300b, x45, 211, etc.

True, Greg. I should have been clearer. There are zero feedback SET's, but I think it's more of a "differential of the circuit" except in the case of direct heated tubes - or am I wrong about that? I do think, for all the noisiness of a direct heated tube, they do have something "special" regardless of their own internal topology. And it's never so with transistors. I disagree with Dr. Chau about the 845, which to me is actually quite a transparent tube. Of course, I've only heard them implemented in Mastersound's amps with a Hyperspace/47 lab phono pre/Cardas cartridge. The thing I find cool about SET's and the reason I love using them, aside from just the fact that they are actually exotic (like a minty Giulietta Veloce, or a vintage 1930's Rollei camera...) is that the holistic approach is the only one that works b/c of the low power; do you use speakers that are really super neutral, like Joseph Audios or go for the total overload of richness like a pair of Proac's or something?

Hi Bio: nope, the speakers were a mess

The tubes were driving a tractrix horn loaded lowther ex4. 120 Hz horn. Then I had to use a suitable bass driver capable of playing the game (that was fine, a Supravox 400 did the trick). Then, not surprisingly, I wanted to get rid of/tame that annoying wizzer-related intermodulation the lowther had by extending the upper end. Of course, wishing to play purist, I couldn't consider sticking a filter onto the lowther, now, could I?
So, I was looking around a tweet to mate well with the lowther. Never found one; many available supertweets, for example, Murata, are low sensitivity -- which in turn means another amp, sonically matched to the lowther amp, but producing ~4x the power or more... a big mess:)

I like your point about the exotic nature of SETs -- similarly a prewar Leica.

Thanks Joe for the kind words, but in all modesty I rarely use my title even in a professional setting. It's too prentious.

For Joe,

The PX-25 is a pre WW-II power triode developed in Europe. Real PX25's used an odd B4 base rather than the more common UX4 base and could put out a bit more power than a 300B. The PX 25 is a bit more linear than the 300B and supposed to have better extension at the extremes. Real PX25's are rare and I have only seen them in Japan. KR started making 'PX25' a few years ago. This tube has curves that are like old PX25's but slightly different electrical ratings. The new PX25 has a lower maximum plate voltage and can be found in both b4 and UX4 basings. The only amp I have heard using the PX25 is the Art Audio PX25.

For Bio,

The main point in my message to Joe is that implementation is more important than the sound characteristics of a particular tube. The Mastersound 845 reference is the best 845 amp I have heard. It is even better with the right driver tubes and some mods. My friend who is a long time Mastersound dealer also runs his with a Hyperspace into JM Lab Utopias. Other 845 amps I have heard (Air Tight, Quadric, etc....) have not been so transparent. I am biased because compared to a 2A3 and 45, I do find the 845 a bit thick.

Even though I agree with Greg about the 300B in general, I have heard 300B amps that do not have those flaws. Try listening to a Shindo 300B limited.

One final point, I think the sensitiviy issue with SET are often overblown. For many types of music, 8 W into a 90dB speaker is fine. I ran a 2A3 for many years into a 92 dB speaker. Except for large orchestral, it was fine. Your own mileage may vary. Remember that it takes double the power to increase the sound pressure by 3 dB.

Rchau - Are you talking about the guy at Taurus Audio in the Bay Area? What a *GREAT* room that is, as in *WOW*. And a cool guy too. The only system I've heard I liked as much was an Airtight (the KT88 one, I can never remember their designations) with a pair of large Opera Piega's. But the turntable was not on par with a Hyperspace. I sort of remember the guy at Taurus saying the phono was 47 lab... but if he's your friend, maybe you know differently...

I started a thread "Lowest power amp with Joseph Audio RM33's" - I have heard of someone using a 2 or 3 watt 45tube SET with a Pair Harbeth 40's (was in 6moons I think) and getting decent, though not spectacularly loud results... but of course I got a guy who told me "100 watts solid state minimum"; which is to me what leads to this sort of view of SET as difficult and unreliable. No one ever checks the ohms on the speakers which even if it's 4 ohms minimum, should be workable at 15 watts, if they're good. It seems to me a pair of Revel M20's at 4.4 ohms minimum would work as well with a Cary 300b as a Levinson, and might possibly have good synergy (crazy rich mids, and extremely neutral speakers), at least for a cheaper system. So 6 ohms minimum to me seems like it should run just about anything... or am I hi-jacking the thread?

Biomimetic, good stuff, keep it coming.