Homebrew Toroidal Output Transformer

[Osvaldo de Banfield]

I want to share some comments about an experience I´m doing.

I´m doing an EL34 class A UL toroidal output transformer, based on some readings from Menno van der Been, but made of my own. I had done the following try: I buy 3 commercial illumination toroidal transformers, 220V to 12V 25Amper, the biggest I could find. I measured all parameters before unwind, primary resistance, inductance, leakage inductance, etc. So, I unwind two of them, and measured turns, core dimensions, so I could estimate core parameters as, power capacity, inductions, etc.

I done a new winding re-calculating the new cutoff frequency based on induction, cross sectional area, peak voltage between plates, etc. I hand wind equally spread around the core, and in 7 turns one over the others to complete circumference of the transformers, but something was wrong. About 8KHz, the -3dB I could find. The windings are done bifilar about 1000 turns between screens taps, and 1500 more from tabs to plate, a total 40% voltage ratio for screen. I believe that winding in such way, the total capacitance inter-winding is too high. So, I´ll try another form. I want to subdivide the circumference in 10 sectors, separate them by a spacer, and wound single wire in each section. In this way I can have the total 3000 turns in sectors phisically isolated, and capacity must be lower. Two sections will be for the screens (600T) and 3 sections more for the plates (600T +900T = 1500T).

Any suggestions or ideas are welcomed.

[Kay Pirinha]

Hi,

bifilarly winding the primaries always will lead to large winding capacitance unless it's an unity coupled transformer according to F. McIntosh. You'd better wind only one wire at once.

Another aspect: Did you provide an air gap to the core?

Best regards, Uwe

[flatheadmurre]

I hope it´s a PP transformer you´re trying to make ?

Another trick when winding toroids is winding some turns then going back some and start winding again.

The turns overlapping a bit.

[AndrewT]

I posted a note about a valve power amplifier that was redesigned simply because version 1 used a toroid for the output transformer.

It turned out that the toroid could not have sufficient inductance.
They issued the modified version as well as a kit of transformer to update the previous version.
The update was to use an EI transformer that could have sufficient inductance to allow the valve amp to perform properly.

[flatheadmurre]

For low Ra tubes like 6as7 a toroid power trafo works surpricingly good.

But brands differs very much.

Depending on winding metod used some works better then other.

it´s trial and error.

[Osvaldo de Banfield]

Originally Posted by Kay Pirinha

Hi,

bifilarly winding the primaries always will lead to large winding capacitance unless it's an unity coupled transformer according to F. McIntosh. You'd better wind only one wire at once.

Another aspect: Did you provide an air gap to the core?

Best regards, Uwe

No, it is designed for push pull class A operation. I buyed in web 2 matched pairs russian EL34 Mullard. And gapping a toroid is a not simple task !!!

[Osvaldo de Banfield]

Originally Posted by flatheadmurre

For low Ra tubes like 6as7 a toroid power trafo works surpricingly good.

But brands differs very much.

Depending on winding metod used some works better then other.

it´s trial and error.

Yes, I have triyed a 110V +110V power transformer driven by a couple of ECL82 and works fine, but it hasn´t screen taps.

[flatheadmurre]

I tried gapping a toroid using a wire arc disharge maschine but tension in the core made it warp.

After several "cuts" the gap still wasn´t parallel so SE toroids are for the Pros.

[geraldfryjr]

This is a very interesting thread !!

Bifilar winding will added much intercapacitance to the transformer.
It is a very bad demon even to the ESL community.
Have done I have done lots of study's on this, as well as a few others.

You should check out this thread on the subject of the using of ready made cores for audio (ESL's) where a particular core gets tested and dissected.

Antek Toroidal power transformer for Step-up, Measurements (part 1/2)

and this is what I have figured out about such bifilar windings,

Antek Toroidal power transformer for Step-up, Measurements (part 1/2)

As I am sure that you already know is that the bigger the core you have to work with the easier and better the low frequency performance you get using the least amount of turns.
Also using a larger core also allows you room to use a very thick insulating material between the first layer and the core and adjacent layers as well.
This will help too greatly reduce the inter-winding capacitance.

I too am getting ready to embark on a project similar to yours, Finally, as I am researching resources for some quality H.V magnet wire.

I have some cores in the 200 watt size that I am working with for now and my lowest frequency is only in the 180Hz to 300hz range so this makes things a little easier on the turns count for now.

At a later time I plan on a large full range transformer using some 2000 to 3000 watt sized cores from Alpha Core.

I am trying to size my wire so that I don't have to have any winding overlap at all but some I guess is inevitable.

Although may application is a bit more demanding due to the much higher voltages the Theory and Techniques are pretty much the same.

Even and complete coverage of the core area is what keeps the leakage inductance down.
So, Splitting the winding into different sections may (or may not) increase this.
This may work well using multiple alternate layers to help keep the DC resistance the same for two identical windings.
I am assuming that this is what you are referring to by splittng the winding up into several sections.

Anyway Good Luck with your project and I am looking forward to seeing your results!



jer

[LinuksGuru]

I designed and ordered from factory a pair of toroidal output transformers for KT88 push-pull 60W project.

1) You don't need bifilar winding, it will increase stray capacitance with no other benefits at all.
2) Audio toroids have very large stray capacitance unless you pay attention to the construction details.
3) Properly designed toroids for push-pull amps don't need air gap, neither fixed neither distributed. In fact, air gap would kill all advantages of toroid core. You can use software simulation with permeability curves and DC bias to evaluate side effect caused by output tube idle current imbalance.
4) Book written by Menno vad der Veen is completely useless if you are going to design your own toroidal audio output transformers. I don't have latest 2011 edition but previous was helpful as I described above.
5) Toroids are being designed with the same laws of physics as EI or double C core, but with different core/coil geometry, so you may simply adapt "standard" techniques.

I've got desired results only on 3rd run of samples. Need to say, it was quite complex, time consuming and expensive enterprise.