RF from the preselector to the 40 meg first IF. Currently this is using a 12AT7 twin triode, but efficiency seems low. The RF amp is a 6BA6, as are the 2 455kc IFs. I stuck with the ECH81 for the tuneable IF, as I am very familiar with these. The detector is an anode bend for AM, and will see the BFO injected into its cathode for SSB/CW. There are still problems with the 40 meg bandpass filter, and am struggling to find a good coupling to prevent double humping, and signal loss. There is no preselection yet, other than a random tuned circuit temporarily hooked up to the first mixer. AGC seems to work fine, this uses a voltage doubler using germanium diodes, giving a negative 10 volts on s9 signsls. The voltages to the BFO, VHF oscillator and associated mixers are regulated by an OA2 neon., giving a stabilised 110 volts. The s meter uses a bridge circuit, one side is the cathode of the third IF valve, the other side derived from a voltage doubler from the heater supply. This needs improving, it drifts all over the place as the set heats up. Another two quick videos graphically show the workings of the Wadley loop. The next few days I will tidy up what I have so far and try to pinpoint the weaknesses. At least with sound, it helps to track these issues.
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dayleedwards
Aug 05, 2022
In Tube Radios
Today having a bit of time have begun work on my WLRX. Originally I intended to design a minimal solid state design using modern ICs for everything, but decided to go with vacuum tubes, I have hundreds laying around, all looking at me sadly, wanting my attention. Going with a rough block diagram of what I need, have bent up an alluminium chassis hopefully big enough to hold enough valves, and around 25 coil cans.. Sorting my valve stock, I can either use miniature 13/15 7 pin types, or the 9 pin european type multis. To decide this, Ill need to have some idea of a schematic and finalise signal flow. Using multi element valves, eg ECF80 triode/pentodes will reduce valve count and real estate needs, but will complicate wiring. With multiple conversion stages it will need the signal doubling back here and there , and without care, it will interfere with itself. For instance it would be folly to use a spare triode in an IF amplifier valve as the BFO oscillator...... The 7 pin valves are probably the way Ill go, but I have not got any 7 pin valve bases as yet. The next issue is heater current, either type take 300mills each at 6.3 volts X 15 = 30 watts or 4.5 amps of heater current, thats one big heavy transformer, and we are gunna need 50 odd mills of HT current too.. Series heater stringing will need 100 volts at 300 mills, still 30 watts but may be possible with a small, light and modern SMPS supply. With the radio covering 100kc to 30 megs, its highly likely this will cause birdies somewhere in the range.... these supplies are filthy with EMI/RFI. Im wanting to replicate a Racal RA17 without the size and weight. If there is any interest, I will document my progress. Im thinking: preselector/ RF amp 6BA6 1st mixer 6BE6... if these things even work at 70 mHz??? harmonic generator 6BA6 VFO 6C4 (mHz tune) 37.5 to 67.5 mHz 2nd mixer 6BE6........ if these things will even work at 70 mHz??? 37.5 meg IF amp 6BA6 40 meg loop amp 6BA6 3rd mixer ECC81 2nd IF 6BA6 4th mixer/osc ECH81 (kHz tune) 3rd IF 6BA6 3rd IF 6BA6 BFO/product detector ECH81 AM/agc detector/1st audio 6AV6.... may omit thisone and just use a semiconductor diode Audio output 6AQ5 Magic Eye EM80..... for bling appeal only..... The front panel is a complicated thing requiring 3 dial scales, one for preselect frequency, another for mHz selection, and the third for kHz tuning. Hoping to use simple dial string arrangement for these. Then add Volume, Mode, some kind of lock indicator and wide/narrow filter selection, AGC fast/slow, and RF gain. If any room left, a front speaker too.
