Winding an output transformer for PL36 push pull

[boater sam]

I'm putting together an amplifier using all TV valves.
Push pull output with PL36s, got loads of them.
The question is, do the primary windings of the transformer get wound on the bobbin/s in the same direction relative to the core or are they wound opposite ways, if so why?
I have never wound a push pull transformer hence the query.
To reduce the capacitance this will be wound on 2 bobbins, half the primary and half the secondary on each bobbin with the windings cross connected in series. I assume I connect end of one to start of the other?
Been reading books on how to do it but can't find answers to these questions.

[ppppenguin]

Same direction, that'e end of 1 to start of other. It's intuitively obvious that this should be so but I can't quite work out a way of putting it in writing. As for the construction of the winding that's one for Ed Dinning. I worry about putting each half on a separate bobbin because you also want best coupling between the 2 halves.

[Terry]

If you consider your primary to be identical to the HT secondary of a mains transformer, it might help the understanding.

If you have, say, a 250-0-250 volt winding, it can also be looked at (or even used) as a single 500V winding with a centre tap so the two halves must be wound in the same direction for that to be possible.

In normal use, the centre tap will be the common, earthed, point. Then, on one half cycle of the mains input, one end of the secondary will go positive relative to the centre tap and the other end will go negative. On the second half cycle, the polarities will reverse.

An output transformer works in exactly the same way as a mains transformer but in reverse. Because of the phase splitting arrangement feeding the output valves, the anode current through one valve will be increasing whilst the current through the other valve is reducing - ie: like the HT winding on the mains transformer, the current flowing in each half of the primary and hence the voltages induced across the two halves relative to the common centre point  which, in this case is connected to HT+, will be in anti-phase exactly the same as before with the situation reversing on each half cycle.

Does that help?

[Refugee]

I have some old photos of one that had a wire come adrift inside it. I opened it up to take photos for a quote to get it rewound but ended up fixing it myself.
You can see the connections for each half of the primary and the secondary turns.
The primary is clearly wound half under the secondary and half over it for good coupling.
Capacitance should not be a problem at audio.

[Kalee20]

Capacitance should not be a problem at audio.

It is though - if you have enough turns for the lowest frequencies (enough turns to not saturate the core, or enough turns to have sufficient inductance, whichever figure kicks in first), then there's usually enough capacitance to start shunting the signal at the highest audio frequencies.

Careful winding, maybe with some interleaving between layers, can help here, although it hurts other aspects.

And, if you have negative feedback around your amplifier, then capacitance in the output transformer is bad news anyway as it can cause instability and oscillation. Never mind whether it is audible or ultrasonic - you just don't want it! Taming it usually involves putting a single dominant HF roll-off somewhere, and if the transformer capacitance/load resistance has a roll-off at not far above AF, then you may have to introduce something within the audible frequency range, to kill it.

[boater sam]

Thank you all for your answers, I now understand that all windings should be in the same direction on the core.

I consulted Langford-Smith Radio Designers Handbook, it is anything but a hand sized book, hernia opp next week As I understand him when he pontificates over segmented windings in audio transformers, by splitting the windings, effectively the capacitances are in series therefore overall smaller.

He seems to think it is a good thing.

[Refugee]

With most amps I have seen there is a filter in the negative feedback network with high pass to get rid of the unwanted ultrasonic component by reducing the active gain at those frequencies.

[ppppenguin]

Dominant pole is a perfectly good way of ensuring that an amp with NFB is stable. Unfortunately it needs a fair bit of spare bandwidth to work. If there's anything odd about the freuqncy and phase response above the wanted frequencies it's all too easy to get an oscillator, or at least a very underdamped loop. Refugee's last post, suggesting a high pass section in the feedback puts in a zero at some frequency above the wanted range to counteract such problems. If the forward path isn't utterly horrendous it's should always be possible to design a NFB loop that gives good results but it might need care and/or complexity.

Those of us who've mainly worked with DC coupled designs mustn't forget that the feedback will also have stability criteria at the low freuwncy end too. Again mainly due to the characteristics of the OP transformer.

[Ed_Dinning]

Hi Sam, Kalee has summaries it correctly.
There was a design for side by side bobbins in PW in the 60's but it was complex and probably didn't work too well.
Sectionalising is a good technique, but is expensive to do commercially. If you are going for a UL connection then you can wind A1 (Sec1) Sc1 - (Sec2) Sc2 (Sec3) A2. Join the ends of the 2 screen sections for the CT. This technique can be used for different speaker impedance tappings, but all windings should be in circuit at all times by series/ parallel connection.
There is lots on the web about this, but generally there should be N sec windings and N+1 pri windings. The famous Williamson amp had about 14 pri windings alone.

Ed

[reset]

I'm putting together an amplifier using all TV valves.
Push pull output with PL36s, got loads of them.
The question is, do the primary windings of the transformer get wound on the bobbin/s in the same direction relative to the core or are they wound opposite ways, if so why?
I have never wound a push pull transformer hence the query.
To reduce the capacitance this will be wound on 2 bobbins, half the primary and half the secondary on each bobbin with the windings cross connected in series. I assume I connect end of one to start of the other?
Been reading books on how to do it but can't find answers to these questions.

   


Sam
If you follow the link and download the book (first line in post) and go to page 33 he give details of winding a transformer with split sections.
http://radio.radiotrician.org/2016/02/co...n-for.html
73
Greg