Harry Lythall - SM0VPO

We are living in a time when true radio components are no-longer available to the amateur constructor. The variable capacitor is rapidly dissapearing; killed by the varicap diode, although the prices are almost as high. Everything is becoming "surface mount", "cost effective" and "throw-away".

Today we can still buy radio valves (vacuum tubes) but there are few suitable support products for them. The tube base is now more difficult to get than the tube iteself, and there are few varicap diodes that can tolerate 67V battery valve HT, let alone the 250VDC used for mains valves. There are companies, such as Colomor Ltd (UK), and Antique Electronic Supply (USA), who are a goldmine of information, together with the availability of valves and valve related parts. You can even still buy those old high-impedance headphone from AES!.

One item that is somewhat lacking everywhere is the old valve IF transformer. I once bought a small pile of valve double-tuned 455KHz IF transformers from AES, but I was advised that they were on their way out. If I wanted any other frequency than 10.7MHz or 455KHz, then there was nothing. I then decided to find a quick, easy and cost-effective (i.e. cheap!) method of making these myself. A typical two stage circuit (simplified) is shown above, and it is the two IF transformers I want. I will also calculate the capacitors marked "C". Here is my solution.

75 years ago you could not buy these components, but radio amateurs still built equipment. They used "bits and bobs" that were available. I have available loads of those plastic tubes for mains wiring, since I am renovating a house. So I took about 5cm of one of the thinest tubes I could find. I then took two slightly larger bits, each about 1cm long. The size was chosen to be a push-fit over the first tube. Next I needed some plastic washers.

I cut my plastic washers out of an old VISA card, but you can use just about anything. The inserts in bottle tops, plastic tokens, plastic card, just use your imagination a bit.

The plastic washers, 5cm tube and two 1cm tubes are fitted together to make two coil formers that can be moved together for adjustment. The washers are fixed to the 1cm tubes using super-glue/crazy-glue/Areldite, etc. The two 1cm coil formers I wedged in place with a short bit of a matchstick trimmed with a scalpel blade.

The next stage is to wind the coils. As one person has recently pointed out to me, if the impedance of the transformer is made equal to the valve anode impedance, then the tuned circuit will be damped and the "Q" falls to "1". Whilst this is great for audio output transformers, it is unacceptable for tuned circuits. Anode impedance divided by the desired Q is the primary impedance required. Remember also that there are two tuned circuits: primary and secondary, so a Q of typically 10 to 20 for both primary and secondary is reasonable. If you choose a higher Q then the losses will increase. If you decide on a single-tuned transformer then you should aim for a higher Q factor, say 50 to 100, depending upon what sort of bandwidth you want. Whilst I am on the subject of bandwidth, there is no reason why you cannot choose a slightly higher Q and then "stagger" the frequencies of the two tuned circuits. This will give a more flat response to the bandwidth, at the expense of loss. Staggered tuning and IF alignment requires more elaborate tuning methods, but this is beyond the subject of this article, as is the degree of coupling between primary and secondary, which also affects the bandwidth.

Now you have a fair idea of load impedance of the transformer primary. This is going
to be the impedance of the coils, since the tuned circuit impedance = inductive reactance
= capacitive reactance at resonance. In the calculator below, just enter the frequency
(KHz) and impedance of the input coil you want. I have also given you the formula, if
you wish to do some finger-counting and work it out manually. When you click the "show"
button, the values you entered will also be copied to the other forms as default values,
but **you may still enter any value you like.**

Now that the inductance value is known, we can calculate how many turns of wire are needed on the coil. Again, I have given you the formula, if you wish to do some finger-counting and work it out manually, but this time you may need to remove your socks.

Formula: | |
---|---|

Where: L=Inductance(uH), a= Winding depth(mm)
+ Former Dia.(mm), b= Winding length(mm), c=winding depth(mm) |

Now you know how many turns to use, all you need to do is to work out the wire size to fill the former. If winding by hand then you can assume that the wire is square, so a winding depth of 5mm and a winding length of 8mm will have 40 square millimeters of winding area or 40 turns of 1mm Dia. magnet wire, or 160 turns of 0.5mm Dia. wire.

For the output coil, find out the input impedance of the next stage. Use the same formulas as above. The input impedance of a component (filter, etc.) is normally given in the specifications of the component you are feeding. If you are feeding a valve grid, then the impedance is equal to the grid-leak resistor (R1), but this should be divided by around five. This is because the higher impedances will be more damped by stray capacitances, and feeding a stage with a lower impedance only improves stability. A better method is to tap the coil to get any ompedance you want without damping it.

The coils must now be mounted and terminated. This I did by deciding the final can size, then cutting out a block of wood as a model. The ends became a template for marking and cutting two pieces of copper-clad board. Drill the centre of the two to fit the coil former.

At this stage you can etch the board if you want to include trimmer capacitors, as I did, but this may not be necessary. More about capacitors and adjustment in a moment.

When the coils have been assembled, as above, fit stout tinned copper wires between the corner holes. If you used copper-clad board then it should have been etched so that pads are available for soldering. Otherwise, just glue them using Areldite, or any other epoxy.

