Actually, one of the most common misconceptions newbies face is the misconception they get from other misconceptions.
It's really hard and I admire you for your interest and willingness to study vacuum tubes.
One of the most common misconceptions by tube newbies and those caught up in the "audiophile" marketing hype is that certain tubes are "audio tubes" while others are "TV tubes", etc. When they were first designed, vacuum or electron tubes were created as general purpose tubes. Depending upon their unique characteristics, raw materials availability, manufacturing costs, etc., some tubes were later assigned to specific purposes. For example, a popular preamp tube - the RCA JAN CRC 12sx7gt - was originally used for radar and avionics by the US military. However and because of its unique characteristics, it later found contemporary use as an audio preamp tube. JAN (Joint-Army-Navy) tubes are particularly sought after now since quality is more consistent in light of the US military's more stringent manufacturing and technical specifications for these.
So if they were all the same then why did they made a 12SX7 variation if the 12SN7 already exist? Why not use the 12SN7 instead? Was RCA making a new variation so that it can ask for more funding from the military?
The answer in the datasheet was so it can survive greater G and a longer service life. Back then, a longer service life correlates to reliability. We all know how crazy the military can get when it comes to reliability.
Technically speaking, there is no such thing as an "audio tube" or a "TV tube", etc since electron tubes do nothing more than amplify ANY electrical signal. Later on, certain tubes were designated as transmission tubes due to their high amplification properties or as audio tubes due to low microphonics.
Most of the so-called audio tubes now are nothing more than general purpose tubes that were later designated for audio use for marketing reasons. Mostly because of high demand from audio enthusiasts from Japan, Taiwan, etc. who wanted to soften the hardness of digital music from CD, etc, vacuum tubes that used to cost mere cents and were later designated through marketing hype as "audio tubes" (such as the NOS 300B, 2A3, etc.) have skyrocketed in price due to market speculation. Once an unknown general purpose tube gains popularity as an "audio tube" and speculation again drives the price up (a Taiwanese once bought up tens of thousands of a particular unknown tube when it gained popularity as an "audio tube" among the DIY community), DIYers who are mostly seasoned electrical engineers like Tony who have been exposed to both the analog and digital side of the technology then search for other tubes to use. You'll see this phenomenon happen often on DIY audio sites.
Once a tube becomes expensive, Tony and the other DIYer's then search for other unknown but still relatively-cheap tubes which can be used for audio and then design and build customized circuits for these. At the end of the day, what truly counts in tube audio is the sound and not the marketing hype surrounding certain tubes since this only serves to fatten those in the business of milking money out of those who don't know any better.
Ah... there may not be an official designation for "audio tubes" and "TV tubes" but there exists a "classification" that most tend to overlook. Especially those who believe that all tubes were made to do the same thing.
Tube designations were given by virtue of "application", transmitting tubes were designated transmitting tubes because they were designed to be one, just like receiving tubes, mixing tubes, amplifying tubes, switching tubes, rectifying tubes (and the list goes on) does.
"Technically speaking" is a big word to use so excuse me if I would not want to apply that here now, why?
Simple.
Audio amplification operates on a certain frequency. So does FM, VHF, UHF, radar and yup, TV circuits as well. If you don't consider this in your designs then you'll have linearity problems that ultimately lead to other problems such as distortion among others.
You may not hear it (good for you), but it's there.
Finally, any "seasoned carpenter" can use a pair of pliers to drive a nail and it won't matter much... as long as the carpenter understands the difference between a plier and a hammer.
Oh well, just my two cents. Everybody have a nice day.
