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Ohm load for SET Amps

I've always heard/read/searched that 8-ohm or higher loads are ideal for tube amps.  Speakers are "tube friendly".  I've never really understood why that is the case.  Could anyone with much more experience/wisdom than myself (most, I'd imagine) explain why this is the case?  What would happen if one were to use 4-ohm speakers with a SET amp?

While I have some foundation of physics and electronics, I'm far from an electrical engineer.  

The main reason I ask is because I have a pair of 845 SET's, and would like to consider speakers rated at 4-ohm.  My 845's have 4, 8, 16 ohm taps.

Thanks for any help!
cyberbob
atmasphere's avatar
atmasphere
11,806 posts
 
Output transformers tend to become less efficient into lower impedance loads. This means that it will absorb a slight amount of power, but the other issue that is often more important is that bandwidth can suffer- by as much as an octave off the bottom from 4 ohms to 8 ohms!
The 845 isn't easy to get bandwidth out of anyway due to output transformer issues.
It really is to your advantage to seek out a 16 ohm speaker- the amplifier will not only have wider bandwidth, but slightly lower distortion too.
roberjerman
4,319 posts
 
Halving the impedance doubles the current (amps) required. Tube amps are current "challenged" compared to SS amps.Therefore the higher the speaker impedance the easier it is for a tube amp to provide enough current. 
roberjerman
4,319 posts
 
A 16 ohm tap means that ALL of the OPT's winding (on the output side) is in use. An 8 ohm tap means that 1/2 of the OPT's winding is in use. A 4 ohm tap means that 1/4 of the OPT's winding is in use. This directly affects damping (control) of the speaker's motion. 
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