Measure tube amplifier output transformer impedance

A tube amplifier output transformer is a crucial component in vacuum tube-based audio amplifiers. It's responsible for matching the high impedance of the vacuum tubes to the low impedance of the speaker, as well as transferring the audio signal from the tubes to the speaker while maintaining fidelity.

WARNING! - The project described in these pages utilizes POTENTIALLY FATAL HIGH VOLTAGES. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.

Imagine you have salvaged one from an old amplifier but you don't know which is the primary side, which is the secondary, and what's the impedance of that transformer.

Finding the primary and secondary side is the first step. You just have to measure resistance between the two wires of each side. In this type of transformer primary side winding has a greater resistance then secondary side.

Once you know which is the primary side you have to measure the impedance of the transformer, this is quite important cause you have to match the impedance of the power tube.

I you take as example a 6V6 tube and take a look at the datasheet, for a push-pull class AB amplifier when the plate voltage is 285 the effective load resistance (plate to plate) has to be 8000ohm. So our aim will be matching that impedance.

We know that the ratio between primary and secondary impedance is equal to the square of the ratio between the turns on each side. So we have to find the turn ratio.

Zp/Zs = (Np/Ns)^2

To find the turn ratio the way I use it to load the primary side with an AC voltage, I use pretty high voltage cause the ratio will lower the voltage down on the secondary side, using an higher voltage makes reading with a standard multimeter more accurate.

So to find the turn ration you just have to measure voltage on the primary side and then voltage on the secondary side, this ratio is equal to the turns ratio.

I prefer to measure the secondary side voltage under load. For my approximation it's enough a resistor even if it's preferable something that is not a "pure" pure resistance load, so a speaker as example. Then I made the mean between the voltage on the secondary loaded and not loaded. So summarizing the procedure

1. Apply an AC voltage on the primary side. Use an high voltage source, like your main amplifier transformer output. Also to be sure not to break anything, put a fuse on the output of your main transformer. I use a 300V AC with 200mA fuse.
2. Measure the voltage on the primary side. Note that it will not be the same voltage of your main transformer output, cause this voltage will drop due to the resistance between the turns. Let's call this value Vp.
3. Measure the voltage on the secondary side without any load. Let's call this value Vs.
4. Measure the voltage on the secondary side with a load with impedance Zs. Let's call this value Vsl.
5. Compute the turns ratio. Let's call this value K. K=Vp/((Vs+Vsl)/2)
6. Now you can find your Zp impedance with the secondary side is on the load of Zs. Zp=Zs*K^2

This way you can match your output transformer with your tube specs.

Notes

• read risk disclaimer
• excuse my bad english