When the coil is wound, it needs capacitance to tune it, but what capacitance do you need? The answer is that capacitance that gives the same impedance as the inductor at the operating frequency. The formula is:

** Reactance = 1 / (2 * Pi * F * C)**

Yet again I have an on-line calculator for you to use. If you have already calculated
the inductor then you should have the default values already entered for you. All
you have to do is to click the "Show" button. **You may, of course, use other
values.**

Bear in mind that all stray capacitance is included in the calculation. If, for example, you needed a fixed capacitor of 140pf for resonance, then you may only need 125pf if there were already 15pf of stray capacitance in your project wiring, or load device (input capacitance).

If you want to do as I did and add a trimmer capacitor for tuning, then you will need to give a little thought as to the value of that variable capacitor. If you needed a frequency ratio of 1.4:1 (eg. 380KHz to 530KHz) and the total capacitance required was calculated to be 150PF, then you would use a 100pf fixed capacitor in parallel with a 100pf preset capacitor. The capacitance ratio is 2:1 but the frequency ratio is sqrt(2):1 = 1.4:1

The 100pf preset capacitor "mid-capacitance" is 50pf, plus the 100pf = 150pf, which is the 455KHz value of capacitance required. Again, don't forget to subrtract stray capacitance from the fixed capacitor. But all this can be worked out roughly on your fingers.

A final comment about capacitance: when you calculate it, take a *practical* look
at the calculated value. If it only 2pf or 10,000pf then something is wrong. Perhaps you
entered a wrong value? Perhaps you need a coil with low impedance tappings? Tappings will
be mentioned farther down the page.

I have been lucky in that I have access to loads of thin aluminium sheet that is about 0.33mm thick. It is used for decorative panels, but I am assured that you can buy it from metal merchants. This I cut out using ordinary scissors, then I bent it around the wooden model and glued the end tab with a dab of crazy-glue.

The bottom tabs are bent to 90 degrees and used to fix the final assembly to the chassis. Leave the can on the wooden model block and it can be used to bend the lid to get a perfect press-fit lid. You can even bore holes in thelid to access any trimmer cap you added. Don't forget to drill a hole at each corner of the lid before bending to avoid that sharp corner.

By the way - I have not shown ANY dimensions in any of these drawings. This is so you can make up your own mind and use whatever size cans you want.

The final coils will look something like this. The "*=see text" points to the method of fixing the tinned copper wire. If you used copper-clad board then it should have been etched so that pads are available for soldering. Otherwise, just glue them using Areldite, or any other epoxy.

Fix the cans over a cut-out in the main project chassis using M3 or 6BA bolts. The can bottom-tabs do not go all the way to the corner so that there is clearance. The can tabs are trapped between the chassis and the bottom PCB. Just take care to ensure that there is adequate clearance between the coil terminal posts and the mounting bolts. You could fit the bolts in the center of 2 of the four sides, but I will leave these details to you and your situation.

When a can is placed around the coil then the inductance changes a little, but by using a large diameter can then effect is minimal. I built a simple valve receiver for the HF bands using a 1.65MHz IF. Here are a couple of shots of the IF cans with the lids removed. In these cans I used a rather large former, and the holes at each corner is so I could push a screwdriver through to fit them to the chassis.

So now you see just what you can do for screened low-frequency coils, what's stopping you? The point I want to make is that you do not follow my suggestion exactly, you edit and change to suit your own needs. Pen tubes? Felt-tipped pens are usually a good cheap source of thin plastic tubes.

There are several possibilities. For example you can add not four, but five, six, or more tinned copper wire terminations to make coils with many different windings, or tappings. Internal capacitors can be soldered inside the can to the copper terminal wires. You could even build complete circuits inside the cans to make the can a complete module, or hide "non antique" components.

You can make the inductors tunable by using a bit of epoxy to glue a nut in the end of the 5cm bif of plastic tube. Through this fit a steel bolt with a ferrite bead glued to the end. The construction will look something like this (but without all the silly colours I added to illustrate things).

As a matter of interest, ferrite will increase the inductance, or lower the tuned circuits frequency. Brass, on the other hand, has exactly the opposite effect. So use a longer brass screw and you will reduce the frequency. You could also add a second nut as a lock-nut to prevent the bolt moving with vibrations. You could also wrap a layer of felt around the ferrite to guide it in the tube. Ikea sell these dirt-cheap sticky felt pads to put on chair legs, to prevent them damaging the floor. These are ideal.

Well, I have presented a solution for you to play with. I hope that this will answer some of the e-mail and occasional messageboard posting questions I receive. So, now all you need are a couple of EF80 tubes, a few pen tubes and a bit of aluminium. I hope soon to present an idea to fabricate variable capacitors. I have been succesful using foil on soda bottles 1/2 full with water, even copper-clad boards pushed between each other. But this is what the hobby is all about. I spend a lot of time building, but very little time actually using the gear I make. In recent years I have precious little time to even build:-(

I hope you, too, have fun, de HARRY, Lunda, Sweden.