Vintage PT newbie question

[lumox0013]

newbie question but using vintage iron primary rated for 117v wall voltage here is 124v is there an easy way to drop it to 117v so my sec. and fillament v. come out right? trying to recycle. any help is greatly appreciated

[paulster]

I really wouldn't worry about it. You're 6% over spec at that voltage and heaters are generally rated with a voltage tolerance of +/-10%, so I'd just run with it.

[lumox0013]

my heater supply is running at 7.4v and my sec. which should be 300-0-300 is running 350-0-350 seems high enough to make a difference + I have had this problem before just checking with those more enlightend than I . thanxx for the feedback

[Wayne]

http://www.geofex.com/Article_Folders/v ... ntvolt.htm

W

[paulster]

Yep, that's very high! Are those loaded voltages, or unloaded?

[lumox0013]

unloaded,loaded the fil. drops to 6.9 which seems safer but as I said I've run into this problem before and was looking for a fix-all ,that wayne was nice enough to provide. thanxxx wayne exactly what I was after. the wealth of experience and imformation on this site never fails to amaze me thanxx to all you guys for inspiring some one who for years thought it not to be posible to build your own tube amp, though the quality isnt there yet I have built many amps now .still practicing. I have listeind to many clips on this site and hope to follow on you alls path to tube heaven. thanxxx again for all the help guys!!!

[Structo]

That is a 6% difference in primary voltage.

I thought that this ratio was constant?

In other words shouldn't the secondary voltage be 6% higher than 300v?

So it would be 318v?

Heater would be 6.7v?

What am I missing? Or is it because there is no load?

[paulster]

unloaded,loaded the fil. drops to 6.9 which seems safer but as I said I've run into this problem before and was looking for a fix-all

The heater winding is still pretty high, which makes me think it was designed for a higher current draw than it's seeing at present, which would have brought it down further.

In this case the vintage voltage adaptor is probably worth the effort.

That is a 6% difference in primary voltage.

I thought that this ratio was constant?

In other words shouldn't the secondary voltage be 6% higher than 300v?

So it would be 318v?

Heater would be 6.7v?

What am I missing? Or is it because there is no load?

With no load the voltages will go up.

Transformers, being lossy, will sag under load. So manufacturers have to work out what the typical load will be, what the losses will be and set the voltages for this operating point. This means that unloaded they will be higher.

It's one of the skills of transformer winding.