Would You Rather Own A Good SET Amp, Or A Great Push Pull Amp?

It is a two camp showdown one camp being group 1, who evaluate through solely having used their experiences of End Sound produced and the impression made on them.

The other camp, group 2, who are much lesser in their number than group 1, and are using their understanding of Math to influence their assessment of End Sound having been experienced or not experienced for the supply of a description of what is occurring or expected to occur to an End Sound as a result of the Math.

@pindac There’s a third camp of which I’m a member, whose position was clearly stated by Daniel VonRecklinghausen decades ago when he said 

“If it measures good and sounds bad, — it is bad. If it sounds good and measures bad, — you’ve measured the wrong thing.”

lack of distortion e.g. via excessive negative feedback - doesn’t always speak of good sound, but significant levels of measurable distortion can never qualify an amplifier as high fidelity.

@yoyoyaya This is true! An amplifier can sound quite dreadful if the feedback is improperly applied!

In my case I want the amp to be neutral and transparent, but also musical (which it should be if its really true to the signal). 

If the amp has unmasked higher ordered harmonics it can sound less than musical since the ear uses those harmonics to sense sound pressure and so is keenly sensitive to their presence. Since the ear also uses harmonics to sense tone color, obviously these harmonics can cause the amp to sound harsher and brighter than the actual signal. This is a common problem if feedback is improperly applied. 

So the amp design, if it is to have distortion, must have a benign distortion signature such that its innocuous to the ear. SETs do that, but so do a lot of other amps including some solid state. 

I do have fond memories of that Dynaco ST35, will have first listening session with the Bendix 6094/EL84 amps this weekend. 

@sns Make sure its properly refurbished it its an older one otherwise all bets are off. If a newer one, quite often they are assembled from kits and might have errors. I bought one off of ebay that had this problem. It took a bit of work to get it to come around. There are a good number of updates for that amp BTW that are quite worthwhile. 

845 SET can drive some speakers not often associated with SET, power supplies with these quite substantial, around 900V plate voltage with these. With the right speakers a nice 300B amp running top tier 300B tube like the new Western Electric is sublime, never heard any pp that can replicate harmonic structure of these.

@sns A typical 845 SET makes about 20 Watts at full power, which will be about 10% distortion. By contrast a Dynaco ST35 can make 17 Watts with about 1% distortion. At any power level, the Dynaco will outperform the 845 not just in terms of distortion by also bandwidth- it will go lower and go higher. 

I get that the raw THD does not tell the whole story. 

So then it comes down to that seductive bit. I've found that distortion obscures detail, and having run an LP mastering operation as well as a recording studio, I have a number of LPs that I know exactly how they are to sound. That's the sticky bit; since I know how they are supposed to sound I've yet to find any SET at any price (including 845 based amps) that can do as well as a PP amp of only 15-20 Watts (usually EL84 based, FWIW). That is, if being faithful to the actual recording/signal is important while being able to present it in a relaxed way with authority that allows one to listen all day (or night). 

I'm still left with the million dollar question, being, 

" what makes a great amp and what makes a good amp? " 

Is there such a choice available? 

@pindac Yes, there is. 

I have loved SETS for a long time, but I simply cannot afford a "great SET". And, being retired, never will. 

@nightfall I’ve yet to hear an SET at any price that could beat a good quality PP tube amplifier. I’ve been investigating SETs since the first started to show up in the early 1990s so I’ve heard quite a lot of them. 

If your speakers can make bass and if you don’t want to bi-amplfy then you will want to pay attention to this next fact:

Most SETs have troubles with bass, since the output transformer has to have a saw cut in its core to reduce DC saturation which otherwise causes a lot of distortion. Sorry if this sounds a bit technical.

The output transformer works due to something called inductance.

Inductance is what allows a long bit of wire that might have only 200 Ohms resistance to have an impedance of 3000 to 9000 Ohms when wound inside an output transformer. Power tubes used in SETs need that kind of load to work properly. That saw cut I mentioned vastly reduces inductance at low frequencies, which in turn means the load on the tube is much lower too. This is bad for the power tube and causes it to make a lot more distortion while also running hotter. 

As a result there’s no SET made that can make full power at 20Hz. But there are a good number of PP tube amps that can; there’s no need for that pesky saw cut in a PP output transformer. 

So let’s imagine that you were considering a powerful 20 Watt SET (that’s a lot of power for an SET BTW). Most SETs, because they don’t use feedback have only about 20-25% usable power before distortion causes them to sound ’dynamic’; IOW above that power level distortion masquerades as dynamics.  That is why you read about how ’dynamic’ SETs are but its really just distortion.

PP tube amps OTOH have about 90-95% usable power. So in the case of the 20 Watt SET, it would have about 5 Watts of clean power where a 20 Watt PP tube amp would have 18 Watts or so. 

A PP tube amp of that power is usually a lot less expensive than an SET of that power, probably several times. If the PP tube amp is properly designed and built, by most audiophile metrics it will ’sound’ better than any SET of the same power and likely a lot of SETs with considerably less power. 

But for PP tube amps 20 Watts is about the golden point where you can get both really wide bandwidth and decently low distortion. A good example of such an amplifier is the Dynaco ST35; if you find one in good condition (there are vintage ones and new ones) you’ll find them causing SETs pain and shame on speakers like the ones you mentioned with no worries. 

They are rare but there are PP tube amps of even less power- like 5 Watts. Any SET of that power level will fall flat on its face in comparison, if musicality and neutrality (so it can play any kind of music easily) is your goal. 

Now if you’re stuck on an SET there is hope. If you limit the bass it sees (like 200Hz and above) that will get rid of most of the problems with the bass. You’ll have to bi-amplify of course. FWIW the most successful systems I’ve heard that use SETs take this approach.