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Author Topic: Winding coils on forms vs toroids  (Read 4249 times)

Posts: 35

« on: February 07, 2012, 11:39:35 AM »

I have a certain fascination with the old homebrew tube rigs.  At some point, I want to build a superhet with the tubes standing proudly on top with their soft glow visible and the guts hidden below... something like how the high end tube audio amps are packaged today.  Why hide all that light and hand blown glass, right?   Grin

To the question... so many of these designs used tank circuits and IF transformers with either hand wound form coils or TV parts.  Is there any reason why a toroid couldn't be used in place of an air core coil?  They are much more elegant IMHO.  If one knows the frequency and capacitance, you could solve for the inductance, use a toroid calculator and wind it in a smaller package, right?  Also, since many of the inductors had the ability to trim (with the LC circuit having fixed capacitance), couldn't you also trim the resonant frequency on the capacitance side rather than the inductor side using a trim capacitor?  My newbie impression is this is an old school versus modern approach but achieves the same end.  Yes or no? 

Any thoughts or comments on the 1960 ARRL DCS-500 Superhet?  This is what got me thinking, modified using a switchable filter selector rather than a rack full of air coils to switch out.  Or, something like the HBR 14.  I realize that either would be a quite ambitious project but dreaming and thinking on it doesn't cost much.

Jon - KJ6TJX

Posts: 13007

« Reply #1 on: February 07, 2012, 01:28:48 PM »

You certainly could use toroid cores in place of solenoid coils.

Wide band circuits where two or more stages have to track as the receiver is tuned require
adjustment of both the coil and capacitor for all stages to match.  You can do this with a
separate core in series, and I've seen cases where an adjustable former was placed beside
the toroid and the turns wound over it as well, giving a small range of adjustment.


Posts: 1044

« Reply #2 on: February 07, 2012, 02:40:27 PM »

You certainly can use toroid coils, if you can get the right inductance. I was looking at a design the other day, thinking I could substitute a toroid for the "pill bottle" tank coil. and found it would require a quite large torus or really small wire.

One trick to make a toroid adjustable is to stack an adjustable-inductance former alongside it and wind the turns over both for at least a portion of the coil.  Otherwise, yes, you could use variable capacitors (trimmers) to accomplish tank tuning as well.

Posts: 4358

« Reply #3 on: February 10, 2012, 05:56:03 AM »

For tank coils, be careful about the magnetic field strength in AT/m - you mustn't approach saturation.

For coupled circuits, such as IF transformers, you will need to change the coupling method, since the toroid has a much more closed field than a solenoid. Again, watch the magnetic field strength with DC flowing when you have a lot of turns. Unless you can get the turns ina single layer, you may have Q problems because of the self capacity once the ends of the toroid approach each other, and certainly if you need so many turns that you end up with a multi layer  coil..

Posts: 2070

« Reply #4 on: February 10, 2012, 07:36:43 AM »

Tube circuits traditionally had lots of room and high impedances. Air wound coils work well as do slug-tuned solenoid coils with low-to-medium permeability materials. Shielding was done with metal shields and physical layout. Transformer coupling of RF stages was often done with a "movable link" coil to get that right, often with rather low mutual inductance and tuned circuits. Air wound coils work great there.

When transistor circuits came along the circuit impedances dropped a lot in most stages. To get lower impedance at a given frequency requires more inductance. High permeability materials became more important. High permeability toroid cores are very much self-shielding reducing the need for metal shields between stages and letting everything be more compact. The trend towards broadband (which happens to coincide with the move to solid state) favors untuned broadband transmission line transformers for coupling RF stages, and high permeability ferrite materials rule there, at very low impedances.

Posts: 707

« Reply #5 on: February 11, 2012, 10:52:40 AM »

I've substituted toroids for traditional slug tuned coils.  The Qs are often
the same or close and at HF they work very well in tube circuits.

The only tough item is when two tuned circuits are used as a coupled
affair in the shielded space.  The solution is two toroids with a link winding,
a common L on the low side or a top coupled C.  It's not as hard as it
might look and the values can be found by experimentation or calculation.

The tough case is IF cans in the VLF range (85, 100, 262, 455, 500, 1650khz)
and Max at:

has a great solution that can be applied to any flavor of "shielded coil".


Posts: 898

« Reply #6 on: March 08, 2012, 07:30:09 AM »

I'd suspect you could get better Q using a toroid vs. a 1/4 inch slug tuned form.

One of the advantages of Ted's Crosley's early HBR receivers was the use of very high Q coils
in the front-end.  So, at least in theory, if you can improve the Q in the frontend you'll have
less loss. And the toroids are self shielding to some degree.

 You can get a bit of tuning range by spreading or compressing the turns. One thing
I would suggest is picking up an AADE cap/inductance meter, and perhaps a cheap Heath
QM-1 Q Meter to assist with the toroid winding and to get some idea of the Q.  I've seen
the Q Meters go for 20 or 30 bucks, and the AADE is pretty essential for any homebrewer's

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