I won't go into technical details here - somebody else can - but I think the
helpful concept to have is that a transformer can transform down "less" or
"more" according to the output taps (if there are any). Put it on its 15 ohm
tap and it transforms down "less". On the 4 ohm tap it transforms down "more".
So with the same speaker, the "less" the transformer transforms down the louder
it will be, the "more" the transformer transforms down the higher the impedence
the valves will see, so the happier they are in terms of less distortion, but
the quieter they are in terms of output. There is an optimum point where it's
both quite loud and quite undistorted. If it goes louder than that it sounds
woolly, if you go the other way it tightens up the sound, which is initially
satisfying but steadily becomes unnecessary in terms of diminishing returns. If
you had 8 ohm speakers, putting the amp on the 4 ohm tap would be cleaner and
tighter, using the 15 ohm tap would be louder and a bit more ragged. Neither
will damage anything, and frankly you can set it at whatever sounds preferable
to your ears.

=== Andy Evans ===
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