Testing an Output Transformers

[dreric]

I have a couple of output transformers I'd like to test before I drill holes in the chassis.

I seem to remember a post telling how to do this or maybe it was a link but now I can't seem to find the source.

Does anyone have a good method to test OTS?

Thanks
Eric

[marcoloco961]

I have a couple of output transformers I'd like to test before I drill holes in the chassis.

I seem to remember a post telling how to do this or maybe it was a link but now I can't seem to find the source.

Does anyone have a good method to test OTS?

Thanks
Eric

I usually check continuity across the leads first to make sure there is no break in the windings. Then I apply a small A/C voltage to the primary. From there Vin/Vout = V+I ratio. From there square the V+I ratio to get your impedance ratio. Then multiply this by the speaker load rating to be used to get the primary impedance.

Example from recent tranny.
Vin = 11.49
Vout = .372 Amps, or 372 mA.
11.49/.372= 30.89 (or V+I ratio)
30.89*30.89= 954.2 (or impedance ratio)

using a 4 ohm speaker, 4*954.2 = 3,816 ohms or 3.8K
Using a 8 ohm speaker 8 * 954.2 = 7,633 ohms, or 7.6K

There are other tests you can do for different safety checks and rates. Hope this helps.

[Andy Le Blanc]

Aiken has a good little write up...

You need a signal generator, a dmm, and a calculator.

say you apply 1v Ac to an opt sec. and see 20v across the pri.,

thats a 20:1 turns ratio, or a 400:1 impedance ratio

plug in a 8ohm load it becomes a 3200:8 ratio which means the the power tubes are
seeing a 3.2k plate or anode impedance reflected by the load.

If I remember, if your plotting a ep ip you want to use 1/4 that value.

This is a static figure, dynamic results based on min and max plate currents will be different.

[Dr-Joned]

It is better to use a small transformer (ie 12vac) to determine the voltage ratio rather than a speaker to measure current. The reason being, that the speaker connection is not a true 8ohm, but nominal. The speaker could be 50 ohms at a rated 60hz and this would surely throw off the measurements.

[selloutrr]

http://www.geofex.com/ampdbug/outtrans.htm

[dreric]

Thanks for the help!!!

Eric