Amp & Speaker matching

How important is it to match impedance on amps and speakers? I own Manley Mahi monoblocks and the specs show power ratings for both 4 and 8 ohm. Can I use 12 ohm or 16 ohm speakers and what would it do - reduce output, damage to amp or speakers...
There is no danger at all with using a speaker of ANY nominal impedance with any tube amp. Your use of even higher impedance speakers than the common 4 or 8 ohm speaker is particularly ideal with tube amps--any possible frequency response variation caused by the typically higher output impedance of tube amps will be lessened when used with such high impedance speakers.
The higher the impedance of the speaker the better the match with most tube amps. The "natural" output impedance of tubes is normally high and tubes don't dump current as easily as most SS amps.
The answers to your specific questions are, the output by the speaker will increase, and not be reduced. Niether the tube amp, nor speakers, will be damaged by the relatively high stated nominal impedance on your speakers.
Not sure I agree with Larryi.

The output transformer in a tube amp reflects the load impedance back to the the tubes. If you use speakers rated at a higher impedance than the output transformer impedance things will be ok. You will just get lower output.

If the speaker impedance is lower than the output transformer impedance then the tubes will draw more current than if the impedances were matched.

Whether or not this is a problem depends on the amp's design limits. Consequences range from none to damage such as blown output transformer, blown tubes, or worse, depending on amp and the amount of the impedance mismatch.

Check with your amp manufacturer for advice.
R Burke,
In my opinion the higher the speaker impedance the better for tube amplifiers,i.e. easier load to drive and generally less distortion. You should not have any problems.Low impedance speakers can be a 'potential' problem for some tube amplifiers.
Manley and VTL set their output transformers to approximately 5.5 ohms so their amps will perform well with 4-8 ohm speakers.
If you run a 16 ohm speaker on an 8 ohm tap, you may get around a 3 decibel loss in volume. On some amps, this can be a risk. It depends how they wound their transformers, and to what impedance they're loading the tubes to, plus other design characteristics in the amp. I would ask first to be safe. Some amps may be fine this way, others won't be. When the speaker impedance gets too high, it could cause the output transformers to arc inside. The worst is no load at all.
Looking at the manual, it appears that there is only one output tap, which is stated to be optimized for 5 ohms, consistent with the comment by Rrog.

My GUESS is that using a 16 ohm speaker on an output tap that is optimized to work into a 5 ohm load does not represent a disparity that is large enough to cause amplifier damage, that can result from "inductive kickback" if a reasonable load is not connected to a tube amp that has an output transformer.

However, Hifitime and Ghostrider are correct that power output will be reduced. The amount of reduction will depend on the feedback setting you use and your choice of UL or triode mode (since both of those things affect the amplifier's output impedance), and the impedance of the speaker. On a percentage basis, the worst case reduction will probably occur where output impedance is lowest, which in this case is 1.28 ohms occurring for the maximum feedback and triode mode settings. According to my calculations the corresponding reduction into a 16 ohm load may be as much as approximately 3.75 db. That represents a loss of more than half of the amp's power capability, compared to a 5 ohm load driven with those settings.

Sonics may also be adversely affected by the mismatched loading of both the output transformer and the output tubes. Ringing may occur in the transformer, at least to a small degree, and the distortion performance of the output tubes may not be optimal.

As has been said above, high speaker impedances can certainly be advantageous, but not if the impedance is severely mismatched to what the amplifier is designed to drive.

All of that said, though, touching base with Manley as suggested earlier would certainly seem to be in order.

-- Al
Depending on the design - if you don't mind a slight reduction in power, using the output taps lower than the impedance of your speakers can be beneficial in some cases. Presently I prefer the 4 ohm tap over the 8 in my system where the spkrs impedance ranges from 8-10 ohm. The difference is easily heard. I've heard it go both ways w/several different tube amps w/my speakers.

In the case of my current amp, Roger Modjeski/Music Reference, the designer in his own words, "Although higher idling current will reduce distortion, it can also be reduced by light loading. Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, and allows for more peak current when needed. The only loss is about 20% of the power rating or 1 dB."
The only loss is about 20% of the power rating or 1 dB
Hi Pehare,

That may be true for some of Roger's amps, but it is by no means true as a general rule. As I indicated earlier, the amount of power loss is dependent on the output impedance of the amplifier, on the degree of light loading (i.e., the relation between the impedance of the speaker and the load impedance that the output tap is optimized for), and on other amplifier-dependent variables.

On a percentage basis, the power reduction resulting from light-loading an amplifier having an output impedance of 1 or 2 or 3 ohms or so, such as the Mahi, is likely to be much greater than it would be, for example, with some SET amps that may have output impedances of perhaps 4 to 8 ohms or more.

Best regards,
-- Al
Hi Al,
I agree that the "light loading" principle is likely to yield varied results and is amp dependent. For example my amplifier has an 8 and 16 ohm speaker tap. My speaker is 14 ohm nominal load and 10 ohm minimum. Without question the sound is better at the 16 ohm tap rather than the 8 ohm. Simply put all music reproduction parameters are just better.I understand that the Music Reference amplifiers could be a different situation and thus benefit from the light load appraoch.If there`s loss of power it unnoticeable, the sound is actually more transparent/open, dynamic,with increase scale, fluidity and flow.