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Posted by Paul Joppa on August 05, 1999 at 21:08:11:

While I certainly don't want to suggest that science is everything, there are two points about SET amps that are highly predictable:

1) They are usually low power. You want typically 102dB capability for fully satisfactory sound.

The amp adds 10*log(watts) dB to the speaker sensitivity. (base 10 logarithm) Some examples:

45 tube 2 watts = 3dB wants 99dB speaker
2A3 tube 3.5 watts = 5dB wants 97dB speaker
300B amp 8 watts = 9dB wants 93dB speaker

88dB speaker wants 14dB = 25 watts
90dB speaker wants 12dB = 16 watts

You can have lots of fun with less power, if your room is tiny and/or you like chamber music or little-girl-with-acoustic-guitar music, but this is the standard minimum.

2) Most SET amps are without feedback. Triode amps without feedback, SE or push-pull, have a damping factor of 2 or so, not the 50-1000 of heaviily fed-back sand amps. This means the bass resonances of the speaker are "loosened up" a little - expect the bass to change! Speakers whose bass is a little too quick and tight with a feedback amp are likely to come alive with a no-feedback amp, while flabby bass will just get worse.