How do you determine primary impediance of an output transformer?

[gtrbryan]

I have a few old OT's.  I do not know their primaries or secondaries.  I have seen on the net how this can be done with a variac.  I don't have one and really don't want to acquire one if I don't need to do so. I thought I remembered, years ago, seeing a video of someone checking an OT with a 9v battery.  Am I way off or can that be done.  I have both single ended and push pull transformers to test.  Any suggestions are appreciated. 

[PRR]

You can figure turns-ratio and impedance ratio. NOT impedance.

Transformers are levers. My prybar can work with small nails or large nails. But much less well with railroad spikes, or for pulling splinters from my finger.

So you can estimate a *range* of probable impedance, but not the impedance it was designed for.

> OT's.  I do not know

It is a bad habit to "salvage iron" without keeping PTs and OTs together, with notes on tube-type and anything you can figure about load.

[HotBluePlates]

I thought I remembered, years ago, seeing a video of someone checking an OT with a 9v battery.  Am I way off or can that be done.

You might see someone test the polarity of a speaker's terminal with a weak 9v battery, but since it is d.c., it won't help you figure out anything useful with an OT, as those work with a.c.

You'd like to apply a volt or two of a.c. to the secondary, and measure the resulting voltage on the primary. The voltage ratio will equal the turns ratio. Say 1vac on the secondary yields 10vac on the primary. 10v/1v => 10:1 voltage ratio, primary to secondary.

The impedance ratio is the square of the turns ratio. 10 squared is 100, so the impedance ratio of our transformer is 100:1.

The primary impedance is the value of the load attached to the secondary multiplied by the turns ratio. If we hook an 8Ω speaker to our secondary, 8*100 = 800Ω will be the reflected primary impedance.

[Colas LeGrippa]

PRR : ''So you can estimate a *range* of probable impedance, but not the impedance it was designed for. ''

Question: if an OT has 3 taps :  20:1 ratio,  28:1 ratio and 40:1 ratio, it is clear that the impedance it was designed for is 6.4K

because 20 squared= 400, 28 squared= 800 ( almost ) and 40 squared = 1600, by hooking respectively a 16ohm, 8 ohm and 4 ohm load, the load reflected on the primary is 6400, no ?

It will work with different tubes and voltages probably, but it was designed for 6.4K, or am I dumb ?

[kagliostro]

@ Gtrbryan

I thought I remembered, years ago, seeing a video of someone checking an OT with a 9v battery

I think you refer to an old trick to test transformer winding as to discover if there are shorts
see attached image

@ Colas

I think that you can't be sure at 100% but what you say is reasonable and the % that your conclusion is correct is very high

K

[Colas LeGrippa]

@ Colas

I think that you can't be sure at 100% but what you say is reasonable and the % that your conclusion is correct is very high

K


  I am dumb, then ?

[gtrbryan]

Thanks for all the suggestions.  I will try searching the numbers  but i am not optomistic.  These look old old old.  Some harvested by me and some given to me already harvested.  Might justify a variac purchase if i can get a couple usable ot out of the deal

[Colas LeGrippa]

You do not need a variac. Hook up a heater transformer to the secondary taps and see what happen in the primary. Dividing the voltages will give you the turn ratio. Suppose you have 7 v on one side and 140 on the other, that means you have a 20 turn ratio on this tap. Square= 400. Continue the same procedure with other taps. You will find that the turn ratio squared multiplied by 2 or 4 or 8 or 16 will give exactly the same primary impedance of, let's say 4,3k. This is your tranny primary impedance, that's it that's all. So, by hooking different speaker impedance to the corresponding tap, you always get the same primary impedance. So you can hook up different speaker impedance to your amp, and by switching the amp to the right speaker impedance giving you the perfect match to the output tubes. I hope this is clear......it's difficult for me tonight to write in english.

Colas

[PRR]

> a 16ohm, 8 ohm and 4 ohm load, the load reflected on the primary is 6400, no ?

4-8-16 is a very common configuration, yes.

But we sometimes see 2-4-8, or occasionally 8-16-32.

And a 150-200-600 connection is possible on a "line" transformer.

There's also "voltage rated" transformers for multi-speaker long-line work: 25V, 70V, 100V, 140V. Your proposed ratios work out as 1-1.4-2, so could be a 70V-100V-140V winding.

So you need an additional clue.

[kagliostro]

First I'll try the way Colas proposed, may be you're luky

if problems (I don't recognize usable impedances) I'll go on for further clues as suggested by PRR

K