SET v. Pushpull

https://parttimeaudiophile.com/2018/03/30/review-mcintosh-ma252-tube-solid-state-integrated-amplifie... 

A number of good points have been made above. An important one that Geek101 mentioned that warrants additional emphasis, IMO, concerns impedance swings.

An impedance curve for the LS50 can be seen in Figure 1 of Stereophile’s measurements. It can be seen that the impedance varies from a low of about 4 ohms at 200 Hz to about 13 ohms at about 1.5 kHz, before descending to around 6 ohms in much of the treble region. That impedance characteristic coupled with the small size of the speaker and its woofer (see the next paragraph for the relevance of that), as well as its low sensitivity (84.5 db at 1 meter for an input of 2.83 volts), makes it clear that the speaker was designed with the expectation that it would usually be used with solid state amplification.

With a few exceptions, nearly all solid state amplifiers have an output impedance of a tiny fraction of an ohm. The interaction of that very low output impedance with the LS50’s impedance variations over the frequency range will work in the direction of augmenting the mid-bass response, relative to the response at higher frequencies.

In contrast, tube amp output impedances differ widely among different amps. Some have low output impedances, in the area of 1 ohm or even less, while some have output impedances of several ohms or even more. And if the LS50 is driven from a high output impedance the interaction of that output impedance with the speaker’s impedance swings will result in a tonal balance that is not what the designer intended, and in particular is likely to be weak in the mid-bass.

You indicated that Reichert obtained good results pairing the LS50 with an LM518ia. Undoubtedly a major reason for that was that the LM518ia has a considerably lower output impedance than most SET amps, especially from its 4 ohm tap. Per Stereophile’s measurements of that amp:

The output impedance varied slightly with frequency and load impedance, but was approximately one-quarter the nominal value of each output-transformer tap: 3.8 ohms (16 ohm tap), 2 ohms (8 ohm tap), and 1.1 ohms (4 ohm tap).

These impedances are relatively low for a single-ended-triode amplifier.

So I would definitely not extrapolate Reichert’s finding to other SET amps, or for that matter to many push-pull tube amps.

Also, regarding power capability, keep in mind that what will **dramatically** affect how much power is required is the dynamic range of the music that is listened to (i.e., the **difference** in volume between the loudest notes and the softest notes). Many and perhaps most pop and rock recordings are dynamically compressed to a dynamic range of less than 10 db, meaning that less than 10 times as much power is required to reproduce the loudest notes as to reproduce the softest notes. While classical symphonic recordings may have dynamic ranges of 30, 40, or (in a few cases I have measured) as much as about 55 db. Those correspond, respectively, to 1000 times, 10000 times, and about 300000 times as much power being required to reproduce the loudest notes as to reproduce the softest notes (and those are not typos). So in those cases if the volume control is turned up high enough to hear the soft notes at reasonable levels vastly more power will be required than would be necessary for recordings having minimal dynamic range, such as most pop and rock recordings.

Finally, regarding the Joseph Audio Pulsar, Stereophile’s measurements show it to have impedance characteristics that are more tube-friendly than the LS50’s, but its sensitivity is even lower than the LS50’s, making power capability an even more important consideration.

Good luck. Regards,
-- Al

Many excellent replies! 
You really want to do some listening to a variety of tube amps. Many tubes sound quite different and of course designs of a given tube do as well. There are a number of push pull amps that allow you to roll EL34s, 6550s and a few other variants.

SETs are a broad group and many are very low power(e.g. <10w). You really need a super friendly speaker(e.g. horn) to get the most of those. It all comes down to matching speaker+amp. 

A couple of personal experiences might be of help. I loved the sound of a number of SETs I heard, but I didn't like many of the horns and other speakers that were the best fit. That led me to 845 SET monoblocks from Dehavilland which run at 35w and allow you to open up your speaker options to many conventional speakers in the 93db neighborhood. Silverline La Folias were my best match with them. Merlin VSM Series also a good match, though lighter in the bottom end. 

When I had the opportunity I ran a pair of Atma-Sphere MA1 OTLs, which are another entirely unique class of tube amps that do many things better than most others again IF AND ONLY IF paired w/the right speakers (i.e. moderately efficient but more importantly with a friendly impedance curve. Some folks say they conquer that issue using items like Speltz Autoformers, to open up the speaker options to theoretically almost any impedance. Personally, the autoformers were not my cup of tea. OTLs had speed and clarity superior to most of the other tube amps I've heard of all other types. 

For me you've got to find the speaker you want first and then ask what amps will be a good fit with it, and consider a wide variety of topologies based on listening. If you can't audition, buy used so you can change gears without losing your shirt. Cheers,
Spencer

Tom,

You have been given very good input here. Almarg's points about impedance was definitely spot on (as usual in his posts): both as a key speaker attribute when considering to match with SET, and in showing your current speakers aren't the best fit with SET. The main points were around impedance variability over the frequency range. I also want to point that impedance magnitude is very important for SET matching as SET amps are constant current sources.

Most other amps are constant voltage sources, so as you halve the speaker impedance they deliver twice as much power, or half as much power as you double impedance. You've probably amps specs saying 200W into 8ohm and 400W into 4ohm. Here's the simplified math behind that:
Power (P) equals voltage (V) times current (I). P=V*I.
Also V=Z*I, where Z is impedance.
You can replace in the first one: P=V^2/Z. If an amp is constant voltage, power is inversely proportional to speaker impedance. Double Z, halved P.

On the other and SET are constant current, so in P=V*I you can replace into P=Z*I^2. As you double speaker impedance the amp delivers twice the power.

Look into speaker designs by SET amp manufacturers. For example Coincident Technology. Their speakers are 95dB-ish sensitive and 10 ohm, low variation impedance. Just looking at the 95dB sensitivity might lead you to believe they aren't good match for a 300B 8W, but the designer had that amp in mind when designing. BTW, note these aren't horns. While horns are indeed a good match for SETs, not only horns are suitable for SETs.

Also, another consideration often brought up by Ralph from Atmasphere: SETs produce extremely low distortion provided you run them at say 25% of their rated power, and low distortion up to 50% of their rated power. Distortion grows fast as you approach rated power. So while it's true SETs produce more distortion, that is true at their rated power, and a well designed system that incorporates SET should take into consideration running them as easily as possible. Hence it's also true SETs produce very low distortion, with the added benefit of producing more pleasing harmonics when they do.

Bottom line: it's all about execution. No blanket statement can be made. SETs could be better or worse. Your existing speakers aren't a good match for SET. But you also mentioned they are too small for your room. Maybe you want to go SET knowing you will replace your speakers with SET friendly later. Or replace your speakers first and then find an amp to match.

Sorry for the length!