Denco (science)
 

Can anyone enlighten me on the process Denco may have used to develop their range of coils?

Whilst I understand the need for an inductance value to give a specific tuning range when used with a given capacitor value as per their use of 300pF in their coil selection chart, what did they do to get the primary/secondary ratios, the number of turns (and the position of the coupling coil) for the feedback circuit etc?

In my simple mind the feedback (in particular) would be fairly irrelevant in that you need only have the right 'polarity' of turns and the necessary signal amplitude would be set by whatever gain you chose for the transistor providing the feedback amplification - so the value of the feedback turns count would be 'irrelevant'?

Same goes for the position of the feedback winding - why is it where is is and how did they get to it?

I'm going to guess that it's a back art that mere mortals can't understand but if I as to wind a coil with the correct primary inductance (to match one of the Denco coils) how far out would anyone be if they simply 'guessed' the coupling turns ratio and the feedback coil ratio? How would this affect the circuit operation?

In fact has anyone ever done this?

I suspect the Denco values are now 'industry standard' and that circuits designed to use them are based on the values Denco programmed into their coils at the time but if anyone was to wind their own on the basis of altering external component values to counter the differences would there be any particular issues other than these variable external component values i.e. something specific to the coil winding itself?

This leads to the inevitable possibility of resurrecting a range of Denco-type coils that, in time, would have their own established component values to make a plug-in-and-go solution that Denco originally provided.

Re: Denco (science)
 

There is a lot of information on RF and IF coil design in the Radioton Designer's handbook , 4th Ed p1025 onwards. The book is not cheap, but there are downloadable copies out there.

It's not for the fainthearted!

Craig

Re: Denco (science)
 

Feedback coil (for reaction) - it depends on the characteristics of the valve being used with it.

You don't want any resonances within the band, of the feedback coil, or you could have a dead-spot, or an unusually lively spot. Particularly for superhet oscillator coils, oscillation amplitude needs to be as constant as possible across the band. And for coils for a regenerative-detector TRF radio, you want the reaction control to be approximately the same all across the band - the same constraints apply.

But, the guys at Denco don't know what valve you're going to use, its characteristics, or its capacitances. So anything they come up with is necessarily a compromise. Fortunately, it's a compromise which works quite well.

It's been found empirically that feedback winding of 1/3 the turns on the main winding usually works. Resonance is then well-removed. As for its position: It needs to be as close as possible to the main winding, for good coupling with 1/3 the turns, but not so close that capacitive coupling between the windings is significant. There's also the practical matters of having holding jigs to support it while any glue sets.

A lot of it is black art, yes, but the maths is so complicated it's not clear what you need to do to optimise what, and if they were the equations would only be solvable by numerical methods, so empirical methods are often resorted to.

Re: Denco (science)
 

Craig - yes, that's the complication I thought would raise its ugly head. Yes, of course there is a totally reliable (probably) theory behind everything we do and coils are no different but..

Kalee20 offers some hope for anyone (me?) that might consider trying. The empirical methods described would be a good starting point and the fact that there are no 'perfect' active elements that a designer could adopt to create a given coil that would work for every application leaves plenty of room for experimentation.

I'd much prefer to keep the maths side to a minimum (resonance calculations etc) so a suck-it-and-see approach works best for me.

Any rule-of-thumb (such as 1/3 the main winding as a feedback) is the kind of stuff I'm looking for if only to reduce the amount of messing around - yes, I will be using the basics of the information already available for some of the Denco range and I have a very large collection of images of the whole range I can refer to - if not simply count the turns (or estimate them for piled windings) - to get me going.

Last thing - the formers. Should I stick with polystyrene or just make some using fibre-glass tubing (easier to source). The dust iron cores might prove to be an issue too. Is there a ready source for them or is it even possible to make some similar material?

Re: Denco (science)
 

Iron cores?

The one firm I know of that still makes iron dust cores of various grades is

https://www.micrometals.com

Bill Amidon of Amidon Associates, Otsego St Hollywood (yes, that Hollywood) is a retailer of their stuff in amateur radio quantities and he's very close to the factory. Micrometals are in Anaheim.

David

Re: Denco (science)
 

Don't know if you've seen this site - https://66pacific.com/calculators/co...alculator.aspx it has various RFey calculators and articles.

Andy.

Re: Denco (science)
 

I rather suspect that the Denco coils were developed empirically. Someone had a radio design and fiddled with the coil windings recipe until they had good results. The dimensions of the former and the tuning slug/screw having been chosen to be convenient and from what was easily obtainable.

From the point of their release onwards, a lot of magazine radio designs were developed to work with the Denco coils, so they became a de-facto standard, and as Ian White put it, you could follow certain components from project to project in the articles of F G Rayer :-)

Be careful with formulae for the inductance of solenoidal coils. Most are for single-layer ones (which rules out the wave windings of Denco's LF/MF parts) and they contain some assumptions which break down for large or small coils. There have been several formulae published over the years with careful explanation of what range of dimensions they are reasonably good over. Various folk have based web calculators on them, or have quoted them in later books, but usually the caveats regarding applicability got lost along the way.

