A quick look on google reveals that Digikey sells dual gate mosfets at 84 cents each, rated to 800 mHz. So, it would make sense to grab 20 of these for future projects. Part number 3SK293. These are surface mount unfortunately, but with only 2 legs a side, should be easy to mount. Aliexpress are asking over $4 for the same thing. Insulated gate FETs make for better RF amps and regens in my experience, the nearest thing you will get to a vacuum tube'

I would expect any germanium transistor to work equally well on low voltages, the base voltage conduction point is also a few hundred millivolts. Getting one that works these days might be a mission, they slowly become more leaky over time. They also are quite temperature sensitive. Im not sure they still even manufacture them?

Hello QRP, another very wet day here in NZ, a computer day inside for me. I would love to see this design simulated to see what is going on, and whether it actually works the way I would like to think it does. I noticed I promised to have a video of it working too, and I dont think this ever happened? The radio still resides here on a dusty shelf, so I have no excuse for not following up on this.

Hello QRP, its been a while since I have heard from you....... The Supergainer uses characteristics with valves that are very hard to replicate using silicon, especially gain and dynamic range. You would have run into this very difficulty using unusually low voltages in your design. The conversion gain of the ECH84 is way above any FET or transistor by virtue of the high tank impedances and free voltage amplification with the coils. Even using IGFETs such as the old 40673 will get close, but still lacks the dynamic range inherent when using high voltages with tubes. The old fashioned pentode makes for the best regen, whether in IF amp/detector or straight RX. These are just my opinions and experience. There are also some unknowns with my supergainer that I have yet to figure out, one of which you mention. Using the ECH81, the usual goto valve for AM mixer service, and using in the supergainer , the regeneration of the IF varied considerably over the 3 to 1 tuning range, (4 to 12 megs) requiring the regen control to be adjusted over the band.  Using the ECH84 cured this. I cannot find much info on the differences between the ECH81 and ECH84, I am assuming the latter is a sharp cutoff heptode as opposed to the remote cutoff of the 81, as this one is designed to take an AGC control voltage. The anode resistance is similar for both, so why the difference here, I just dont know. The next issue is conversion gain, valves have TONS. The negative here is the potential overload of the IF amp/detector. We need the added regenerative selectivity, but not the gain. Unfortunately, we need one to get the other. Therefore, the IF amp needs lots of dynamic range, but this is destroyed the tighter the bandwidth before oscillation sets in. One thing in our favour, stong signals require less regeneration and can have wider bandwidths, hiding the issue.  Nothing is easy.  This design relies on an RF attenuation control at the aerial input rather than a volume control, to keep the signal levels in bounds. Now we have the problem with mixer noise increasing as we reduce the input signal. Solid state is MUCH quieter, that would be a big advantage . The more control grids in a valve, the noisier it will be, thats why triodes were used at VHF. The regen IF would be hampered using silicon because of the low impedances, the mixer stage would require an RF amp to get similar sensitivity, not benefitting from the mixer load Q multiplication from the reflected IF resonance..... think Q multiplier. Audio wise, silicon beats valves anytime, but by using ridiculously high turns ratio transformers, a triode can acheive very high gain in one stage. One interesting valve to fiddle with I think would be the triode section of the 6AV6. These have a anode resistance of 80000 ohms, with a 500k anode load have a voltage gain of round 70 with few hundred volts. Thats lot of gain in one stage, around 40 dB I think . So, I dont think a solid state supergainer with a similar number of parts would perform without disappointment, it would need more complexity for a similar performance to the two valve version. As always, I love being wrong, thats how I learn new stuff.

The purpose of the GDO seems to escape you. The GDO is designed to be pulled , and MUST be pulled by an external tuned or resonant circuit. The external circuit will absorb energy from the GDO oscillator, hence causing the dip in grid current, or , with solid state equivalents, a decrease in oscillator energy giving the indication. Buffering the GDO from its immediate environment will negate it as a useful measuring tool, instead turning it into a not very good signal generator. The whole reason to use a GDO is to quickly find a resonant dip indication of a passive tuned circuit, or alternatively find the approximate frequency of an externaly energised oscillator tank. They can serve as a field strength indicator, check for spurious and harmonic artifacts in a TX, serve as a quick and dirty signal RF source, can give a very good indication of inductor "Q" with experience of that particular GDO. These devices are second in usefullness to a multimeter to the RF designer in my opinion. In essence, they NEED to interact with the local environment to have any uses at all.

