Customized off-the-shelf ... toroid

Making the right toroid

The voltage of my toroid transformer is too small. It is lacking 1,5 V AC. The output voltage is too low. Can I increase it ? 
Yes, of course you can - provided that the central hole of your toroidal transformer is not sealed, filled up with some epoxy resin, so that you have the freedom to thread something through the middle of this "doughnut".

If the center of the doughnut is not flooded with resin - then you can add your own additional winding,
make a few turns with some spare wire and then add this "delta" voltage, from this extra winding,
to the winding that you are complaining about, that it's output voltage is too low. Easy.
  I've actually been practicing it, as you might have noticed on one of the photos in the text relating to the "rectifier loop" antenna. 

Such DIY extra windings - work just fine. You need to take care and wind them tightly though, applying a big amount of tension ( but don't rip the wire apart, while you are at it).
If they are wound loosely, they will be a nuisance, as they may start buzzing during operation ( = emitting a buzzing sound, which is infuriating - especially if you are listening to music).

If you shall be winding more that one extra winding - it is worth to add some inter-winding insulation,
as a protection against the voltage potential differences that might exist between the windings in your specific future application.
You can use some simple scotch tape, or better - cut some strips off a roll of baking paper - which tends to have very good voltage insulating properties.

From my observations: every single turn of the wire around the core of the dough-nut gives you about 0.1 ~ 0.15 Volts AC. 

Of course, this is only an approximation. It varies somewhat - depending on the type and make of the toroid. 

But generally - that is what you can reasonably expect. As for the Current Density - assume it to be not bigger than:

Density = 2.5 Amperes per square millimeter of the cross section area of the wire.
Knowing the amount of current that you will be pulling off this winding - you can easily calculate the required minimum diameter of the wire.

Just to remind you: the area of a circular cross section of your wire is equal = Pi x Diameter x Diameter / 4. ( ... because Diameter = 2 x Radius ... )
Therefore: Diameter = SQRT( 4 x Area / Pi ).
BUT: Area = Current / Density
Therefore: Diameter = SQRT( 4 x Current / Pi / Density ).

Of course you can go with a thicker wire. 
But if the winding is to stay cool ( in the sense of temperature ) - avoid using a diameter that is less that the one you calculated here.

Once you have all your extra windings in place, you probably should wrap an extra, "exterior" layer,
with some heavy duty tape, so as to provide the wire with some protection against external mechanical injuries that it may be subjected to in the future.

You will also use some sticky tape to secure the endings of your windings, before routing them "to the outside". 
If these are thick wires - you can route them to the outside simply "as is", probably inserting them in some additional protective sleeve, like pieces of polyvinyl insulation that you took off some other wires, that were de-insulated ( at some other time, other project...).

If the wire used for the winding is thin - you should probably solder some more hefty, stranded leads to these fragile endings of your winding, and then secure these leads (as by making a half knot and heavy duty sticky tape). Then - route these flexible leads to the outside of the toroid.

When you will be "winding" the windings, which essentially comes down to threading a piece of long wire through the middle hole within the doughnut, please take care to evenly distribute the individual turns around the whole perimeter of the doughnut. 

At best, if you manage to make the "beginning" and "end" of the winding .. meet at one and the same region of the perimeter,  so as to conveniently secure the leads, route them to the outside, as a "pair" ... and affix them with some heavy duty tape.

Oh, By the way ... there's a trick which you can use, so as to minimize the amount of wasted wire.
Start making the winding from a length of wire which is about 80% ~ 90% of the length that is actually needed.

First - calculate the average length of one winding. Simply take a shoelace wrap it once around the core.

Now: mark it, remove it and measure the resulting length with a ruler. This is your "single-turn-length".
Multiply this single-turn-length by the number of turns that you need to implement.
OK, OK, OK, ..... but how do I know, how many turns of wire am I supposed to apply, in order to achieve the necessary ''target" voltage ?

To determine this, I suggest that you take some spare length of wire, and make exactly 10 windings. For the sake of a test. 
Now, switch the toroid on, into the mains, and measure the resulting voltage that you obtain on those ten windings. 
Divide this result by ten - and you have the "voltage increment" of a single turn.
Now, you can simply divide your requested voltage by this number - and you have a good idea of the number of turns that you need to apply.

Now, multiply this "number-of-required-turns" by the "single-turn-length", and you will come up with the necessary total_length of wire to make the winding.

And here it comes ...
Now, calculate what is 80%~90% of this "total-length", take exactly such a length of wire off from your drum, thread it through the middle of the doughnut and start threading your turns, until you use up the whole length of this wire (from this end). 

