Sometimes there may be no difference. Otherwise the quality of mfr and parameters
like mu, gm, rp, can vary. Sometimes one tube type is relabeled as another type.
Also the crosstalk between the two triodes in one envelope can vary.
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Wait until the moment you'll have to compare different rectifier tubes with almost equivalent Rp, but different brands. Especially the solid vs. mesh plate comparison of AZ rectifier types. 
I have no idea, to be honest. Probably something to do with tube emission, purity, construction, materials used?
From my personal experience, my favorite sounding brands are Telefunken, Philips and Siemens. But there are "sleeper" tubes too. Ruskies such as the 4P1L, the 6P45S and the 6N6P. RTC (Philips) TV tubes.
I have no idea, to be honest. Probably something to do with tube emission, purity, construction, materials used?
From my personal experience, my favorite sounding brands are Telefunken, Philips and Siemens. But there are "sleeper" tubes too. Ruskies such as the 4P1L, the 6P45S and the 6N6P. RTC (Philips) TV tubes.
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No Physics involved, but Psychology.
Tubes vary wildly, but one to the next, brand alone is no reliable parameter.
After you spend some time around this subject, and find that during the tube heyday different tube manufacturers contracted with their competitors back and forth and around the industry to build tubes for each other but put the buyers brand on them to put into devices, you might talk to someone who wants to tout one brand as special but that company may have made tubes for 6 different companies and the touter will snub the others that are exactly the same.
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Differences in geometry can have a significant and measurable effect on the linearity of a tube.
Curve tracing will often reveal some differences, and an amplifier test circuit with FFT often will show very significant differences in total distortion and the harmonic make up of that distortion.
Perhaps someone has a few FFTs they can share, I unfortunately don't.
Curve tracing will often reveal some differences, and an amplifier test circuit with FFT often will show very significant differences in total distortion and the harmonic make up of that distortion.
Perhaps someone has a few FFTs they can share, I unfortunately don't.
A lot of this may be audio-foolishness. Typically, Telefunken, Mullard, etc VTs cost a lot more, and therefore should sound better.
There may be a basis in fact for this as well. VT's are low gain devices, unlike transistors, and so the actual performance is more dependent on the active device used. Transistors, even in low gain applications, are inherently high gain devices, and so SS performance is more dependent on the circuit, and less so on the actual device. It doesn't matter much what transistor you use so long as it can handle the frequencies involved, and no special properties like an unusually low noise figure, are involved.
Not so with VTs, and especially in circuits that don't include NFB.
If you take for example 6P3S from the same Svetlana, of the same data code, I bet in an amp with zero feedback they would sound differently.
But I had once metal 6L6 made by RCA back in 1942. All of them were like twin brothers. Measured identically, and better than spec sheets.
In amps with properly calculated regimes and with feedbacks (you know I prefer nested feedbacks) I sometimes use mix of tubes from RCA, GE, Sylvania, etc., no difference. All sound as nice as possible, when they are alive and well.
But I had once metal 6L6 made by RCA back in 1942. All of them were like twin brothers. Measured identically, and better than spec sheets.
In amps with properly calculated regimes and with feedbacks (you know I prefer nested feedbacks) I sometimes use mix of tubes from RCA, GE, Sylvania, etc., no difference. All sound as nice as possible, when they are alive and well.
Some makes of Tubes certainly last much longer than others; One of my friends used Russian 811A's in his amps for close to 20 years Still measured as new; then he tried Chinese 572B's after 18 moths they were as good as dead. I have heard of USA made 805's lasting for 20+ years in transmitters; Must just be better materials.
It was widely known through out the electronics industry of 40 plus years ago that Telefunken tubes were virtually the quietest available. Test equipment manufactures like Tektronix, Ballantine and others used them in certain locations in their products. Boonton Electronics used them exclusively in their early RF millivoltmeters because of their low noise properties. And I'm talking about the real Telefunken tubes that were labeled "made in West Germany" with the diamond shape in the bottom of the glass. I think Marantz supplied them in some of their early equipment as well.
People that claim Mullard and other Euro tubes to be better "sounding" are pretty much correct. Exactly why this is I cannot say. But it obviously has something to do with the way they were made. As a long time active member of the New Jersey Audio Society over the past 30 years I have sat through many tube rolling sessions. And as a result I can truthfully say that tubes like Mullards, Tellies, and Amperex Bugle Boys are usually always the clear winners. Sure there are others, but these are the most popular. At least here in America.
So how do they sound "better". To be honest the differences are subtle. Sometimes a particular instrument just sounds cleaner or clearer. Sometimes it's a voice that's better defined. Or it could be better low end (or high end) response or presentation. Subjective? Perhaps. But when agreed by virtually all listeners there's less chance of that.
Valves can add even harmonics which is good to the ear.
Many genre guitarists love the harmonics given out by a valve amplifier.
Sounds better can mean adds distortion but not all distortion is bad.
I saw a demo on YouTube about how how the harmonics in tubes added to the music.
Some may like the sound, but others may not, it's all about personal preference.
When one listens to live music, especially in a defined space like in a medium sized room or even a concert hall, you are hearing harmonics produced by the instruments. Especially stringed instruments. They all produce overtones. (I don't mean loud screaming rock guitars) My ears tell me that some of these are lost when recording. Probably beginning with the microphones themselves. Yet we try our best to reduce harmonics from the output of our amplifiers. In this light it seems wrong. Yea, I know we're talking about internally generated harmonics. But how many of the original good harmonics are we loosing? Not even the best direct-to-disc recordings can capture and reproduce this exquisite sound.
I believe only people who have impaired hearing can love intermods.
This myth about "people loving harmonics" was invented by sellers of horrible transistor amps that claimed their superiority. I can simulate you the same result using one opamp with diodes in feedback. The same curves, the same sound, like from a "no feedback tube amp"
This myth about "people loving harmonics" was invented by sellers of horrible transistor amps that claimed their superiority. I can simulate you the same result using one opamp with diodes in feedback. The same curves, the same sound, like from a "no feedback tube amp"
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There is a big difference between harmonics generated by each musical instrument, and harmonics generated by an amplifier. When an orchestra is playing, each instrument generates its own characteristic harmonics, but there is no intermodulation distortion in the concert hall resulting from these individual instruments' harmonic "distortions" interacting with each other.
By contrast, if the amplifier in the home introduces harmonic distortion then there will be IM products resulting from the interactions between the various components in the audio signal. You cannot really re-create what might be "lost" in the recording by introducing distortion in the home amplifier, at least if more than one instrument is playing.
There are no real benefits to be derived from having a distorting amplifier. Even if it generates only second-harmonic distortion this will still lead to unpleasant IM products, especially if the musical signal is a complex one with multiple instruments.
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Brand of valves means nothing, as most of them happily rebranded valves from their competitors. Place of manufacture and exact design details matter to some extent, as this can affect microphony, hum and noise. In most cases a good circuit can avoid differences caused by brands etc. as the circuit behaviour will be set by resistors rather than valves.
Some brands have their "mark seals" and are difficult/impossible to be copied. For example, Telefunken noval tubes have their trademark logo stamped in the glass during manufacture, while it being in a soft state. Philips tubes have marks etched on their glass bulb. Siemens tubes have an inner metal plaque with the date manufacture stamped on them.
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