Ultralinear vs. Triode vs. SET

Droz,
If you have not done so already, search the site for high effiency speakers this topic has been well covered on audiogon. The old threads contain much information and many good suggestions, there`s no shortage of very good speakers of this type.In contrast to Sparky`s statement above, in my experience the high effiency speakers sound more real/natural and less HiFi in many cases.

Push-pull amplifiers make more power, but often do so with increased 5th harmonic distortion, which causes them to sound brighter. This is not always the case though, if the amplifier is fully balanced from input to output the only distortion component will be the 3rd harmonic, which is considered one of the lower ordered harmonics and is more innocuous.

A pentode amp will make the most power for the tubes involved, but will also be the least linear, and so will need negative feedback to linearize it. There is a price paid here- you get most forms of distortion reduced, but with an increase in the odd ordered harmonics, which the ear finds unpleasant.

Ultra-Linear is a method of using the output transformer as a very local feedback loop to linearize the output section. The result is an output section that that has nearly triode linearity, and nearly pentode efficiency. This is a very nice way to set up a push-pull amplifier, but if the amp were actually using *real* triodes, it would sound even better as it would be more linear.

Output transformers are such that the bigger you make them, the harder it is to get bandwidth. This is particularly true of SETs, and is a primary (no pun intended) limitation in the higher-powered examples. In most cases, the 7W of a 300b is about as big as you can get and expect bandwidth that might be considered 'hifi'. This is also why the smaller SETs, the 2A3s, 45s and the like have the reputation for sounding better- they have greater bandwidth. Now you can fudge things a little- optimize for bass or highs (and since most high efficiency speakers have troubles making deep bass, the tendency is to go for the highs rather than the bass) since you aren't going to get both.

Push-pull overcame a lot of these issues to a certain degree, with its own set of trade-offs. For example, an SET will have nearly unmeasurable distortion as the signal level heads towards zero, whereas a push-pull amplifier will usually see increased distortion (this is commonly attributed to an additional circuit in P-P amps called a phase splitter; its worthy of note that not *all* P-P amps have to have this as a separate circuit). This is where SETs get the magical midrange that they are known for- when distortion is reduced, detail is revealed.

You still have the issue of bandwidth with P-P, but in general you can have about 10X more power available compared to an SET with equivalent bandwidth.

The DC that the power tube draws through the transformer of an SET can cause saturation (distortion) issues with the transformer. This is a major design consideration of the OPT for an SET. You don't have the DC saturation issue with P-P, as the DC currents used by the power tubes cancel in the core of the transformer. On top of that, even-ordered harmonics are also canceled in the load, making for an amp that should be more transparent at greater power outputs.

The distortion of an SET is mostly lower orders until the output power becomes significant. At that point the higher orders become significant (although measurably insignificant) to the human ear. The 5th, 7th and 9th harmonics are used by the human ear to determine sound pressure, so when they are distorted (even so slightly that it is difficult to measure) the effect is that of increased loudness. Since more power is usually used on transients, what you have is the loudness cues occurring on the transients. The result is that SETs will *seem* to be very dynamic for the amount of power that they have, and often people will say its all the power that they need- they don't want to turn it up any higher.

So now you can see that in conversations about such, that if you substitute the word 'distortion' for that of 'dynamics', the meaning of the conversation will not be altered! It happens that if you can eliminate the odd ordered distortions, the amplifier will have no loudness cues, and the immediate effect will be that of turn the volume up higher, as it does not sound 'loud'.

Now IMO and IME, 'loud' is something that a stereo best not do, **regardless of the actual volume**. That can be a bit of a trick, but is well worthwhile.

I can't qualify my preferences like Atmasphere, but like many, I find the limitations of SET's too restricting. My adventures in audio began with solid state, but as soon as I realized that tube amplification was the common thread shared by the systems I liked, I've been devotee, and spent time with several examples of your subject designs. All choices involve some compromise, which then boils down to your personal musical values. The best compromise, for me, has been a well executed 300b push-pull design of moderate power. My tastes in music are pretty wide ranging and this design has allowed me to enjoy the entire range without extreme limitations being placed on my choice of speakers.

I currently own an Atma-sphere OTL (S30), 4 SET amps (45,2A3,300B,GM70), a SEP amp (single ended pentode using 6CL6 tubes), a reburbed Dynaco ST-70 and a Class D amp. I have owned a myriad of other SS, PP and SET amps over the years.

With appropriate speakers, Single ended and OTL topologies bring me far closer to the music than any other type of amplification. The clarity and presence of these type of amps can be stunning. I'll leave the technical explanations to more qualified folks like Atmasphere. To my ears, the distinction between good SE/OTL and other amps is not subtle. It's readily noticeable.

However, saying that, there are a few important considerations when using these amps. First, you must match your speakers accordingly. For the most part, SET's will not perform optimally with inefficient speakers. If you love large planars and Electrostatics, then you are probably going to need to look elsewhere. Also, your listening tastes come into play. A 45 or 2A3 SET is wonderful with intimate vocals, small scale accoustics and jazz. Realistically, not enough muscle for Bruckner or Mahler symphonies. If you love large scale orchestral music, you probably want to look for a high powered SET (211,845,GM70) or OTL's.

If your speakers match well with either SET or OTL, I really think you owe it to yourself to investigate these amps. It can be a major ear opener.

Droz,
As predictable many opinions and experiences expressed here, as each type of amp design has some level of compromise(SET,PP,OTL,etc.) The 300b PP route is ok but won`t offer anymore speaker flexibility than say a 845 or 211 tube used single ended. There are`nt extreme limitations of speaker choices either( even if you choose a 300b SET). With a PP 300b you will gain power, but now you`ve introduce phase splitters to the circuit. The SET approach will generally sound purer and even more natural(very simpler circuits tend to sound more natural) Since the original post was an inquiry of SET AMPS i sincerely hope you have the chance to hear them in a well thought out system and judge with your own ears. Just for the record I can play any type of music I want with my 8 watt Coincident Frankenstein. Don`t think you have to limit your music selection at all.
Best Wishes