The second solid state design will perform very well, although it does require lots of parts. It may need to be optimised, JFETs are pretty loose in their specs, but it will work right out of the box. If you are looking to build either, choose this one.

There appears nothing unusual in that first circuit. The aerial is connected to the grounded grid amp V1a, the regenerative detector is V3a. V2 b takes the RF from the V3a cathode tap , rectifies it and passes it on to the audio stages. There are so many gain stages to account for the low 12v HT, and therefore reduced gain in each stage. The articles description of its operation is not quite correct. This design is too complicated to no advantage in performance in my opinion. There are better and simpler versions.

Thanks Larry. These forums are great to share knowledge and learn , I learn lots from what others have done and often get ideas and "why didnt I think of that" moments. I just wish we had the internet in the 1960s when I was young, I think my life would have been very different. SINAD meters are very good to standardise signal to noise ratio measurements from one radio to another, the old fashioned human ear is poorly equipped to measure anything. The one shown in the video you referenced would be a very nice addition to a home workshop. The software version would likely give variable results because of radiated computer hash getting back into the radio under test, and different computers would also radiate differently. This is a major problem with any computer used as the basis of test equipment. My fancy digital oscilloscope radiates so much power supply hash, and after the initial excitement of purchasing a modern scope, its pretty much useless when dealing with sensitive radio receivers. The manufacturer actually used an unshielded switched mode power supply. I am very disappointed with it.

Have spent even more time experimenting with this radio. It seems it has reached the limit of its performance insofar as sensitivity and stability is concerned. The biggest ongoing issue is drift in the tuneable 2.455 to 3.455 mhz oscillator in the final IF section. As the radio warms up, it drifts out of the narrow 455kc xtal IF filter passband, only stabilising after 10/15 minutes. The 2mm alloy chassis has lots of thermal inertia, ie, it heats slowly. I am wondering if a series of holes drilled through the chassis would create a thermal "island" in the ECH81 3rd mixer area to help isolate the external heating, especially from the mains transformer. I have now settled on 6BE6s pentagrids for the first and 2nd signal mixers, with low screen voltages and high value cathode resistors, the mixer noise has reduced considerably, conversion gain still remains high, the signal and injection grid isolation is a welcome bonus. A quick 12 dB SINAD test reveals -107 dBm AM at 30 percent mod, thats around 1 microvolt across 50 ohms, this is quite good, I think up from the -92dBm earlier. This sensitivity holds from 3 mHz through 30mHz... more than adequate for HF.. AGC is now applied to the RF, ist IF, and second IF amplifiers all using 6BA6 remote cutoffs, the RF amplifier now using a 6BZ6, these seem quieter in operation. Cosmic noise is now audible over all bands. Another slight issue is BCFM breakthrough in some higher bands, this would be eliminated with a LPF after the RF amplifier. Another annoying issue, I cannot quite get the new valve VFO to exactly coincide with the existing FRG7 tuning drum markings, so have opted for a digital display. This is due to arrive from China in a few days. Unfortunately it uses a six digit 7 segment display, so may be a tight fit in the front panel. These modules have a +- programmable IF offset so setting this to -3.455 should allow it to read 0 to 100000 as the VFO tunes 3.455 to 2.455 mHz, ie, the Wadley loop 2nd IF runs backwards in frequency as it tunes the RX frequency higher. Its a cheats way of doing it, I have spent far too much time on this project already. With my current fascination with Wadley loop receivers, and before I forget how they work, have picked up a realistic DX300. I am wondering why the reviews were so bad, and just how bad are they, can it be improved etc. I may do a write up if anyone is interested.

