Some notes for making one-off power and audio push pull plate transformers transformers using kraft paper and press-board.
Paper insulation in storage will take moisture according to relative humidity which will reduce dielectric strength
to essentially zero kV/mm and increase dielectric loss angle (tan_delta)
At 6 % water content , tan_delta is about 0.2 and that reading can be used to back calculate moisture content.
The insulation can not be used without drying then impregnating after the winding is wound.
For a small LV rectangular concentric windings, that was usually done by oven bake, vacuum down, vac-impregnate, then oven bake.
As paper structure deteriorates above 100 C, the windings will be limited to Class A service temperature rise.
Typical varnish was thermosetting like the well used Isonel 51 and 31. Although these alone have a dielectric strength of more than 100kV/mm,
the combination of impregnated paper will be a lot less, as it is impossible to remove all the moisture before impregnation.
For a bobbin made out of pressboard, it has to be strong enough to withstand the winding process, and not distort in service.
The core of the bobbin will be at winding temperature ( Class A).
In such a transformer, interlayer insulation (within each winding) was usually not used, with enamelled wire insulation supporting the voltage. ( These transformers do not have impulse voltage ratings, nor short circuit forces.)
So,for the plate windinges, the enamelled wire insulation grade should be selected for more than say, 3 times the plate DC voltage
The kraft paper would be used between windings, that is, between primary and secondaries.
Usually at least two wraps would be used. Excessive insulation leads to higher leakage reactance.
A possible failure mode of push pull transformers is when the secondary is accidentally run open circuit by disconnected speakers.
This can cause interlayer flashover within the audio push-pull plate winding.
Maybe a final note is that by changing to a fibreglass bobbin and Nomex interlayer, most of the above difficulties can be avoided for the hobby builder.
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