Tube amps have a signature sound

The most consistent sonic tube signature is not the sluggish droopy ill defined sound of the golden era of "hi fi"!
Instead I am convinced it is a much more pronounced 3D soundstage. That is if you are talking of the currently produced tube amps. The degree of upper end sparkle and controlled bass, is very variable. That is clearly audible however there is in addition a very important psycho-acoutical sound.
It is referred to as a distortion, but you can't pick it out it is sort of invisible, if you will. I think is 2nd order distortion. Again I can not describe it but it is there and most people really like it. I can say what it is not.It is NOT poorly defined, fuzzy, rolled off, sound or anything of the sort. It is a highly desirable harmonic phenomena.
If you want to hear what I think people refer to as tubey, then you might try to find a tube amp from the "golden era of Hi Fi" i.e. the 50s until the mid 60s". That would be the extreme of the misapprehension of what a typical modern tube amp sound. It is by all means coming from a tubed amp you are thinking of what you heard 40+ years ago. And for all of you who think I don't know that current tube amps vary widely from directley heated triodes to push pull to SETs and they all sound different you are incorrect. However if you have the experience of listening to a current tube amp of any stripe and say that the usual perception/recollection of tubey, then surely you jest.
I happen to enjoy that Hi Fi sound when feeling nostolgic, which is only occcasionally. If you really want to hear the old tubey sound IMHO, Don't forget you will probably hear it best with the same era speakers. For some odd reason I find the source doesn't seem to have the impact it does on modern systems, but clearly a "record player" is appropriate.
Most of these amps are integrated but if component the pre will already have a "phono" stage input, MM BTW. The only other choices were a tuner and reel to reel for the truly insane. I attribute my ability using curse to the Reel to Reel we owned. My father would become increasing frustrated until his rather expansive cursing vocabulary spewed forth. I recall it was about a 50 - 50 chance that it would get tangled up, off track or some other other calamity would happen.
Or I could have simply said just go out and listen to a !@#%%^&* highly regarded, modern tube amp and you will hear what a tube amp sounds like..

What experts have to say has nothing to do with one's decision if you really spent the time to listen to live music.
For example, going to concert, listen to how a live piano and violin play.
The human ears are very sensitive and very adaptive at the same time; if one tells the brain that this is closer to live music, that would be closer to live music.
I do not believe any of the so-called "experts", for example, those "tin-ears, but big money marketing people" would give you, or the poor so called "audiophile" honest answer.
They always recommend the big advertising money sponsor's equipment.
TRUST YOUR OWN EARS !!!!!!!!!!!!!!!!!!

If there is a 'signature sound' it is the low levels of odd-ordered harmonic distortion which our ears are evolved/created to detect. Odd-ordered harmonics are how the ear detects loudness. Tubes make far less of this than transistors. Transistors don't make much either, but the hundredths of a percent that they *do* make is a demonstration of how sensitive our ears are to this type of distortion.

By contrast our ears do not care so much about even-ordered harmonics, but a properly set up push-pull tube amp is not going to make a lot of those either. So if the amplifier is set up right, and other important design considerations are met, a tube amplifier will sound more natural to our ears than a transistor amplifier will.

So IMO, the 'signature sound' is music.

There are technical reasons why tubes sound different from transistors, usually. Why there is so much controversy surrounding it is a mystery to me when there is plenty of objective reasoning to show they are different animals.

First, the tube is a higher impedance, higher voltage, lower current device than the transistor in almost all cases. This has to alter the design topology and interface between components when dealing with those differences alone.

Second, the triode is a unique device not duplicated in the transistor world in terms of its characteristic curves. There is a tiny but mostly unused and ignored portion of the curve of a FET (JFET, MOSFET, it doesn't matter) called the "triode" region. This is the only place it mimmics the behavior of a triode, and is basically so small an area as to be unusable. The FET is used in the "saturation" or "constant current" region, and mimmics more closely the pentode. A BJT is a current amplifying device that is also different than these others.

The triode has curves that have the following properties: nearly infinite DC impedance in common cathode configuration except for the biasing resistance, and current increases with increased plate voltage over the entire range of operation. Pentodes, like FETs, don't act that way because they go into constant current for increased plate (drain) voltage. In both cases I'm talking about holding the input voltage constant when raising the plate (drain) voltages.

This triode characteristic curve does an interesting service in a typical common cathode amplifer stage: as plate current increases from increased input voltage the plate voltage decreases as any inverting amp stage should, but with the decreasing plate voltage the tube wants to counter that increased plate current. This is local negative feedback built in without even trying.

So the triode typically has lower gain and lower distortion than a pentode or FET that does not have this local negative feedback working on its own.

You can make this triode amp sound more like a transistor amp by applying the same global negative feedback techniques that most of the transistor amps use, and you might like that. But there are limitations: triodes are lower gain and lower bandwidth usually so less NFB can be applied for similar overall gain. Thus you are left with some residual triode sound even if you try to squash its signature distortion.

It's not that there's a frequency response rolloff that accentuates the midrange. That midrange emphasis is more likely a feature of the distortion spectrum of the devices in the midrange compared to the extremes from two likely sources: A loose midbass with lowered NFB and poor PS regulation can produce harmonics into the midrange that decreases with increasing frequency. And then in the highs there is again a decrease in overall NFB which changes the uniformity of this distortion spectrum. The result is a frequency dependent distortion of unique triode-produced distortions, be it considered good or bad.

And while some people claim that distortions are not audible when "linear enough", also realize that in a bottle of wine that contains 14% alcohol and a lot of other complex chemicals, the whole bottle can be ruined by 5 parts per BILLION of the chemical that makes it taste "corked".

There are three kinds of distortions: audibly pleasant, audibly unpleasant, and inaudible. How much is audible of what kind of distortion is very difficult to find out through experimentation since it's almost impossible to alter just one kind of distortion at a time.

What is considered "most accurate" is still a judgment call. Many people say the triode amp sounds "most accurate" when compared overall to live, in most implementations, and I still agree with that. Why? I don't know and I can tell no one else has that down to a solid science yet.

Kurt