Remember that the winding of a coil will have inter-turn capacitances all over the place. This leads to the coil having one (or more) self-resonant frequencies. Even below these frequencies, the negative reactance of capacitances will counteract some of the positive reactance of the inductance. So if you try measuring (or using!) a coil, the inductance will act as if it varied with frequency.

So a formula will give you a start, but you'd better check the result by making one, and being prepared to twiddle the design a bit.

Inductors are probably the least-perfect and also least-well understood components, and therefore the scariest.

Do a course on electronic engineering and you'll be taught first about electric fields and capacitors, then they'll get halfway through magnetics and inductors and hey! the exams are almost here, time to start revising!

This means the engineering profession is poorly grounded in matters inductive, and the foibles of inductors provide additional aversion-therapy.

Useful beggars, though.

David

Re: Denco (science)
 

Other issues with inductors are that the core material (unless air) tends to be a lot less linear than the dielectric of a capacitor. The inductance of a coil depends on the current passing through it a lot of the time, the capacitance of a good capacitor does not normally depend much on the applied voltage. And inductors are much harder to screen from each other than capacitors are to screen.

For those reasons an inductance bridge will normally be designed to balance the inductor + resistor against a resistor + capacitor rather than trying to use a 'standard' inductor.

Re: Denco (science)
 

A mix of very helpful and often 'scary' comments and replies! They all go towards helping so are very much appreciated.

I expect an awful lot of suck-it-and-see experimentation and under this expectation any rule of thumb tips could save a lot of time.

The one thing that will (maybe) always throw anyone off course will be the 'piled windings' which, although looking very neat and practical, seem difficult to achieve in any normal manner without using specifically designed tools. In this case, is winding a 'lump' of a coil between two 'washers' a suitable alternative or is the actual need for a neat - probably low inter capacitance? - method preferred?

Re: Denco (science)
 

That's exactly the reason for wave winding.

Have a search in the forum. A few wave winders have changed hands and a few people have built their own from scratch.

DAvid

Re: Denco (science)
 

I once heard magnetics described as just like electrics except you have carbon for your conductor and salt water for the insulator.

Re: Denco (science)
 

Have you seen the "Coil Design and Construction Manual" by B. B. Babini, Bernards (publishers) Limited, No.160? It's one of their booklets approx A5 size intended for home constructors, so may have some content of interest to you.

Section 1 has three chapters to do with design and construction of RF coils and RF chokes.

There is a copy here:- https://worldradiohistory.com/UK/Ber...ion-Manual.pdf

Ron

Re: Denco (science)
 

It's a fair book, but uses 'old' units, and it contains a few glaring errors. The screenshot below is one, from Chapter 3 about HF chokes, which flummoxed me in my formative years - definitely NOT the case!

Re: Denco (science)
 

If you wanted to capture the RF properties of a typical Denco coil to use as a template (to copy) then one way would be to measure each coil on a 4 port network analyser as in the image below.

This would capture the behaviour as a 4 port s-parameter (black box) model and this could be used in a simulator to compare against any attempts to clone the original. It could also be used in an RF simulator to accurately model how the Denco coil operates in a typical circuit designed for a Denco coil. The feedback in a regenerative receiver could be analysed vs tuning across the frequency band for example.

Re: Denco (science)
 

It's one thing to to discover or calculate the inductances of the Denco coil range, it's quite another thing to use a wave-winder to faithfully replicate the windings. I've spent a lot of time experimenting and searching internet for home-brew coil winder designs and examples of wave would coils which have the density of wave windings that Denco and other professionally made coils had. Neither have I seen a single example of a good wave-wound coil made on a commercial winder.

Indeed, most videos of even solenoid coils (transformers) in which properly done would have the turns neatly close wound side-by-side, seem to create and untidy winding, possibly because the coils are would at high speed. Maybe that's no too critical as the turns ratio is a bigger consideration for transformers than the inductance, which is more to the fore with RF coils.

Hears and example of beautifully made winder, which only winds solenoid coils and if you look at the end result, the turns aren't side-by-side and no better than I can achieve on my sewing machine powered coil winder on which I guide the wire feed back and forth by hand and eye:

https://www.youtube.com/watch?v=46af_KBC68I

It isn't difficult to make what looks like a 'wave wound' coil, of which I've made quite a lot. What is difficult is to get the waves to lie side by side to create a high density coil, and I've seen only one example of that, and a very good example it is too, but unfortunately, there is little detail about how it's programmed. I'll mention it below.

Several years ago I became intrigued by the Morris Gingery Hand Wave Winder, which was a reproduction of a 1930s machine. It's been mentioned many times, but for anyone who hasn't seen it, it can be seen here:

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

If you look at the finished coil, you'll see that the density of the turns is low and hence, so will be the inductance relative to the physical size of the coil. Indeed, he coil has probably got more air in it than wire.