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dayleedwards
Jul 08, 2022
In Solid State Radios
6 Transistor regenerative SW RX with modified feedback. This radio from my dusty archives remains the best performing transistorised regen I have built to date. It is only suitable for SW AM listening, for this it is very stable and tunes easily. The RF signal arrives at the input buffer via the simple attenuator control. This reduces overload on strong signals and acts as a volume control. The 40673 detector transistor is a historic relic from the dinosaur era, I see these are still available at a ridiculous price, but any DG Mosfet will suffice. The signal from the input buffer , once resonated in the tuned circuit is introduced into gate 1 of the mosfet. The gain of the mosfet is varied by adjusting the voltage on gate 2, a similar situation to adjusting screen voltage on a pentode valve. This reduces any frequency shift with gain. The feedback is from source to the tapped tuning coil. The diode is connected directly to the DC bias on the drain, and is lightly bypassed to ground via a small capacitor, and forward biased by the 470k to ground.. It is not often realised, but this capacitor connects the Drain back to the tuned circuit via the ground path, and is part of the regenerative feedback circuit. As regeneration increases, the Drain current increases, as a result the drain voltage drops slightly, decreasing the minute current through the diode. Ideally, this diode is sitting on its "knee" of its turn on conduction angle. As the current reduces , its dynamic impedance increases, reducing this feedback path. The 1N4148 is a switching diode with a very sharp turn on. Also, as the detected audio waveform modulates the dynamic resistance, negative feedback increases on modulation peaks, giving a cycle by cycle correction, the small bypass capacitor holding the diode cathode at an average level, rather like the action of an automatic diode noise limiter, that clips any spike above the average audio level.... this reduces gain above any average RF level. The amplified RF at the drain is also rectified by this diode, its operating point continually kept at the knee by the interaction of the Mosfet and the charge on the bypass capacitor, and also the first audio coupling capacitor, the latter is effectively DC grounded by the forward biased transistor junction. The net result is a secondary negative feedback loop tending to stabilise the operating point. Its not perfect, but seems to work well for AM, SSB will drive the detector away from oscillation on carrier peaks, preventing good demodulation. The rest of the radio is just audio stuff, high gain required as the recovered audio from the diode rectifier is of necessity at a very low level. I fiddled with the 470k diode load for a very long time to get the best results. A 78L05 regulator supplies the RF and front end audio circuits, current drain is a few milliwatts. With the coil as shown , and a paralleled polyvaricon AM type tuning type, coverage is just over 4:1, or in this example, 2.5 to 12 megs. A large tuning reduction is required.
6 transistor AM SW regenerative RX with modified feedback. content media
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dayleedwards
Jul 07, 2022
In Tube Radios
For Triode mainly..... he is to blame for this .... After researching that 6BN6 you mentioned, and the homodyne you mentioned, here is a circuit that im very sure will work first time. Because its a new topic, I have made it a, ummmm, new topic The RF input to detector and mixer is presented to the control grids of both. The 6BN6 is configured as a limiting quadrature detector as per the data sheet. The ECH81 is also the data sheet The VFO is also presented to the control grids of both the ECH81 mixer and 6BN6 detector. The error signal from the 6BN6 anode is used to control a varicap diode in the common VFO. The varicap needs its anode lifted from ground to cancel the standing DC voltage on the 6BN6 anode, this hovers from around 1/2 supply to 3/4 supply, this accomplished by the adjustable screen dropper tap, this puts the varicap in its linear capacitance slope, needs to be say +2volts to 30 or so. As the tuned frequency deviates from the Fin, this error voltage will force/drive the VFO back to center. The 6BN6 limiting action should improve locking performance on modulation peaks. Both oscillator and RF circuits use similar tuned circuits , cover the same frequencies, so perfect tracking is assured. With approx the +20dB conversion gain of the ECH81, audio level should be sufficient to drive a simple triode/pentode output stage directly to a goodly volume, ECL82 or similar. Locking range is dependent on the ratio Varicap C/Tuning C. Too much will make tuning "snappy", tending to jump from one close station to another, too little will make tuning "fiddly" to lock. Adding a tuned RF amplifier stage will improve weak signal performnce. A simple passive audio LPF will likely be required to reduce adjacent 5kC hets.
AM broadcast homodyne
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dayleedwards
Jul 06, 2022
In Other Electronic Projects
Someone with more time than me may like to research the above valve and construct a receiver using it. Its a very unusual valve originally intended as a FM limiter/detector, but I reckon is good for many more things. My thoughts are, to build a simple RF amplifier centered on a ham band, the output then going to the EQ80 first control grid. For full limiting, the grids need a 14 volt p/p signal as I understand it.... easy to acheive with a regenerative amplifier, the EQ80 grid can simply be connected to the top of the tank. The second EQ80 control grid is connected to a VFO also centered on a ham band..... this becomes the main tuning. The EQ80 is a limiter, in which case it will supress any AM modulation, ie noise and much sideband interference. Limiters, by their very nature will "lock" onto the strongest coherent signal in the passband, in our case, the tuned one. RF sensitivity is increased, and RF bandwidth is narrowed with increasing regeneration. As SSB demodulation is a mixing process only, the audio will appear as a PWM signal on the EQ80 anode, and can be recovered by a simple LPF. Any AM station will cause only a heterodyne.... BUT, by filtering the recovered DC from the EQ80 anode, and using this error voltage to control the VFO frequency, it will be possible to synch detect AM with selectable sidebands. This will require another mixer in tandem.... Because very good isolation is achieved between RF and VFO, there will be minimal, if no pulling with tuning. One knob tuning and perfect tracking is easily achieved as VFO and signal circuits are identical in frequency. In essence, we should have a stable, regenerative , sensitive, potentially quiet direct conversion RX, with good frequency stability, no requirement to have fiddly threshold settings, good post detection filtering with a suitable audio filter not requiring huge and generally problematic audio gain. The EQ80/6BE7, can be thought of as an analog NAND gate, where for the anode voltage to go lower, the two control grids must be positive together. At 90 degree delta, the anode is at high voltage, as the phase rotates between the two inputs, anode voltage drops to a maximum at zero degrees. Between the two, the "pulse width" varies as a function of the phase difference., or the "overlap" of input voltages. I bought a box of these on ebay.... they are very cheap.