Yes,, its pretty much all there is to change, but the change is very small, less than 1kc. The issue I think is the impedance mismatch. These ceramic resonators expect in/out impedances of a few hundred ohms, hence the large value capacitors, these effectively load the thing down with capacitive reactance. Increasing the capacitance values will lower the frequency, but a point is reached where the thing will stop oscillating, and thats not very useful. There is also quite a large variation in the center frequency of any one sample of these resonators, they are cheap to make, and cheap to buy, much cheaper than dedicated 2 pin ceramic resonators normally used for this application, explaining the spread. It was just an easy way to create a stable BFO cheaply and using "junkbox" parts. If for instance this BFO was installed in an AM only radio, then the actual IF frequency can be moved to suit the BFO oscillator center frequency, its easier to do this, and will not effect the radio performance to any great degree.. Just why each and every resonator/filter is 5 KC too high, I have no clear idea, I have not investigated it. I assume it oscillates in some sporadic mode, although the frequency is and does remain very stable.

By far the easiest way to design a coil to resonante on a specific frequency is using a Grid Dip Oscillator. These are largely forgotten these days, but to the RF designer, they are as useful as a multimeter is to an electrician. By using a similar capacitor to what the radio will use, either fixed or variable, a coil is wound, temporarily connected to the capacitor and "dipped" with the GDO. This will give the frequency range, high and low, and also an indication of the "Q" or quality factor by noting the depth and width of the dip indication. By adding or removing turns, the resonant freq can be changed as well as guaging the effect of different tuning slugs. Both solenoid and torroids can be measured, coupled coils can also be designed , and the degree of coupling also found, from under coupled, to over coupled double humping. The GDO will also detect an oscillating circuit , either by accident or design,and indicate the frequency. The GDO can also inject a frequency into a circuit to test operation, similar to a signal generator. The GDO is a quick way to design coils and unlike claculators, will take into account stray capacitances, losses, and other real world variables.

And a trap for us young players..... I had a pulsing effect on my secondary HT rail, this one feeds the tuneable IF section and demodulators with regulation performed by an OB2 regulator neon. These stike at around 150 volts and then shunt regulate at around 105 volts. The rail was pulsing between the above figures, cycling at around 2 cycles/second. Turns out, I had mistakenly put a 50 mFd cap across the neon to decouple that rail to ground. This turned the neon into a relaxation oscillator, it would strike at 150v, then pull the rail down where it would cease conduction, the rail then returning to 150v, then striking once again......the time constant being the series HT feed resistor, and capacitor. We rememeber something every day.....

A short video of the Deltahet receiving American CB on 27 megs, over 8000 miles from here. A careful alignment has netted an MDS of -110 dBm, AM, 30 percent mod, and -92 dBm for a SINAD of 12 dB. These figures are not dramatic, but perfectly adequate for good HF. Deltahet receiving American CB on AM, 8000 miles distant.,https://youtu.be/C8uwzP-Gqs4

Yes, the ECH81 is by far the best bottled mixer made, and easy to use. . I dont know why, but these valves never got used in the USA, the only triode/hexode that I am aware of that had any traction was the older 6K8. The 6BE6 was used extensively, from the AA5s through to those big Collins radios. I think Hallicrafter used 6K8s extensively, but Im not really up on American stuff. Europe was a leader in valve tech, especially Philips, they printed many books of radio designs for the hobbyist, and gave out very complete application notes for all their products. I hated repairing anything made by Phillips , most of their passive components were rubbish, remember those horrid IF cans that were held together with wax, and would break if you even looked at them. The "lollipop" capacitors that would self disintegrate over time, weirdly made potentiometers that could not be replaced by anything else, even their knobs were different. Philips TVs were the same. But, to be fair, their stuff performed really well. Your circuit would have been a fairly standard use of preferred valves, and that radio would have performed very well. Even today, valves easily outperform transistors in simple radio circuits, and are definitely easy to use. The endless fun, to build a simple radio, and then try improving the performance by tweaking, adding and modifying is an awesome hobby..... everyone smiles as they receive that very first signal on that collection of just a few random parts.

