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Vacuum Tubes they have a warmer rich sound.
VACUUM TUBES - Are They Warmer Sounding?
In a word: YES.
Why? The phenomena is simple in theory but complex to explain fully. I'll make
an attempt here to explain. We are talking about the difference between vacuum
tubes and solid-state devices, e.g. bipolar transistors and semiconductor IC's.
Note that I did not include FET devices, which are a form of solid-state device - I'll cover these later. The answer is rooted in the harmonic content that the device can pass or generate. What's known is the human ear finds even harmonics pleasing and odd harmonics annoying or grating, or in the least unmusical. First a word about harmonics. Harmonics’, for those who are not versed in wave theory, are simply frequencies which are related to the fundamental frequency being reproduced. As an example, a 100Hz wave may generate harmonics out to the nth order. How strong and how many of these harmonics are present will determine the harmonic content, or harmonic distortion (THD) as it is characterized (when deemed undesirable). An example: a square wave has 100% harmonic distortion, the fundamental plus all its harmonics are present in equal amounts.
In a square wave (e.g. digital) harmonics are very desirable. In fact the more the better. In the analog world and musically speaking, the inverse is true. What are even and odd harmonics? It just refers to their respective mathematical relationship to the fundamental. Using the same 100Hz, the 2nd, 4th & 6th harmonics (even) would be 200, 400 and 600Hz, the 3rd, 5th and 7th harmonics (odd) would be 300, 500 and 700Hz, etc. In low distortion equipment (i.e. audio gear) these harmonics will decrease exponentially as the order goes up and the faster the better, resulting in lower THD Tubes: there are a couple of important characteristics that separate tubes and bipolar transistors. First, by nature vacuum tubes are not very fast elements. That's a good thing for music lovers. Being relatively slow, the tube cannot generate huge amounts of harmonics even if it wanted to so they act as a kind of intrinsic filter to our benefit. Second, tubes favor even harmonics and are less inclined toward odd ones. In fact tubes a rich in second order harmonics and by the time the 3rd shows up, it is greatly reduced by the tubes natural characteristic.
This is what is attributed to a tubes "warmth" . And lastly tubes are voltage based devices, they amplify voltage. That I'll save for another time.
Bipolar transistors and IC's are very fast, making them ideal digital devices. They operate in two modes: switch mode, e.g. ON or OFF for making square waves or pulses for the digital world, and linear mode, meaning what comes out is proportional to the signal that is put in and is how they are used in amplifiers (power and preamps). They can and will generate harmonics waaaay out there.
They are also rich in odd harmonics which results in the unpleasing or harsh, edgy quality often attributed to them. These devices are, in linear mode [that is, as they are used in amplifiers] current amplifiers. Side note; The first IC's and transistor based opamps that came into being around 1969/1970 sounded awful - very thin and "card boardy".
I remember the first recording console delivered to the studio [a very famous one back then] where I was working at the time and had these opamps in it. It sounded really bad. This was at a point in time where the world was changing from discreet transistors to opamps (amplifiers on a chip) in the professional audio equipment world. Also one inch 8 track tape recorders (not the consumer cassette machines) were a big deal too. Anyway I digress.
In the photo above are five made in the USA Curtis Mathes Vacuum Tubes # 6HS6. These were made at the Austin Texas Curtis Mathes Plant . These are tubes known as old new stock in boxes, because they are just as fresh as the day they were made at the Austin Plant before the tragic fire.
Finally, earlier I mentioned FET's or field effect transistors. Like tubes they are voltage amplifiers and generate predominately even harmonics, making them a very good substitute for the vacuum tube with the same pleasing mucical sound. That is why you see some amp manufactures making a big deal of MOS-FET and other similar devices in their products. Hope that helps and makes some sense.
The invention of the vacuum tube or thermionic valve brought the dawn of the age of electronics. Its invention enabled the wireless technology of the day to move forward. Many new and exciting applications were found for these devices, first as telephone repeater amplifiers and then many other applications that were not always linked to wireless and as a result the new area of electronics was born.