transformers/output impedance

Transformer is basically an impedance converter. To set proper conversion ratio we need to know speaker's impedance and recommended tube "plate load impedance". If we don't know speaker's impedance we can measure DC resistance and multiply it by about 1.25 (impedance is mostly resistive). Plate loading we can find in tube's datasheet (or plate-to-plate loading impedance in Push-Pull configuration).
We need our transformer to reflect speaker impedance (secondary side) to plate loading impedance (primary side). Transformer impedance ratio is a square of turns ratio.

Example: Plate load impedance is 10,000 ohms and speaker impedance is 4 ohms. Necessary transformer impedance ratio will be 2,500. Turns ratio will be square root of 2500 = 50. Voltage and current ratios will follow turns ratio. Secondary voltage change will be 50 times smaller than primary but secondary (speaker's) load current will be 50 times smaller on primary side.

As you can see it would require very big voltage changes on primary to get decent output voltage, hence power delivered to speaker. We can lower this ratio by finding tubes operating at lower plate loading impedance or to put tubes in parallel.

Thank YOU,
In your example, we say this
is a transformer 1-50? The final voltage, after transformer will decrease 50 times, but amperage will increase 50 times. The power (watts) will remain the same.
The same power, but with an lower output impedance?

160562 .... you are wandering in the same technical mind (pun intended) field that I found myself. What Kijanki said is spot on. You may find it helpful to retrace the steps I have taken to avoid the land "minds" I stepped on by pulling some of my posts.

One interesting post in particular dealt with "back impedance matching." IOW, output trannies work both ways. The output tranny steps down output tube voltage and impedance characteristics to the match speaker load. At the same time, a speaker's impedance characteristics, which vary as a function of frequency, are stepped up on the primary side of the tranny. This is the load the output tube "see."

As Ralph (Atmasphere) and Al (Almarg) have explained, output tubes operate best (i.e., low distortion and power delivery) if presented with a load (impedance) that falls within the tubes "sweet spot." Since speaker loads vary as a function of frequency, the output tubes may operate outside their sweet spot. If so, power delivery and distortion may suffer to some degree.

The optimum solution: if using a tube amp, (i) select speakers with a ruler flat impedance curve that corresponds to the output taps (e.g., 4, 8 or 16 ohms; preferably 4 or 8), (ii) )zero phase angle characteristics and (iii) high sensitivity. Good luck with finding a speaker possessing all of these characteristics at the same time ... and that sounds good.

Bottom line: life is about compromise and trade-offs. Ce la vie.

That's right, except for the losses in transformer. This impedance cannot be simply measured with ohmmeter because transformer does not operate at DC but behaves according to ohm laws for the CHANGES in voltage. It is called reflected impedance. Transformer has to be large enough to carry desired power. Cross section of the center of the core is proportional to square root of power and is roughly 1 square inch for 50W (for 200W it will be 2 sq inch etc.). We want as few turns as possible to avoid losses in copper but we have to provide minimum number of turns per volt. This number is inversely proportional to square area of the core and is smaller for large transformers. Knowing maximum voltage and turns per volt we can establish absolute number of turns. We select core material, size and number of turns to prevent core saturation, when magnetic flux is higher than max flux density rated for given core.

All this is also related to frequency. It is easier to saturate transformer at lower frequencies and it has to be taken into account for size of core and number of turns per volt.