After listening to the Deltahet over the last weeks, I decided to address some annoying issues. Firstly, tuning the megacycle selector was much too critical, the bandwidth was too sharp. This too affected the second mixer, the CIO injection level was too low This was why that mixer originally used the "leaky grid" method, where the 37.5 and 40 megahertz were applied to the same grid of the 6AU6, These two signals are so close together in frequency that they interfere with each other, not a good design. .... tuning the 40 meg BPF would pull the 37.5 meg tuning. A 6BE6 was tried here, having separate control grids, but the signal levels were far from optimum to achieve good mixing. So, another amplification stage was in order here at the 37.5, allowing steeper selectivity skirts, and a final amplitude increase from 350 mV to around 1 volt. The coils could now be stagger tuned to increase the effective bandwidth, the final tuned circuit was tapped down to allow cathode injection to the second mixer, thereby further isolating the RF and injection ports. This has made tuning easier and the circuit more robust. There are now 3 double tuned circuits at 37.5 megs, with a 4th single tuned at the output, the 40 meg signal BPF retains the 8 top coupled tuned circuits, the coupling reduced from 2.2 to 1 pF per section to give the 800 KC bandwidth with minimal ripple . The existing second mixer was repurposed as the third amplifier stage, the ECC81 twin triode first audio was replaced with a 6BL8 /ECF80, the triode section remaining as the audio, the pentode now becoming the new second mixer.... there was limited room to use another valve, the 7 pin miniatures have only limited possibilities The second annoyance was the many different valves used, I figured it could be improved here as well. The EF80s were replaced with 6AU6 RF pentodes, with only a slight decrease in conversion gain. The radio now uses 6AU6s in all the mixer and loop amplifier circuits, and the remote cutoff 6BA6s in the signal circuits using AGC control. The 4th mixer and product detector still uses the ECH81s.... these operate at higher signal levels. The ECF80 is now out of place, but needs must. Fortunately with valved equipment, many mods can be made without too much major surgery. The performance of the radio is very good, as far as sensitivity and stability is concerned. The off signal noise level is high, this is mainly mixer noise, this reduces quickly on signal. It is also exaggerated by the very wide first IF bandwidth. The wider the window opens, the more dirt blows in...... I have managed to receive the American truckers at 27.025 for long periods, just as stable as say, WWV on 5 megs, thats a distance of 8000 miles or so...... sensitivity is in the 1 microvolt region right now.... Once I am happy with the radio, I will measure the actual performance, drift, sensitivity, dynamic range etc, and come up with a few figures.... I am especially interested in the dynamic range and image rejection figures.

I would check the output transformer, these are notorious for failing after these years, Check the anode voltages on the output EL84s. Alternatively, just look at them in a darkened room to see if the screens grids are glowing red.....they will if the outputs are deprived of anode volts. With a faulty output transformer, the tweeters can still make noise because of the leakage capacitance between the windings. Also check the screen voltages, the transformer may be fine, and the screens are deprived of current, this will have a similar effect on the woofers.. Edit, Having just looked at the diagram, the btweetwers appear to be piezo devices, these are driven via the screen grids, being capacitive, they act as a screen bypass. These are working fine, so, Im putting my money on the output transformer. Caution.... Never fire up any valve amp without speakers connected, the output transformer will arc and destroy itself in seconds

Band scan of my homemade Deltahet Triple conversion Wadley loop drift cancelling RX .,https://www.youtube.com/watch?v=EIw-fE6IAZc

