Considering A 20-40 Watt Stereo SET Amp

I like the el84 tube and it is often used in pretty nice sounding low-cost amps.  It is an indirectly heated pentode tube, but, the tube type does not determine anything particular about its sound.

to put what @atmasphere has so eloquently stated into a nutshell - it‘s all in the implementation…

there are many many out there, new and old, i cannot say i have kept up with the newer options but in my past travels i have really enjoyed dennis had's cary 300b series ... in particular i still have an original 300b sei... with lovely 6sn7 input and driver tubes -- really lovely sound with speakers that are efficient enough

good luck on your search

I have multiple systems. But I pretty much have had a SET-based system since the late 1990s. Owned many many SET and speakers that can run on them. With SET the lower power models are mostly the best sounding. But you have to carefully select loudspeakers and today not many truly SET-friendly designs exist in the market. I joke with others about SETing one’s system up for failure and when I see SET amps it’s mostly not on a SET-friendly loudspeaker. I design my own loudspeakers for SET use and have done so for nearly 30 years. SETs are amazing but all your ducks need to be in order to hear this and once done you don’t have any real limits on music types played and SPL. A common complaint from those who SET their systems up to fail.

I believe that Canary Audio makes both zero NFB SET and push-pull 300b amplifiers. I don’t know if anyone has done a direct comparison between these two versions.

@charles1dad I have to imagine Canary has... PP amps, just like SETs, vary quite a lot depending on design concerns! For example, single-ended circuits exhibit in mathematical terms a quadratic non-linearity, which describes how the harmonic distortion arises. When measured, it is found that the math accurately predicts the harmonic content.

OTOH, a fully balanced differential circuit exhibits a cubic non-linearity. In this case the even orders are cancelled; ideally only odd orders remain, but they fall off at a much faster rate as the order of the harmonic is increased (with the 3rd harmonic masking them). This again is accurately predicted by the math.

So this is where I'm going with this: when you combine both single-ended and PP circuits in the same amp (such as seen in almost any traditional PP amp, like a Dyna ST70), you get both non-linearities. This tends to exacerbate the 5th harmonic (Norman Crowhurst also wrote about this problem).

So if the comparison is between an SET and and a traditional PP amp, you are getting more 5th than you would if the PP amp was fully balanced. I've no doubt this is a lot of why SETs can be preferred.

These are all readily solved engineering issues though- its not hard to build a PP amp that does not have that problem! This is part of what I meant when I used the phrase 'cast a wider net'.