Wadley Loop RX

Small and slow progress lately, having completed the 37.5 Mc Wadley loop section. The first iteration gave only a few millivolts of RF at the 6BE6 second mixer, nowhere near enough on grid 3 to get any decent mixing action. Frustratingly, the grid input coils, and anode loads were VERY broad in tuning for a peak, heavily loaded by the valve, a good sign that they were having trouble with transition time issues. Turns out, changing the two IF amplifiers from 6BA6 to EF80 frame grid valves, the gain increased dramatically , I now have 2v p/p , this over a bandwidth of around 400kc, all coils now with sharp tuning peaks.. The 7 pin pentodes appear to be very limited at higher frequencies, possibly loose construction tolerances? I may also need to replace the 6BE6 heptode mixers s with something better also. ... perhaps ECC81s?

Any drift in the first VFO will be cancelled over this bandwidth, ie, +/- 200kc or slightly less. The coils required were wound on ex TV formers, two 10 turn coils on each former separated by 1.5 inches to get the right coupling. By injecting a 37.5 meg signal into the first mixer, mixing products were heard at the expected plus and minus via the tuneable IF, so it is working so far. Now its time to build the harmonic generator, and the VFO. We are coming into summer, so progress will probably slow.

Can you use a series combination of a high pass filter cutoff at 39 MHz and a low pass filter with a cutoff at 40 MHz to get the desired 1 MHz band pass?

It might eliminate a lot of the tedious fiddling with the coupling and make steeper skirts on the filter.

73,

Win W5JAG

L/C Filter

Thanks Jag, all sorted.

The filter block experimental prototype took HOURS, there is a lot happening with few parts. Obviously, the coils need to resonate at the correct frequency. Then, because each individual tuned circuit resonates at just one frequency, we need several to widen the passband. If each tuned circuit is too higher "Q" then there will be a gap between each peak, this is called ripple, so we need more TCs to "fill in the gaps". Then they need to all be coupled together somehow. Proximity will couple these inductively, but too close will cause "double humping", too far apart will increase "Q" of each and cause ripple. Capacitive coupling is tricky, each coil at resonance is very high impedance, and even vey small stray capacitance has a very large effect. But, we need some capacitance to swamp out the strays, to make the design more robust and more repeatable.. The individual "Q" of each TC is largely controlled by the L/C ratio. More coil, less capacitance makes for a higher "Q", less coil, more capacitance will have the opposite effect. Phase changes too between inductive and capacitive coupling can cause dips and peaks in the passband.

I see no way to model these things, I chose the "suck it and see" method.

Using a spectrum analyser with a noise source seemed the only way to actually see what is going on.

First problem, the SA has 50 ohm input/output impedances, so direct coupling will short out the filter, A temporary two turn coupling coill was wound on the two outer filter coils as an artificial tap. With each TC having 13 turns, that gives an impedance ratio of around 50 to 1, or 2500 ohms , probably around the same loading as a pentode control grid at 40 megs or so.

The coils were wound on the formers and resonated with a 15pf cap , the slugs allowing fine tuning. The spacing between each TC was changed until the individual peaks began merging into one. The coupling capacitances were then varied to massage the response curve into what is required.

Thats why it took 5 hours to get it right, and keeping it as simple as possible.

At this stage we have 5 tuned circuits at 13 turns on a 1/.4 inch former, spaced at 18mm between centers. The resonating caps are 15pF, as are the interstage coupling caps. The input TC is inductively coupled only to the second TC, removing a pronounced dip in the passband center point. Once I add a screening box around this, its all likely to change.....

Perhaps QRP could try modelling this on his computer simulator??? Is it even possible???

Those last two photos are crap, I have no idea how to remove them????

5 LONG hours and

a swear word or three got me this.

Finally have designed a reasonably simple bandpass filter, 1 meg wide, down 20dB at 500kC either end, no passband ripple, just the ticket for both the receiver first IF and the Wadley loop amp, the same design will do both, one being tuned to 40 meg Cf, the other at 37.5 megs. Two cascaded should net another 10/15 dB attenuation on the skirts.

There are two gain stages, one between this filter and the next, and the second to match into the second IF mixer. The signal here must be over 5 v p/p to give good high level mixing. Looking at the response curve, just one may be enough for the receive IFstrip, Each harmonic of the VFO is at 1 meg intervals, so the loop should not have any response to the ones each side, so at Fc of 40 megs, we want it deaf to anything on 39megs and at 41 megs, otherwise these signals will cause bleed through of any station on adjacent bands above and below causing birdies. Two of these filters cascaded should be enough. Here is the prototype experimental filter......

I added a temporary coupling coil front and back to allow the 50 ohm spectrum analyzer inputs/outputs not to load the filter, being valve stages these are high impedance, so a quick and dirty impedance match was needed. At 40 megs, valve signal grid impedances are beginning to drop, I may need to tap these down on the input coil..

And, just when you think we have seen it all, I came across this......

https://www.youtube.com/watch?v=Mfly1pGgpqY

A little progress today, but ran out of solder......

Now the tuneable IF is working, its time for the wadley front end. A few pics prior to wiring. There are many coils to wind, fortunately they are all the same in having only around 9 turns, resonated with a trimmer cap on each. Bought a bag of these from China, old fashioned ceramic type, a decent size that wont fall apart if they are adjusted more than once. My experiments using miniature coils in a shielded box was a failure as I had no control over the inductance coupling between them, this caused multiple humps and dips in the passband response. The el cheapo ceramic fixed caps were more temperature sensitive than I would like, and that didnt help either. The problem I have now is using repurposed IF transformer formers, the slugs are powdered iron that have a vicious effect on frequency setting, and the formers are a coarse thread, compounding the issue. But, they is all I have so Ill need to be patient when setting it up. Im still concerned that those 6BE6 heptode thingies wont work well at 40 megs, if not, will use something else. The tuneable IF did drift slightly with mains fluctuations, so have added an OB2 120 volt regulator tube , this will supply the VFOs and the BFO, which should help. If this persists, will use a Xtal locked BFO. I think now Im up to 14 vacuum tubes, without the magic eye. Will the mains tranny get hot with around 5 amps of heater current?

