How to shop for speakers?

Yes, good point.I don't know about the figure given by the prior post,but nobody ever seems to mention this for a tube amp.Its common to see the double the wattage for a solid state amp.I tell my friends to try to shop for 8 ohm speakers for use with a tube amp,but tell them if they find a stable 4 ohm, expect some volume drop,so shop for a little more efficient speaker,going the 4 ohm route.

Speaker typically lean toward having a inductive load or capacitive load, these are normally called phase angles, they are measured in degrees. Phasing that tends toward the plus side(inductive) are tube friendly, speakers that lean toward the negative side (capacitive) are not. This is the more important factor, most tube amps will handle a 4 ohm load just fine with an inductive load. And many will handle an 8 ohm load that leads capacitive ok, but hard capacitive spikes are hard on tubes,even on an 8 ohm load and not tolerated on a 4 ohm load. Many impedance charts also include phasing on those charts. On one side of the chart you will see numbers that indicate impedance 2-4-8-12-16 etc. The other side will have a 0, then degrees going
up (inductive) and degrees going down (capacitive). If using tubes, it is almost imperative to get phasing charts on a speaker. I hope this helps, Tim

Sensitivity won't matter, if you have inductive speakers and enough power to get to the listening levels that you require. I looked up the Mac MC275, it is 75 watts, you should be fine with a minimum of 86db sensitivity. That will give you 101db output @ 32 watts. Not rock concert levels, but certainly live Jazz venue volume levels.

HiFiTime & HiFiHvn, most tube amps (that have output transformers) are designed to provide approximately the same maximum amount of power when a 4 ohm speaker is connected to the 4 ohm tap, as when an 8 ohm speaker is connected to the 8 ohm tap. There are several things to watch out for, though:

1)If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a 3db volume drop (compared to the 8 ohm speaker being connected to the 8 ohm tap).

2)If the speaker sensitivity is rated with respect to 2.83 volts, rather than with respect to 1 watt (2.83 volts corresponds to 1 watt into 8 ohms), and if the speaker's nominal impedance is 4 ohms, then a 90db/2.83V/1m sensitivity would be only 87db/1W/1m.

3)Fluctuations in the speaker's impedance vs. frequency curve will assume different significance for a 4 ohm speaker connected to the 4 ohm tap than for an 8 ohm speaker connected to the 8 ohm tap, if the fluctuations are not in the same proportion. For example, if the impedance of a nominally 4 ohm speaker varies from 2 ohms to 8 ohms over the frequency range, a greater impact on frequency response would result (with a tube amp) than in the case of an 8 ohm speaker whose actual impedance had the same tolerance (-2/+4 ohms, meaning impedance varying between 6 ohms and 12 ohms).

Along the lines of Ralph's comment the matchup of the speaker's variations of impedance as a function of frequency, and the amplifier's output impedance, should be consistent with the designer's intention.

Also, I second Tim's comments about the importance of the phase angle of the impedance, and its variations with frequency. A low impedance magnitude coupled with a highly capacitive phase angle is a worst case load.

Regards,
-- Al

Thanks guys.I guess I'll still keep using the 3 db for reference.Sometimes the 4ohm tap does sound better with some 8 ohm nominal rated speakers. That was just a guess.I had a couple of different Conrad Johnson amps,and from what I remember,they wired them all for 4 ohms out of their factory.They told me that they sound the best this way.I see an amp for sale now that has a 6 ohm,and 16 ohm output.Might be a compromise between the two.