Output transformer re-winding question - where best to bring out the primary ends?

[Voxophone]

Since I have no experience at all of rewinding transformers, I’ve pretty much been ready to follow accepted practice. I also don’t see why insulation strength between ‘slipped’ turns would be insufficient with modern materials, but I don’t want to have to rewind the transformer twice!

Incidentally, the AC91 transformer primary only drops 15V between start and finish (275V h.t. to 260V anode). Surely bringing a turn at 275V near to one at 260V wouldn’t be too great a cause for concern as far as insulation goes, or am I looking at this the wrong way?

Regarding wire gauge, from the ratio between the resistances of the design and new sizes, I estimated about half the original primary resistance (i.e. 250 Ohms). As far as I’m aware this should not be too much of a problem, but if it is I could always add a series resistor to make up the remaining resistance.

[dominicbeesley]

As I understand it the transient voltages in the primary circuit of an output transformer can be a lot higher than the supply voltage which is why a layered, insulated construction should be employed (and why the tone capacitor is often a 1000V part).

I also didn't "see" the need for extra insulation, until my first attempt at a frame output transformer running on 50V burnt through its insulation in about half an hour...

It might have a few eccentricities but the Babani book https://www.amazon.co.uk/Coil-Design.../dp/0859340503 is a handy companion and can be read through quickly to get the ideas. The data are handy for rule of thumb stuff.

[kalee20]

David is generally right. Modern enamels are pretty good.

Capacitance issues caused by turns of one layer being adjacent to turns of another, with quite a lot of volts between, can be a disadvantage, but in an AM radio this may be a non-issue. Having said that, if you are going to the trouble of rewinding, you might as well do as good a job as possible (within reason!)

[Voxophone]

Originally Posted by trobbins

Would you wind each layer to the insulation edge? Margin tape may be a help, and avoid your concern about varnishing each layer perhaps. Any extra media such as varnish will increase stray capacitances.

Since the original design had media between the layers in the form of paper, would it not have had an increased level of stray capacitance anyway?

As to the calculator, I stand to be corrected, but I think all calculators have that facility.

For RPN calculators it is 1 enter enter then + + +...

[Ed_Dinning]

Hi Gents, my normal practice with this type of transformer that was probably "stick" wound would be to make up paxolin end cheeks and fix with araldite.
Insulate between pri and sec with type 56 tape, but no paper is needed on either winding.
I normally use grade 2 wire (double enamel coated and solderable)

Ed

[Voxophone]

This is the wire I have: http://www.brocott.co.uk/0-125mm-36-...lderable-250g/

It's described as single coat. Is this likely to cause a problem?

[TrevorG3VLF]

Originally Posted by Voxophone

Since I have no experience at all of rewinding transformers, I’ve pretty much been ready to follow accepted practice. I also don’t see why insulation strength between ‘slipped’ turns would be insufficient with modern materials, but I don’t want to have to rewind the transformer twice!

Incidentally, the AC91 transformer primary only drops 15V between start and finish (275V h.t. to 260V anode). Surely bringing a turn at 275V near to one at 260V wouldn’t be too great a cause for concern as far as insulation goes, or am I looking at this the wrong way?

If the HT is at 250V, the valve will pull the anode down to say 50V. When the valve turns off, the voltage will rise to 450V. Thus interturn voltage can be quite high.

If there is no load or a very light load, then the voltage on turn-off could rise very high, even to sparking across the terminals or in the valve.
Always run a valve amplifier with a resistive load.

Do not add a resistor to get the resistance up to the normal level. All it does is waste power. You want minimum resistance.

A well designed transformer will have an equal area for primary and secondary to minimise total resistance.

[kalee20]

I've looked at the link to the wire type. It should be OK. Temperature rating is good (incidentally, 180 degree wire, although solderable, needs a really hot iron).

However, I am still worried about using 0.125mm wire!

I note from your earlier posts that you want to get it right first time and have no intention of doing it again. I think you might be optimistic there! Expect to start with the best intentions, but make a few mistakes, and end up thinking that you might as well finish it and try it but resigned that you will need to strip and rewind it, having worked out how you SHOULD have gone about it. But at the expense of a few hours, it'll be worth it!