I bought the book from Camden Publications, built the winder and tried various ratios of the number of waves per turn to try to get the waves to lie side-by-side, but my efforts came to nothing. Part of the problem is that a cam to create a straight line wave back and forth needs to be heart shaped, the shape of which needs to be worked out geometrically, rather than simply and eccentric cam moving the wire feed back and forth. Alternatively, in a more complex winder using stepper motors, the software would need to programmed simulate that.

Here are two more examples of 'wave-winders' which feature low density coils of little use as RF coils and couldn't possibly replicate the physically small and dense as Denco (or Osmor, WeyRad, Repanco etc):

https://www.youtube.com/watch?v=Jv7Gsx-kuUw

https://www.youtube.com/watch?https:...EXzRwww&t=162s

This is the only example I've seen of a high density coil and impressive it is too, in the one at this link. As a test, the designer created an RF choke with several closely wave wound piles of quite large diameter, and about 1mm wide, without the turns collapsing:

https://www.bing.com/videos/search?q...6FORM%3DVDMHRS

Therein lies the challenge!

Here's how the 'big boys' wave-wind high density coils (92% density):

https://www.youtube.com/watch?v=JNFZp0uxsOU

In terms of testing the band coverage of a coil, I built a little 'coil coverage test unit' from a Radio Constructor 'Suggested Circuit' which I wrote up for the BVWS Bulletin and featured in this forum post in 2015:

https://www.vintage-radio.net/forum/...d.php?t=114238

Apart from the general discussion about the theory of how Denco may have designed their coils, I'd be interested to know if anyone on the forum has created any wave-wound RF coils using either a home-brew or commercial wave winder. There's also the question of 'Q' but let's not go there!

Pic 1: 500 turns on my hand wave-winder. I you look closely, like those in the videos at the link, the turns are spaced.

Pic 2: A test coil, which turned out to be 3mH. Again, you'll see that the coil has a lot of air in it - like me I guess, except mind is hot air! :)

I think if I were to wind any RF coils for say a receiver project, I'd use Amidon toroids - not try to replicate Denco coils.

Hope these musings are of interest.

Re: Denco (science)
 

Hi Folks, I believe one of the Denco guys had talked about setting up the factory again for production but nothing ever came of it. If he can be contacted (his name escapes me) he may well tell us what equipment he used. Failing that there may be some information in the patent office if anyone fancies doing a bit of research.
I suspect that TOKO and the like also had special machines and techniques

Ed

Re: Denco (science)
 

As I recall, for a brief time after Denco closed down, the son of the founder used the remnants of what stock of materials were left over, to make a limited quantity of coils, using whatever coloured formers remained. The green dual purpose LW/MW coils were the most popular and I believe remain the most sought after. When we look back to adverts in the late 60s, the coils were about 60p, which sounds cheap but accounting for inflation, would equate to about £6.00 per coil. Today. Thus, to make a basic superhet, along with IFTs tuning cap, valves or transistors, and other items would be quite a costly afair.

In 1967 I built a valved double superhet which specified a Denco coil pack (or was it a turret tuner?). It was so expensive that I built the radio while saving up to buy the coil pack. Within months the pack/turret went out of production and was unobtainable, so the set never got finished and was later dismantled for bits.

Re: Denco (science)
 

David - what a very comprehensive, interesting and useful set of links and information! Thank you so much for the input.

As much as I'd like to replicate the Denco range I'm very much aware of the wave winding issues but my initial attempts would be directed towards the high frequency coils that don't employ layered turns anyway. The full range would be a high target to aim for and would only be suited to those looking for legacy replacement or circuit replication anyway as those low frequency bands are very quiet these days.

If I was desperate for such coils I know they are available via sellers on eBay - there have been a large number of such for sale recently - but the costs involved far exceed their original value and reflect the (seemingly) high demand instead.

Anyway, full 'replication' is less of a requirement (for me) as just usable alternatives would be far easier (I think) to achieve. If I was going to go full speed on this it would be for reselling anyway and therefore low cost to the buyer would be my aim (I'm not really interested in a full-on business model!).

Thanks for all the valuable replies and input. Plenty of information to keep me occupied for a few months.

Re: Denco (science)
 

If anyone would like a better quality copy of "Coil Design and Construction Manual" by B. B. Babini”, Have a look at post 14 of "Getting a VA Rating from a Transformer’s Mass". The book was scanned and converted to a PDF file which has been expanded to fill an A4 sheet of paper to make it easier to read for me. Its located in this section of the forum.
Regards Stan.

Re: Denco (science)
 

I have a few Denco oscillator coils for valves. I cannot use them because their specs do not match my own superhet design calculations. They come in different versions based on specific designed values of padder capacitors and air variable capacitor tuning ratios used in the three-tracking solutions. They quoted the specs in their user sheets. There is nothing special about them.

I wound my oscillators (both Hartley and Armstrong) after studying a few samples of oscillator coils from different manufacturers using scramble windings to reduce distributed capacitance. Homemade one performs as good as the stock coils. You can find blank formers with adjustable ferrite cores on ebay.