Ohm load for SET Amps

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.

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.

P.S. to my previous post: The statement I quoted at the beginning of that post is incorrect. The reference to 1/2 of the OPT’s (output transformer’s) winding being in use for the 8 ohm tap should have said 0.707 (i.e., 1/(square root of 2)), assuming there is a 16 ohm tap, and the reference to 1/4 of the OPT’s winding being in use for the 4 ohm tap should have said 0.5.

Tube amps having output transformers are usually designed to have approximately the same maximum power capability when a 4 ohm load is connected to the 4 ohm tap as when an 8 ohm load is connected to the 8 ohm tap, etc.

For a resistive load: Power = (Voltage squared)/Resistance

... so to provide equal power into a halved resistance voltage squared must be halved. Which means that the voltage and hence the turns ratio of the transformer must be reduced by a factor of 0.707.

Regards,
-- Al

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.

The 4 ohm tap allows the amp to make the current needed. The problem is that limitations in the output transformer often mean that the transformer runs a bit hotter when running the 4 ohm tap, and this extra heat comes from the power of the output tube itself. IOW, the amp will make slightly less power due to its being less efficient. But it will make the current needed to drive 4 ohms.

IMO/IME there really is no good reason in high end audio for 4 ohm speakers regardless of your amp. They allow you 3 db more output if you have a solid state amp, but **all** amps make more distortion into 4 ohms as opposed to 8 or 16 ohms, and that distortion is audible as brightness and additional harshness, neither of which is desirable especially in high end audio.

If you really want to do your amplifier investment dollar justice, a higher impedance speaker will do that better all other things being equal. In the case of an SET, I recommend a higher efficiency speaker too; an 845 doesn't make a lot of power as amps in general go, so about 98db 1 watt/1 meter would be my minimum recommendation if you really want to hear what the amp can do.

Thanks for all the input guys!  It seems like the main issue is not getting the most out of your amp, whether from increased distortion, or reduced range, or reduced efficiency, but there isn't anything inherently harmful to the amp, per se.

The concept of impedance curves and matching is an interesting one, and helps clarify a bit in my mind why some amps/speakers may "play well together".

I do agree, and wish, all high end audio speakers were minimum 8-ohm.  My problem is that I've been out of the game for a long time.  All of the high efficiency speakers I've heard were not to my liking, regardless of the amp, and I really liked the overall package of the 845 tube.  It's been a bit difficult to find a decently efficient 8-ohm speaker *sigh*

What sort of problems have you heard in those speakers and what sort of budget do you have?