Researching SET Amps

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What speakers are you using?   That has a lot to do with what tube you can use.   2a3 amps  are around 4 watts , 45 are something like 1.5 watts, 300b around 8

211 or 845 would be a high power SET....  

I'm having a 300b built and its got a lot of options including vintage CTC power transformer and chokes along with Hitachi Fine Met Amorphous Core Output Transformers.   I also have a pair of Quicksilver amps and Mike of QS has always used quality transformers so my 300b is getting the optional ones...I think it is what separates good amplifiers from great ones

what’s important in an SET amp: 1) Power tube mating to the proper speaker, and 2) Design and manufacture of the transformer.

The first question isn’t a thing.

You don’t mate a power tube to a particular speaker, unless you’re talking about total power- for example you’ll need at least 100 dB efficiency in the speaker to be successful with a 300b (which only makes about 7 watts at full power).

The output transformer design is paramount. It mates the tube to the speaker so it has to have bandwidth and the proper matching so the tube makes the least distortion and best power combination.

Keep in mind that with any SET you are up against several limitations:

1) the output power needed by the speaker should not exceed about 20-25% of the full power of the SET if you want to hear what the SET is all about.

2) as you increase the output power of the output transformer, the bandwidth decreases rapidly. The tube has nothing at all to do with this! In most cases, to get ’hifi’ bandwidth (20-20,000Hz) the maximum power is probably limited to 7 watts.

3) since bandwidth is so hard to achieve, the bottom octaves are usually sacrificed in favor of the highs. In practice this works out alright since the high efficiency loudspeakers needed often trade the lows for higher efficiency- so you’ll need a sub and subwoofer amp if you really want the bass to be right. Put another way its really hard to find a speaker of over 100dB that is flat to 20Hz. They do exist, but only in the form of horns that are literally built into the house since the horn needs to be over 20 feet long.

The excessive distortion of SETs prevents them from having the ultimate in detail since detail is obscured by distortion. If the SET sounds really ’dynamic’ its simply due to distortion interacting with how our ears pick up on loudness cues (higher ordered harmonics). Since the transients in music is where the power is, if higher ordered harmonics start to show up in the transients this will impart a sense of ’dynamics’. Quite often when audiophiles talk about ’dynamics’ you can safely substitute the word for ’distortion’ without changing the meaning of the conversation. When SETs sound dynamic you are pushing them a bit too hard. If this goes on all the time you need a more efficient loudspeaker.

Iron rules! My Sanei 2A3 SE amp has all Tango iron - power transformer, choke and OPTs. This is way more important than the brand of tubes!

Here are a few thoughts from a user with a setup that should not work as well as it does ---

My JBL 4430 monitors are only 93dB efficient.

I'm driving them with a relatively old, and humble, 300B amplifier. My amp was originally sold as a kit by Audio Electronic Supply, the kit division of Cary, back in the 1990s. It was not an expensive kit which suggests that the transformers were not of a high pedigree. However, they seem to work just fine. 

This amp was reviewed in Sound Practices back in the day. Actually, there were two reviews of the amp in one issue. One of those reviews was by Herb Reichert, though perhaps that doesn't really matter. He liked the amp. The other builder/reviewer/listener didn't think it was as good as the other kit he built.

In Dennis Had's "manufacturer response" to Herb's review he suggested a circuit mod to incorporate a 12AU7 into the driving circuit. I only mention this to fully 'splain what my amp is like, because my amp has this modification.

Each of us has our own personal listening habits and preferences. I listen in the near field. My listening volumes are typically well below 80dB. Within the context of my listening preferences, in my room, with my speakers, my humble little amp sounds truly spectacular. Audiophile guests have concurred (not that it matters).

My point here is not to disagree or take issue with what @atmasphere has stated. He has forgotten more about amplifiers than I'll ever know and I have tremendous respect for him and the marvelous equipment he builds. He's the real deal, and very generous in sharing his thoughts on this board and for this we should all be grateful. I know I am. 

I'm simply saying that the technical considerations and limitations he shares above do not necessarily diminish the enjoyment that can be derived from a system.

I'm sure my equipment could be better. I know there are better speakers out there and better amplifiers than what I use. But, to the OPs original question my response is that even equipment that is relatively compromised can provide tremendous listening satisfaction. 

Good luck in all you undertake.