how much tube power is needed?

The answer from Mr. Jslager is in error. You only need to double the power for each 3dB increase, not ten times. His table should read: 40 Watts/86dB; 20 Watts/89dB; 10 Watts/92dB. A 10dB increase needs 10 times the power. So with 10 watts RMS a 92 dB/1 watt/1 meter speaker will produce 102dB in SPL (sound pressure level). The three above values will all result in 102 dB peaks. However, musical signals also have 10 dB peaks on the average, so all of the above listed amplifier wattages will produce only a 92dB AVERAGE SPL before clipping on the peaks of 102 dB. Remember that tubes, when driven into clipping, produce THD of over 10%, at best they produce around 1%, so if you wish to put an equalizer, capacitor and distortion unit in series with your loudspeakers then by all means buy a tube amp, the wattage is practically immaterial as they will ALL sound distorted. To figure how much amplifier power you need, take your loudest average listening level, say 102dB, figure the wattage needed and multiply by ten, for the 10dB of overhead needed for the peaks. In the case of a 92dB/watt/m speaker, a 100 watt RMS amplifier is needed. Add to this low distortion like 0.1% or less, low internal resistance (e.g. high damping factor, over 100), reliability and reasonable price, and you will see that no tube amplifier can fill this requirement. Good luck.

Not meaning to confuse the issue here, but I noticed reference in the responses to 3dB being a level difference where doubling the power is necessary. This is true of course, but I wanted to point out that it takes about 6 to 7 dB of boost to double the PERCEIVED LOUDNESS (for the midrange of the music spectrum). This translates roughly into QUARDUPLING the amps power output to double the perceived loudness for a given speaker. Many feel it's the FIRST watt of output that counts most toward listening satisfaction, and those in the Triode camp take pleasure in that. Hope this is useful.

this is in response to fotopress . He claims that no tube amp can drive your speakers properly. I am quite sure that a large VTL,AR or CJ would beat hands down ANY 100 watt transistor amp.Now to facts.I am driving electrostatic Acoustat 2+2 with a custom built tube amp(full triode parallel single ended class A 60 watt)to measured levels of 105db peaks without clipping or distortion.My room is comparatively large(12X20 feet).The sinus wave is perfect(measured on the speaker terminals)at all listening levels and with all kinds of music.Measurements are very good even with Sonus Faber electa amator loudspeakers.Hopefully,in about three weeks i will do the same on a friends Martin Logan SL3

In response to user35@ibm.net, I fully agree that a 60 watt tube amplifier will drive the Acoustat 2's to 105dB peaks, but maybe only in the midrange. As this is a discussion forum so that others may understand the issue, the Acoustat 2's, a "full-range" electrostatic which were first introduced in 1980, have an efficiency of 85dB/watt/M. It would take a 100 watt signal to drive these speakers to 105dB, and since a tube amplifier typically has 2 dB peak headroom, the 60 watt tube amplifier can drive the Acoustat at some frequencies to exactly 105dB peaks. This of course, as with all tube amplifiers, with the inevitable high distortion, limited damping and bandwidth, noise, unreliability, inability to match impedence or drive reactive speakers, etc. The 105dB peaks at 20Hz and 20kHz with low distortion that a true "high fidelity" system SHOULD produce are just not possible with user35's system, and that is also a fact. As for testing, a square wave is much more revealing of sonic flaws such as bandwidth limiting or reaction to capacitance, and a spectrum analyzer would allow simple measurement of harmonic distortion and noise. However, the sound of user35's system obviously is his preference, and I certainly respect his right to love his system! But I digress, and to answer zkis' original question of "how much tube power is needed?" my answer is: NONE!!!