From time to time - during the winding process - you can take a break, connect the toroid to the mains and check what is the current reading of the voltage achieved thus far.
Just to be in the "know".

Please observe that when using this method, up until now, it was **NOT NECESSARY** to cut the wire off the drum, as you were working only with the "loose-end". 
The toroid is ** STILL ** connected to the drum reel - the wire is still a single continuous length, and you still have the option of taking "more" wire off the drum reel, as there is no such thing, as yet, as the "other end" of the wire - you still are hanging off with the wire off that drum reel. The wire is still not cut.

As now you are probably in the whereabouts of 80%~90% of the "target" voltage - use a voltmeter, measure the outcome, calculate the difference - i. e. - how much is still missing, in terms of "missing" voltage, and then recalculate that into the "missing" length of wire.

Now you can fairly accurately and confidently take the missing length of wire off from the drum, add to that some extra slack, and now you can confidently cut off this length off of the drum.
Now you shall "finish" threading the winding by working with this "newly established" other end of the wire. 
Keep on threading, winding, occasionally testing the voltage, until you achieve your desired "target" voltage.

Keeping in mind that this is a voltage that you are measuring under "no-load" conditions,
you may wish consider adding some extra turns as "spare", so as to compensate for the anticipated voltage drop that is bound to be observed, once the winding is operating under real load conditions.

This is a nice trick, as described above, enables you to avoid wasting too much wire.
You have a better chance of cutting a length of wire that is actually exact and "what is really needed", with no extra crazy slack.

So. I hear those voices that this method is of limited use - because we are limited to low voltages only
(in practical terms - due to the count of windings necessary to achieve a higher voltage).

Well ... not exactly.

This method may also be used to make a correction of an existing "anode" voltage, as supplied to a tube amplifier from a mains toroid,
especially if you have an off-the-shelf toroid, which unfortunately is not optimal, as it has a voltage slightly too low, ..... or (YES!) .... slightly too high.

What if I were to tell you that your DIY extra winding is not a "Voltage" ... but a "Difference" ?

You can treat your DIY winding as a small "delta" to the anode voltage, and it can work BOTH ways.
This delta can be either ** ADDED ** to your anode voltage winding,
or it can be **SUBTRACTED** from your anode voltage winding.
It is just an issue of how you connect the two windings in series together.
In the former case ("addition") - you connect them "in-phase" (... i.e. the end of the anode winding to the beginning of the delta winding).
In the latter case (subtraction) - you connect them in "anti-phase" (... i.e. the end of the anode winding with the end of the delta winding).

Such a "compound" winding - now delivers exactly the proper "target" voltage.

Perhaps such a "subtraction" of the delta voltage is not too elegant a solution, and some may consider it as a dirty trick, but actually, it perfectly does the job.
And hey, after all .... the resistors that you have there in your High Voltage DC filter, behind those HV rectifiers, ...
... these resistors are of two or three orders of magnitude bigger in value than the lousy one or two ohms, by which the impedance of the compound winding increases. These few ohms extra - believe me, they do not make any difference .

As an inter-winding insulation - I highly recommend that you seriously consider kitchen baking paper - like the one that women use to bake a cake. 

You can take a roll of such paper, and cut it with a bread knife - slice off a whole section off that roll - 1cm wide - and use this roll of baking paper (a strip) to make the inter-winding insulation.
This is for voltage protection, so that the voltage of one winding, possibly at a different DC potential than the voltage of the adjacent winding - are well separated and insulated from each other.

Such baking paper is very thin, but at the same time - in terms of voltage insulation - it is very efficient.
Use the paper strips to completely cover the preceding (and completed) winding that is already on the toroid.
Try to achieve a uniform cover, of no less than two layers of paper. 
Obviously, the bigger the anticipated DC offset voltage difference between the windings - the thicker you go with the layers of the paper.

If this insulation is to endure more than a few hundred volts - it might even be worth it to moisten, or soak, the paper in transformer oil. 
But that tends to turn out to be a total mess ... I did not use the transformer oil - kept it all dry.
Now I am using the toroid, and as yet - nothing popped (so far).

And mind you - I have some distinct voltage potential differences between the windings.

Have fun at extending the functionality of you existing toroid.

Best Regards,

Matej Isak. Mono and Stereo ultra high end audio magazine. All rights reserved. 2006-2013. ..:: None of the original text, pictures, that were taken by me, links or my original files can be re-printed or used in any way without prior permission! ::..