Output Transformers, why are they made like this?

[Kalee20]

In a single-ended transformer, the primary carries DC which causes a temperature rise all the time, whereas the secondary only generates a bit of heat within itself when there is a signal. (When there's a signal, the primary power loss rises too, because the RMS value of the primary current rises - but it's probably negligible).

A decent output transformer should hardly get warm anyway, it turns out that functional constraints dictate a minimum size. such that temperature rise is not a concern.

If you're seeing blackened windings, I'd suggest it's down to insulation failure, and a steady (and high) drain from HT+ through the breakdown, to the core.

[Mike Watterson]

A leaky grid capacitor.

[boater sam]

If you're seeing blackened windings, I'd suggest it's down to insulation failure, and a steady (and high)  drain from HT+ through the breakdown, to the core.

Which is why I considered winding the primary on the outside, away from the core.  The potential difference between the core and the secondary will be negligible.

 I'm going to try it at my next winding session.

[Mike Watterson]

The primary is better at the core due to shorter turns and easier to wind the secondary on top. Almost all failures in valve o/p transformers are due to excess current. Either from a "tone correction" cap or tone control cap from anode to 0V failing (which is why more often across primary) or the o/p valve grid coupling capacitor failing or on old battery sets the grid bias battery disconnected or dead.

[Kalee20]

Which is why I considered winding the primary on the outside, away from the core.  The potential difference between the core and the secondary will be negligible.

 I'm going to try it at my next winding session.

Do! It'll still work.

But, granted that the pd between secondary and core will be negligible... you've still got the pd between primary and secondary! And that won't go away.

(Connect it so that the primary start goes to HT+ and the outermost of the winding, in free air, goes to the anode, so that the high peak voltages are well away from everything).

[Ed_Dinning]

Hi Gents, lots of valid points raised.
Blackening of the primary can certainly be due to leaky caps etc, but can also be caused by "green spot" corrosion and leakage between turns due to paper insulation and single insulated wire being used.
Much better to use modern Grade 2 (double insulated) wire and omit the interlayer insulation.

Note that on high spec transformers, used on amps with feedback applied, problems can be caused if the winding sequence is altered due to varaitions in interwinding capacitance


Ed