Is a 12AX7 a TV tube?
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.
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.