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dayleedwards
Jul 05, 2022
In Tube Radios
This was my original prototype design to see how well a simple superhet could be made to function, and is still the "guinea pig" for testing new ideas. In order to derive the maximum performance, i decided to go for a fairly high IF frequency to eliminate images. The usual 1700kC is low enough to achieve reasonable gain, but selectivity suffers. Both these issues are nicely avoided by using IF regeneration. Its also out of the range of interferring AM broadcast stations. The best and latest in valve mixer technology was in the 60s when Philips created the ECH81 triode/heptode. This valve had little frequency pulling between mixer and oscillator, and is fairly tolerant of oscillator voltage level into grid 3. By using a variation of the Vackar type oscillator, thus keeping the Oscillator level constant over the tuning range, the effective gain is also kept constant over a wide tuning range. The triode oscillator section is fairly low gM, and has to be driven hard to get into the higher SW bands.. The ECH84 came later, and although designed primarily for synch seperation and gated AGC in TV service, appears to exhibit slightly better performance as a mixer in this application. Because of the very high impedance of the dynamic load on the mixer anode, a result of the "Q" multiplication effect of the regenerative IF, the conversion gain is increased dramatically, therefore, an RF attenuator is required to limit overloading in the IF. The high conversion gain also allows a relatively lossy passive bottom coupled dual tuned bandpass filter on the input, again improving mixer performance by narrowing the RF bandwidth, thereby reducing out of band noise. This is quite noticeable when comparing this to my simpler RX having just the one RF tuned circuit. Using the Pentode screen voltage to vary the IF gain, results in very smooth and controllable regeneration with little frequency shifting. I did try a second IF bandpass filter between mixer and IF in an attempt to improve the IF shape factor. This worked to a point, but something I had not envisaged was the "double humping" when adjusting the regeneration. As the "Q" increased, the effective coupling between the two coils increased, causing extreme interaction and effectively splitting the signal into two several kCs apart at the critical threshold. I would like a 3:1 transformer in the detector anode to gain a free voltage stepup to increase the audio gain, have yet to find anything suitable. This radio I used 5 pin DIN sockets , and PVC plug in coils, an easy way to change bands and makes superhet tracking design slightly easier. Air cored coils and air spaced tuning capacitors are the ultimate for stable and "drift free" oscillators. The best tuning capacitors are made from brass, these have less thermal expansion than aluminium vanes, good luck finding these. Polyvaricons are abysmal, only slightly better than varicap diodes. Having no tuning slugs improve the thermal effects on the coils, but also makes adjustment very time consuming, needing to add or remove turns to get it just right, but its worth it to maintain the high "Q". This set of coils covers 5 to 15megs, tuning is touchy. More coils and less tuning range would be the next goal with this one. Some may find this approach interesting, I know back then i had lots of fun building and experimenting with it.
Another 2 valve superhet
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dayleedwards
Jul 03, 2022
In Tube Radios
This compact radio was constructed quite a few years ago, it has proven very reliable and performs very well. It covers 4,.5 to 13.5 megs, AM and SSB. Controls left to right are ATT, fine tune/clarifier, IF regeneration, and main tuning at the top, using a slomo drive. After the attenuator control, the signal arrives at the 1st grid of V1, this is an ECH 84. The triode section is configured as a semi Vackar type oscillator, this type gives a relatively consistent amplitude over large tuning ranges.. L3 is the main regenerative IF coil tuned to around 1700kC, here I used a AM BC oscillator coil. The cathode of the detector is tapped down via a capacitance divider to provide feedback. The high IF frequency suppresses image responses at the higher received frequencies. The audio output stage uses the triode section of the ECF80/6BL8 . Regeneration is controlled via screen grid voltage on the detector. Audio output is very loud in headphones, but not quite enough for comfortable speaker listening. Sensitivity is around 5 microvolts, limited largely by mixer noise. The radio draws 800 mA from a 12 volt supply. Edit. I forgot to draw in the fine tune capacitor, this is a 30pF variable that is paralleled across the 820pF padder in the oscillator section..
