The AC impedance of an output transformer, seen by the anode of a vacuum tube when a load (a speaker in our case) is connected to the secondary, depends on the ratio between the number of turns, of the primary and the secondary.
Let np and ns be respectively the number of turns of the primary and secondary. Let Zp, and Zs, be respectively the impedance seen at the primary and the impedance applied to the secondary (that is the impedance of the speaker). We have that the ratio between the primary and secondary impedances is equal to the square of the ratio between the primary and secondary turns. More formally:
It is worth noting that, according to this equation, we can change the impedance, seen by the anode of the vacuum tube, which is the impedance of the primary of the transformer, by changing the impedance of the speaker. The impedance of the speaker is reflected to the primary of the transformer, according to the square of the ratio between primary and secondary turns.
It is also worth mentioning that speakers, generally do not have a constant impedance. Rather, the speaker’s impedance varies with the frequency of the signal being reproduced. Figure 18 shows the real impedance of speakers rated for 11 Ohm.
In this example, the impedance is very high around 100 Hz. It goes to 6 Ohm at 400 Hz, then jumps to 11 Ohm at 3K Hz, and then back at 4 Ohm to 20K Hz. This means that the slope of the loadline might vary significantly depending on the reproduced frequencies. The loadline, which is typically plotted as single line, is in fact a blurred area around the thin line.