Today had a MAJOR breakthrough. I had suspicions with the first mixer being inefficient. The radio was sensitive but struggling even with fairly strong signals, and appeared to overload easily despite the AGC working its little heart out. While messing with the mixer and even changing it to a different design, I turned my attention to the RF amplifier. Turns out, the geriatric moron that assembled this thing fitted a 47k cathode resistor instead of a 470 ohm..... yes, I am a bloody moron. Now this radio is really alive. Later when the sun goes down, I will do a decent band scan on video. The ssb detector is now functioning correctly, the s meter is behaving itself, and ALL the birdies have vanished. The good thing I guess is I have optimised the weak signal handling of the first mixer......... always look at the positives. All for the sake of ONE bloody resistor. I temporarily placed a small fan under the chassis to increase ventilation, this works a treat. The long term oscillator drift in the second IF was driving me nuts,, this slowly increased as the chassis heated up, not much, but enough to be a nuisance. I dont know whether lifting the power transformer slightly off the chassis will help here, this will break the thermal transfer to the chassis, and then to the coil cans. Oddly, the 40 to 70 meg oscillator sits within 100kc of its setting, it drifts very little considering the higher frequency, and of course matters not, as its drift is cancelled anyway. The tuneable IF drifts a few 100s of cycles , on;y a pain on SSB.. The BFO uses a ceramic filter, this is relatively stable, despite not liking to work into high impedance valves, they are low impedance devices. I modified the AGC slightly to give a fast attack and slow decay, and added a fast/slow, this will use one of the missing front panel switches. Also added a triode audio preamp stage to give a tad more audio grunt, had an unused spare.. Its pretty much finished. The Kilohertz tuning is slightly non linear and is not quite accurate on the Yaesy dial drum, that is annoying and needs looking into. The ECH81 product detector works well, but having the BFO centered in the 3rd IF passband iis not ideal, the audio image makes copy difficult at times. This needs improving too. I will now do a final video of it running......

Yes Larry, many thanks. Thing is, I have HUNDREDS of assorted variable caps, from single to 5 gangs, some with slomo drives, have been collecting these for years. Its easy for me to use what I have, but this wont work for others. Polyvaricons are still easily obtainable, China will sell you a box of 10 for just a few dollars, and free postage, but they are very fiddly to create some kind of decent and robust slomo tuning arrangement, other than perhaps a dial drum and cord system. I am hoping someone else will decide to replicate the things I make or design, for this to happen, several things are needed. The biggest seems to be cost, and , anything mechanical requires mounting work, drilling, cutting etc, metalwork is usually beyond the reach of many hobbiests. Alluminium chassis are horrendously expensive, even from China, and can make or break a project. And, if a design is too complex, many lack the self confidence to even contemplate building it. I am trying to use commonly available parts that anyone can easily buy without overloading the credit card or getting tangled in divorce proceedings. I would rather have a radio than a wife, but thats just me............. Thinking about this project further, I can possibly do without any tuning capacitors completely, using a second sythesiser working on a slightly different principle, adding another couple of chips. That will dispense with any mechanical work entirely. By using harmonic locking in the first synthesiser, thereby removing any need for programmable dividers, most of the complexity disappears. The second synthesiser can be equally as simple, using a simple mixing arrangement, where an oscillator with a 100 Hz raster at say 30 megs can be divided by 10 to give 10Hz tuning steps at 3 megs, again using the same harmonic generator as a reference....... I think? It all depends on whether the 74HT4046 will cooperate, this I will breadboard in the next weeks.;.......

After looking through this kit of parts, its actually quite amazing the mechanicals involved, everything is built in a big and robust fashion. All components have screw terminals, so no soldering required. The audio transformers are very nicely made in their own metal sheilding containers. The valve holders are designed to screw down to a base board, 4 contacts and a bayonet fitting. Circuit wise, there is very little to it. The problem really was despite the very low gain of these early triodes, it was not possible to achieve any meaningful amount of stage gain without them becoming unstable,, especially at RF. The stage would oscillate. Any neutralization was a band aid, so heavy loading and or staggered tuning of each stage was required to keep the thing working. The selectivity was very broad, sensitivity low. The advertising standards of the 20s were very lax, so anything was a pass, buyers really had no idea what they were getting, being mislead by the advertising hype. I guess with nothing better to compare, most were reasonably satisfied with any noises it would make, being at the forefront of the technological age. More technical savvy people would probably have built/experimented with their own designs using parts available at the time with slightly better results, but requiring even more stages and money.... it was an expensive hobby. Once the screen grid valve made its appearance, radio performance really leapt ahead.