Two valve SW superhet
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dayleedwards
Jun 30, 2022
In Solid State Radios
Solid state superregenerative RX with squelch.... the superregenerative detector is designed around TR1, a BF191 VHF transistor. The base has a tight fixed bias by the diode string, forcing a stable operating point over a wide tuning range, aiding the squelch to maintain good performance. The audio is low pass filtered to remove the 30kC quench frequency and amplified in TR2. IC1a is the main gain block, its low output impedance preventing any unwanted interaction between the squelch and volume controls. The audio then is amplified in the LM386 gated audio power amplifier. The signal from IC1a is also passed to the multiple feedback bandpass filter around IC1b, the second half of the LM358. The filter has zero gain at the Fc of 12kC, dropping rapidly from each side. This filters out any residual quench, and any audio modulation. The output is rectified and the recovered vo ltage used to gate the LM386 off, muting the radio. Set correctly, the squelch will "spit" off signal, and fully open on even the weakest transmission. This design is the classic "noise" squelch method, relying on the "quieting" effect a transmission has on the severe background hiss of any high gain RF amplifying system. Purists will note the absence of any isolation between detector and aerial. My experiments have found little to no attenuation of SR hash with a buffer stage. Measured on my old spectrum analyser, the 6 microwatts of RF hash is spread over a roughly 200kC spectrum at the quench frequency , reducing any radiated power at any one point. My Yupiteru 7200 is struggling to detect any interference at a 3 meter distance, and that is one sensitive receiver. It is also interesting to note, the performance of this version is very similar to the three valve version of the same thing, the former using a high active component count, the valved set using just 6 active stages. Edit.... I forgot the pin numbering for the LM386... the transistor collector connects to pin 7, when switched on, it deprives the preamp stages within the IC of operating current. The capacitor bypass on pin 8 slows the voltage change to remove switching clicks, making the squelch "softer" with the attack and decay.
Solid state superregenerative RX with squelch content media
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dayleedwards
Jun 30, 2022
In Tube Radios
Three Tube superregenerator with squelch This is my latest airband RX. I was motivated to try this after stumbling on to a youtube channel by Mikrowave, and thought to give it a crack. The original had problems with insufficient squelch sensitivity , opening only on very strong signals, limiting the radios usefulness. V1a is the RF buffer stage, V1b the superregenerative detector. The 330k gridleak and 56pf coupling capacitor set the quench frequency at around 80 kC. The detected output is amplified at V3a after the passive LPF removes any residual quench. To simplfy the filtering here, the quench frequency is much higher than is usual. After passing through the squelch gating diode, the final audio stage V3b amplifies to speaker level. The high frequency audio components are amplified by V2b, audio modulation response is limited by the small 150pF coupling caps , detection and level shifting is accomplished in V2a by driving the grid negative by the diodes lifting the cathode by 1.4 volts. A negative going waveform at V2a anode ,with noise, keeps the .47 mfd capacitor negatively charged, reverse biasing the squelch gating diode, thereby cutting the audio. The screen of V2a is varied by the squelch threshold control to vary squelch gain and valve DC bias conditions, ie the anode standing voltage. The SR sensitivity is -107 dBm, this is also near the minimum signal to open the squelch, so is adequately sensitive for weak VHF signals These sensitivity figures are standard with these common designs. With the values shown, rx coverage is from 110 to 144 mc. Audio stages use the ECF80/6BL8, the SR uses a 12AT7, ECC81. The squelch is effectively a noise squelch, full limiting occurs at around 2 microvolts RF input. Total DC current draw with the solid state "vibrator" HT supply is 1.2 amps at 12 volts, otherwise the radio can use a 12v AC supply, the backwards mains transformer then stepping it up to around 150 volts to provide mains isolation.Audio output is approx 400mW, adequate for a quietish room, Running the heaters on AC will probably introduce hum becuase of the very high audio gain, if so, rectify and filter this supply. Most of my valved designs use the ECF80, a triode/pentode TV valve, these are easily got and are cheap.
Three valve superregen with squelch
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