Speaker ohm's?????

Chances are that the amp will be okay. A decent 8 ohm amp would be able to drive 6 ohm speakers without getting too mad at you. My amp is rated at 200 Watts per channel into 8 ohms, and 350 Watts per channel into 4 ohms. My speakers are nominal 4 ohms but at some frequencies actually have an impedance of about 2.3 ohms. Great amplifiers are able to double their power as you cut the impedance in half. Now, at worst case, an amplifier could be designed for 8 ohms and really get overheated and mad at you if you run anything with a lower impedance. An amplifier like that would fizzle out and die on you over some unknown period of time. In general, there is no direct answer to your question since it depends on the amplifier, the speakers, and how much demand (i.e., volume and listening duration) you put on the amplifier.

It's always a good idea to match your amp to your speakers. As far as I know the damage will only occur to the amp if you are requiring it to drive a lower impedance load than it is designed for. Nominal means average, which means the speakers can drop lower usually a couple more ohms.

-aj

Its unlikely that "6 ohm nominal speakers" would hurt any amp. Also, you are calling it an "8 ohm amp" probably because the specs rate its power at 8 ohms and don't bother to list its rating for lower impedances, but really, all amps will drive any impedance down to maybe 4 ohms, but possibly at less power. Don't worry about it.

All SS amps should show measurable increase as impedance is dropped. Theoretically, they should all "double down" in power output as impedance is halved i.e. 50 wpc @ 8, 100 wpc @ 4 and 200 wpc @ 2, etc.... Even though many "hi-end" amps are rated this way, reality shows that this is not the case in most situations though. One really needs to check the power at the point of clipping into the various impedances to see how "beefy" an amp really is and not just look at published specs. The less that power increases as impedance is dropped, the more that the power supply and output devices have become choked. If a stereo amp of SS design can't deal with one pair of "normal" speakers ( non esoteric design ) and drive them to "reasonable" listening levels in an average sized room, it should be disposed of. This is true whether the speakers are rated for 4, 6 or 8 ohms.

All tube amps with output transformers should remain relatively constant due to the loading mechanism known as the output transformer. While you will see some differences ( possibly in both power output AND frequency response ) when changing taps with various impedances and speaker loads, the main reason for having the output transformer is to help the amp remain consistent in performance regardless of the load that it sees. Some designs come a lot closer than others do in this respect.

Output Transformerless ( OTL ) tube amps will typically produce less power as impedance is reduced. There are probably some exceptions to this generalization though, but for the most part, i would consider it true. This has to do with the output impedance of the amp and the inability of most tubes and their associated power supplies to supply large amounts of current. As such, these designs would normally prefer to see a speaker with a relatively benign load that maintained a somewhat higher nominal impedance. They do have the potential for more pleasant sonics ( in my opinion ) as you have removed a major source of signal degradation from between the output devices and the speaker. Sean
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There was a good thread on this not too long ago:
http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1026181509