what does it do when you try different taps thanks
I agree with the preceding responses, but to elaborate a bit further:
The different taps represent different points on the secondary winding (the output side) of the output transformer. For a given signal condition, the 8 ohm tap provides the greatest voltage; the 4 ohm tap provides a smaller voltage; and the 2 ohm tap provides a still smaller voltage.
I believe that a typical design will provide at the 4 ohm tap approximately 0.707 times the voltage of the 8 ohm tap, and at the 2 ohm tap approximately 0.707 times the voltage of the 4 ohm tap. The figure 0.707 is 1 divided by the square root of 2.
The result of designing the taps that way is that exactly the same power (voltage x current) will be delivered into an 8 ohm load as into a 4 ohm load as into a 2 ohm load, assuming the load is purely resistive and independent of frequency.
An additional result, which is important, is that the plate circuit of the output tubes will then see a load impedance (equal to the speaker impedance times the square of the turns ratio of the output transformer) which is the same regardless of whether an 8 ohm, 4 ohm or 2 ohm speaker is connected. That allows the voltage and current operating points of the output tubes to remain at or near what the designer chose as optimal, regardless of speaker load.
But along the lines of what the others have said, speaker impedances are not purely resistive and can vary considerably with frequency. So I agree with Newbee that it would be best to start with the 4 ohm taps, which match the speaker's rated nominal impedance, and evaluate the possibility of using 2 ohms based on what you hear. But keep in mind that the 2 ohm tap will output less voltage, and therefore less power at frequencies for which the speaker's impedance is higher.
Regards,
-- Al
