"Impedance" of the output transformer

[tatchoune]

What I'm telling here is very little known: all dealers output transformers for tube amplifiers exhibit the characteristics impedance input, eg 8000 ohms and 4-8-16 ohm output.

A transformer is only one ... impedance transformer, that is to say there is no question of the proper impedance of the windings, the impedance presented by the primary is equal to the connected load on the secondary multiplied by the square of ratio.

An example of a transformer with a transformation ratio of 20, its input impedance will be:

•Zp = 20 000 ohms pour Zs de 50 ohms Zp = 20 000 ohms 50 ohms Zs
•Zp = 10 000 ohms pour Zs de 25 ohms Zp = 10 000 ohms 25 ohms Zs
•Zp = 3 200 ohms pour Zs de 8 ohms Zp = 3200 ohms 8 ohms Zs
•Zp = 1 000 ohms pour Zs de 2,5 ohms, etc Zp = Zs 1000 ohms 2.5 ohms, etc.

Obviously, it is extremely important to respect the speaker connections to the proper terminals, otherwise it is completely the tube output stage door-to-fake!

[Gilgy]

Originally Posted by tatchoune

What I'm telling here is very little known:

Let me stop you there ....

[danFrank]

I don't know if I understand your statement correctly, but, yes, transformers are impedence matching devices.
But, the ratings of the transformer (example: 4K primary to 4-8-16 ohm secondary) is where the transformer in question works at it's best. IE. Best frequency response, lowest loss, etc...
Daniel

[DF96]

I'm not clear exactly what you are claiming as your insight that is "very little known" by others. The idea that, to a first approximation, a transformer merely transforms impedances is very widely known yet that is what you appear to say. Have I misunderstood? Or have you just discovered this for yourself and feel we might need to discover it too?

[Mr. Zenith]

Huh? (!)

[DF96]

Let me float an idea. OPTs are often described as, say, matching 8k to 8 ohms. The OP has just discovered that there is no such thing as an "8k" transformer, it would do 6k to 6 ohms almost as well. As it is a revelation to him, he wishes to share this with us.

If I can give some advice, such things are better shared in the form of a question; to check that the new understanding is correct, and to see if it is already widely known.

[kavermei]

You'll have to pardon my countryman

Of course, and I think most of us here know this already: many design factors do indeed depend on the absolute impedances for which the transformer is designed. Daniel already said it. Just think of the number of windings, for example.

The transformer will sort of work at other impedances, but performance will not be optimal.

Kenneth

[tatchoune]

I said a transformer hasn't its own impedance, the impedance on the primary depends of the impedance connected to the secondary.
For example, for à 8k/16-8-4 if there is 8 ohms on the secondary, the impedance is 8000 K on the primary. The ratio is +- 32. I then we connect a load of 6 ohm on the secondary instead of 8 ohms, the impedance of the primary becomes +- 6000 ohms. The impedance of the primary changes and the tubes are badly loaded.
Many people believe that the impedance always stays 8000 ohms because it's a caracteristic of the transformer and it's false.

[Wavebourn]

Actually you have reflected impedance in some frequency band. It is the basic function of transformers. Out of this band inductances and capacitances play more and more their role. Also, on lower frequencies, the lower they are, the more distortions you get from core non-linearity. Also, the lower impedances you transform, the closer they are to resistances of own transformer's wires. That means more of thermal losses in the transformer.
Anyway, the transformer is a device that is not ideal, and designing them and their usage you always have to think about equilibrium, in order to achieve optimal results.

[danFrank]

Originally Posted by kavermei

You'll have to pardon my countryman

Daniel already said it. Just think of the number of windings, for example.

The transformer will sort of work at other impedances, but performance will not be optimal.

Kenneth

I LOVE it when someone takes note of my vast knowledge. Thank you, Thank you, Thank you!!!
He he he...