Ohm load for SET Amps

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. 

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. 

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.

It should be said, though, that for a given speaker impedance the damping factor, and hence "control" of the speaker’s motion, will be greatest from the 4 ohm tap, and lowest from the 16 ohm tap. Although the damping factor will usually be fairly similar for a 4 ohm load connected to the 4 ohm tap compared to an 8 ohm load connected to the 8 ohm tap, etc.

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.

To add to the other responses, pairing a SET amp with a low impedance speaker is especially likely to be problematical if the impedance of the speaker varies a lot as a function of frequency. That is because tonal balance will be affected by the interaction of the relatively high output impedance of most SETs (and many other tube amps) and that varying speaker impedance.

A higher impedance speaker having an impedance that varies in a similar proportion will exhibit that effect to a lesser degree, everything else being equal, since the output impedance of the amp will be a smaller fraction of the speaker’s impedance at all frequencies.

Regards,
-- Al

What brand are your monos? My Shuguang monos struggled greatly with 4 ohm speakers but were great with 8 ohm. I have found the MastersounD amps to not have a similar issue. 

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.

Actually this isn't true. The 4 ohm tap has half the windings of the 16 ohm tap. The impedance goes up by the square of the number of turns- so the 4 ohm connection is actually the center tap if there is a 16 ohm tap.