[SteveRB]

Question:
what are the down falls of running speakers with a total impedance load greater or less than what the tube amp is wired for?


example 1, a 16 ohm speaker playing on a system wired for 8 ohms?
example 2, a 4 ohm speaker playing on a system wired for 8 ohms?
example 3, a 2 ohm speaker playing on a system wired for 8 ohms?

thanks again, for any help in clarifying these basic questions...

[David Weil]

If the impedances of output transformer and speaker are not matched, efficiency is lost. More of the amplifiers output power is converted into heat and less is available at the speakers. In short, your achievable SPL is lower -> less dynamic headroom.

[SteveRB]

is the efficiency problem the same for both scenarios? over and under-rated?  I assume that will shorten tube life; are there any other equipment damage issues?

[Niteshade]

A 1:1 match is best, as was already stated. However, you can drive an 8 ohm speaker with a 4 ohm tap and it will work OK, just not perfect. It doesn't work well at all to try and drive a 4 ohm speaker with an 8 ohm tap.

[Michael V]

is the efficiency problem the same for both scenarios? over and under-rated?  I assume that will shorten tube life; are there any other equipment damage issues?

The issue here is what load is being reflected to the output tube.  Looking back at the output stage, this tube will operate differently depending on what load it is driving.  The turns ratio of my output transformer will make an 8ohm speaker appear as a load more like 3,000 ohms, from the output tube's perspective.

Every tube has different parameters.  But for triodes, a lower impedance means more power output and more distortion.  A higher impedance means less power output but lower distortion.  So if you connect a 8ohm speaker to a 16ohm tap on your amplifier's output transformer, the output tube is now seeing a lower impedance.  It will be slightly more efficient - producing a little more power - but at the cost of added distortion.  The opposite will happen when you connect a 16ohm speaker to an 8 ohm tap.

Neither mismatch will harm your output tubes.  Even in the second case, when the tube is producing slightly less power, it is possible that you will crank up the volume knob to produce more power.  When the tube is operating in Class A, it is actually working less hard when it's amplifying higher input signals, so it's not damaging anything.  If your amp is not Class A, this may not be true.

Hope this makes sense 

[richidoo]

The label on the tap is not significant of the actual output impedance of the amplifier when using that tap. The label just represents the mfg's idea of what is acceptable load for the amplifier using that tap. It is his idea of how much woofer distortion he is willing to tolerate as characteristic sound of his amp. Various mfgs' ideas of this will differ wildly.... 

Read more about tube amp output impedance and how to calculate your amp's real Z(o).
http://www.transcendentsound.com/amplifier_output_impedance.htm

If you can get a damping factor 10:1 or better is great. Hard to do with tubes and modern speakers, whch is why ss amps are so much more popular and cheaper. Output transformers which lower tubes output impedance to usable levels are usually the most expensive part of the amp. Eliminating them make ss amps a lot less money for similar HP.  But tube amps have refinement that inexpensive SS amps usually lack. There are always exceptions. Just depends where the talented designer aims his imagination.  But so far none have been able to create low output impedance beautiful sounding tube amp at ss prices.

ELectrical ultility transmission requires "matched impedance" to maximize current trasmission efficiency. They want zero headroom. Audio requires as much headroom as possible to prevent dynamic compression and current clipping, so you (in theory) want the lowest source impedance (amp's output impedance) and highest load impedance (speaker nominal impedance) possible. This is the same for signal transmission from source or preamp to amp. When the ratio is too low, you will have problems transmitting the signal without distortion. You need extra unused current heardroom in audio amps to maintain integrity of the signal. The current amplifier's distortion rises  as it gets closer to max current, so the less current is used the better. Some preamps have enough current headroom to power a power amp. They have very low distortion. It is the same principle with amp/speaker.   The electric utility doesn't care how distorted its power signal is.   They care more about minimizing infrastructure cost and loading wire and transformers with maximum efficient load.

You can overcome the output impedance handicap of tube amps several ways. By increasing the current headroom of the amp with a higher power rated amp, so a 1.5 ohm output impedance tube amp can effortlessly driver a 4 ohm nominal impedance speaker with a 1.9ohm dip as long as the amp can make the needed current to control the drivers without current clipping or going up to high into the current headroom. You can also raise the sensitivity of the speaker so it plays louder at same amp voltage, thus allowing turn down the volume which improves current headroom. Increasing the speaker impedance is another way, using a different speaker, or using a Speltz zeroformer speaker impedance transformer.

BUT some lower powered tube amps prefer a moderate load more than a very light load. They sound best when pushing a good load. You have to be careful to apply enough stress, but not too much to cause clipping. SS amps are so much easier, but lack tube refinement which is more important to some people.....
Rich

[SteveRB]

wow,
thanks,
im going to sift through this a bit and come back with a couple more questions -- specific to my open baffle builds.

thanks again,

[carusoracer]

Very informative answer Richadoo