A quick update here. Yesterday spent considerable time in winding coils and fiddling capacitor values to obtain a nice linear kHz tuning dial readout, and after LOTS of work, have arrived at an exact 1 mHz sweep with linear increments allowing me to use the existing Yaesu dial mechanism. As from now, the tuneable IF section and audio stages are complete. Now I will tackle the Wadley loop front end. Given I have a HP spectrum analyser with noise source, the filter modules should be fairly straightforward to get working.

Well, spent hours tracking down motorboating in the 455 kc IF strip, finally solved by bypassing the AGC line. The radio so far uses a 6BA6 tuneable IF 3 to 2 megs amplifier. Mixer/osc uses an ECH81. The output of this at 455kc passes through a xtal filter to a 6BA6 first IF amp, then to the ECF82 pentode section 2nd IF 455kc amp to a double tuned IF coil for AM demod via xtal diodes, and AGC via a second diode voltage doubler. AM audio doubles back to the ECF82 triode section, then to the 6AQ5 audio out. The double tuned last IF also couples into a second ECH81 mixer/osc as the product detector for CW/SSB. Its running nice finally with tons of gain and is now stable.

A few pics, will do a video at some stage.

Today, managed to mount most of the tuneable IF parts including the kHz tuning capacitor. Because the trimmer caps are mounted on the wrong side, it needed turning 90 degrees to get them in an accessable position, blocked by the xtal filter. Still waiting on the valve sockets, so began winding coils. The tuneable IF needs to cover 3.5 to 2.5 megs, so after experimenting using a GDO and similar tuning cap, 20 turns on the oscillator coil shunted with 390 pf is looking good. Problem here is, the tuning gang is a 370pf unit per section, when really it needs to be around 70pf, and i am too whimpish to pull a few plates out. High capacitance low inductance oscillators tend to be more stable, so thats a good thing. Also, to get a reasonably linear dial scale, the greater the ratio between fixed and variable capacitance, the more linear the tuning. Slugging it back should work fine, IF the crappy triode section in the ECH81 has enough grunt to oscillate with such a large L/C ratio. Also wound the 40 meg IF coils, have to guess the coupling between the two tuned coils on each former. There are two ways I could get the 1 meg bandwidth, either stagger tune or resistor damping. Stagger tuning is/was used in video IFs in television, so I pulled a 37 meg TV IF coil out to study so copied the spacing. Being a pessimist, I have added a second IF amplifier in case more gain is needed, here.

The 37.5 mHz loop IF needs to have a 5 volt p/p on its output, as this is effectively the local oscillator signal for the second mixer. Here I have used a 6BA6 pentode as the first amplifier, through 4 bandpass filters to a 6BN8. These valves can be used as an FM limiter, the high gain should improve the LO signal in amplitude and in purity. A limiter by nature will lock on to the strongest signal and ignore all others, exactly what is needed here. The mixing harmonics are 1 meg apart, so this will improve the selectivity of the loop. I did find a EBF81, a dual diode pentode. This is now the final 455kc IF amplifier and AM/AGC detector. These are quite rare, I only have two. Also, was suprised to learn, the miniature 6AQ5 output valve is equivalent electrically to the 6V6, I never knew that. The first 455kc IF amp is a common old EF83 pentode, both this and the EBF81 are remote cuttoffs, so good AGC. The osc/mixer is the ECH81 already mentioned, and the 3,5 to 2.5 tuneable IF amp is a 6BA6. Once this is all up and running, the rest should be quite easy I hope. The HF VFO covers 40 to 70 megs thereabouts, winding a coil of 8 turns across a 35 pf tuning cap gives just the right amount of swing, so this section too has been sorted , I need to make brackets to hold that capacitor.

The last bit is the CW/SSB detector, here I have opted to use another ECH81 triode/heptode. The triode section will oscillate at 455kc, the mixer will ....er... mix that down to audio. The xtal filter will be bypassed on AM wide, with 8 tuned circuits at 455kcs, that should make a nice sounding audio with steep skirts and a wide passband.

Today marked out and drilled the chassis wit

h what I hope is a reasonably workable and logical layout.

I need to guess quite a few things ahead of time, like, how will the dial cords route thru to the front panel, and will anything interfere given the compact nature of the chassis..... its gunna be tight.

Assuming everything is good, power of positive thinking, I should know better by now, a quick bubble in draino to give it a sheen finish

Now I need to wait for the valve sockets to arrive

Next job I guess is a front panel.....

Interestingly, after a quick read, Ian Pogson has also identified similar shortcomings as I described in the original prototype, with steps to fix these. I remembered, many many years ago, Silicon Chip magazine offered a CD ROM for sale containing every R/H and E/A issue from the 50s through to the 80s, and I purchased it. After a search, it turned up, and with JAGs help on dates, here it all is........... I am guessing, seeing as I purchased it, I can share it for comment and educational purposes.......

and this

Try this

The Deltahet is apparently described in the September and October 1964 issues of Electronics Australia, but a DMCA copyright infringement demand 😐 has removed all of the issues from the world radio history web site.

The solid state successor was described in 1971.

73,

Win W5JAG

Absolutely interested - sounds like a fantastic ( and formidable ) project. Particularly with tubes.

I'm not sure I've ever seen a homebrew Wadley Loop receiver published.

73,

Win W5JAG