Re: Winding a Filament Transformer
Sulaiman, Thu Jun 08 2006, 09:10PM

A 'rule of thumb' is 2.5 A / sq.mm for transformer winding.
(1.5 to 10 A/sq.mm depending on circumstances)

but specifically, decide how many turns are required
and use the thickest wire that will fit the winding area.

Operating the primary with a little saturation gives some regulation
and protection from severe overvoltage.

Re: Winding a Filament Transformer
cbfull, Fri Jun 09 2006, 07:46PM

Do you mean design it to operate a certain margin below saturation? Not familiar with all tranny tech talk, but I thought you were supposed to design it for a higher voltage than what you intend to supply? Sorry if this is exactly what you mean.

Also, if you are powering an 811a or the drop in 542b (or whatever, I can't remember it's number), I would recommend preparing for add-ons. Maybe wind it with two wires instead of one, in case you later want to add more tubes.

Just a thought.

Re: Winding a Filament Transformer
Sulaiman, Sat Jun 10 2006, 09:12AM

Winding for higher secondary (filament) voltage than required is to compensate for "regulation"
Basically it's to compensate for IxR voltage drop in both the secondary and primary windings.
A typical "regulation" figure is 10%
For a 'typical' transformer about 5% is 'lost' in the primary windings and 5% in the secondary windings.
So for a 6.3 Vac filament I'd expect to measure about 7 Vac no-load.

As the line/mains input voltage rises so will the secondary voltage (it's a transformer)
If the primary runs slightly saturated at nominal input voltage (e.g. 230 Vac)
then if the line voltage increases the secondary voltage increases less due to saturation.
This way you get maximum power for a given core size
with a degree of over-voltage limiting.
The transformer will run hot and will buzz/humm quite a bit.