300b, x45, 211, etc.

Here's a great offline response I got from Dr. Chau that I'm posting with permission:

*******

First, you have to realize there are no absolute rules to the sound of a
particular tube. The end result does depend on implementation. However,
you can breakdown trends in two ways. Each family of tubes does have a
commonly accepted 'sound'. Also, many SET manufacturers have a house sound
including Cary.

300B: the most recognized SET tube. Often said to have a rich, warm and
glorious midrange. Also often said to be a bit rolled off on the top and a
bit loose in the bass. Still, it's reputation is from the glorious
midrange. The first popular directly heated tube in the US (had been
popular in Asia and Europe for much longer).

2A3: similar to the 300B but much less power (typically 3W). A bit cleaner
in the mids, more top end, and much better in the bass. The bass from a
2A3 is really good depsite the low power. This became popular in the US in
the late nineties since it was cheaper than the 300B and considered to be
cleaner sounding than the 300B. FYI-I currently use a 2A3 at home although
that will change in a week or so.

45: an even older tube than the 2A3 or 300B and even less power (2W). This
is the current hot tube in SET. Considered even more transparent than the
2A3. Supposed to be better in every way than the 2A3.

The above tubes are directly heated triodes but use an oxide cathode like
most small signal tubes. The next group of tubes are transmitting tubes
and use a thoriated tungsten heater/cathode

211: a popular transmitting tube. Can put out 12-15 W on a single tube.
I've only listened to a 211 on several occasions. I find it a bit thicker
sounding than the 2A3 and 45, more like a 300B.

845: the current hot high power SET tube. can go up to about 18 W on a
single tube. I find it somewhat less transparent than the 211. Many
commercial amps are available with this tube now.

The bass on the 211 and 845 is probably a bit bigger than the 45,2A3, or
300B but not as quick.

Other tubes:

212, 807, 811, 833A-other transmitting tubes. Some are really large like
the 212 and 833A. These can put out a lot of power (100W for a 833A) but
can be dangerous as they need very high voltages on the plate to achieve
max power. The 811 and 807 are smaller and in between the 845/211 family
and the 45/2A3/300B family.

10, 101D, VT52, etc. other small triodes in the vein of the 45. These
often are considered even better than the 45 but have really low output and
are rare. Very few commercial designs are available.

The above comments are gross generalizations and based on my own
experiences. Again, a good design can change the characteristics. A
current thread in the forums is to run the 845 at much lower than normal
voltages (i.e. lower power) which changes the sound.

Also the above tubes are all triodes. One can get good single ended sound
from pentodes and tetrodes. I just purchased a Shindo Cortese to replace
my 2A3 amps. These run a F2a pentode.

In addition to a specific tube sound, manufacturers have a characteristic
sound. Cary certainly has a family sound. Even though the first SET I
heard was a Cary and I used to own a Cary, I'm not a fan of their sound.

As to you question about moving to another tube, you will can find big a
difference even if you move to another manufacturer's 300B amp. It all
depends on what you want in terms of sound. For SET amps, I think you have
to consider the amp and speaker as one unit. This is especially true for
the lower power tubes. If you can, try to listen to a 2A3 or 45 amp since
I think you'll hear the biggest difference in these tube tubes compared to
a 300B. You can only answer you question by listening.
****

For SET amps, I think you have
to consider the amp and speaker as one unit.

Very nicely put! Indeed, in the old times, the speaker came with its amp -- together. Remember those old radiogrammes (or whatever they were called) and radios?

The 1610 is nothing like the 300b. It's huge and made by a company called KR. They also have an amp using it (which is where I heard it). In single tube/channel config they manage +20W / channel. I think they must be using transistor drivers -- but am not sure.
Similarly, the 211 requires a different circuit than the 300b. But it's seriously more powerful.

Of course Biomimetic is right about feedback which can and is used to stabilise circuits... however, one can stabilise a circuit without feedback if one is into that sort of thing. One can't with a transistor for example.

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.