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.
