WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I worked in TV repair from 1968 to 1973 and shopped the trash dump for tubes, transformers, and parts way back to the early 60's. I never saw a 12AX7 in a TV. 12AU7's yes, they were used for vertical sweep or horizontal oscillators in small black and white TV's. Ditto the 6CG7 and 6SN7.
The only place I found 12AX7's was HiFi or stereo sets, guitar amps, and occasionally an electronic organ, although most of them used 12AU7 or 6SN7.
Pinout compatible tubes you *may* find in a few TVs are 12AT7 or 12AV7 or 12AZ7 .
You won't have 12AX7 there because although they are perfectly fine for Audio, are lacking at RF and VHF frequencies, vital in TVs.
But if you get any of these, they are fine, only none has the same gain as a 12AX7 , which was optimized for Audio.
Sometimes I find a stash of TV tubes and design something specifically for them, but I need at least 100 or 200 units to make it worthwhile.
I once bought 1000 ECC189 and made a ton of excellent pedals with them, go figure.
In the early days of the internet I had learned the brand names to watch out for, and I pulled a 12AX7 out of a TV at the curb, and was excited to find it was an Amperex Bugle Boy. Too bad it tested worn-out.