Here is a chart I made in response to a question about direct 12ax7 substitutions. There are more than most people realize. The RCA manual designates the pinout as "9A", so I've included all "9A" pinout tubes here.
I've included all the industrial and military designations of these tubes also (where applicable), and arranged them in order of amplification factor. I did not include European designations like ECC83, etc. Hope this helps someone with their experimentation. As for which of these will work in certain applications, you'll have to do more research and conduct your own experiments. Generally, if a 12ax7 is called for in a certain position in one of your amps, probably any of these tubes will work as a drop-in substitute. The lower the amp factor, the less gain you'll get from that stage, which might be desirable. If the stage is a cathode follower and calls for a 12AT7 or 12AU7, subbing a 12AX7 or another of these tubes might create issues. I encourage you to experiment. This would actually make for a nice video comparing as many of these as possible. I think I have most of them in my tube collection already. If you'd like to see a video like that, let me know.
Here are some unused outtakes from my recent video. This is me jamming to see what comes out, while in the process of writing the demo piece at the end of the video. At one point, I did use a section of this jam as the intro of the demo, but unfortunately my AT4040 mic into the interface malfunctioned and rendered most of that stuff unusable in the end. This is just what was picked up by the onboard camera mic. Some of it was actually pretty decent. I felt inspired anyway. When I feel like I'm jelling with an instrument or amp or whatever, I like to just let the camera and mics roll and see if anything is worth keeping. Sometimes magic happens, sometimes nothing seems to be worth a single solitary shit. Most of the time, it's a mixed bag. It's funny listening back to stuff like this as the guy who did it, I can always predict where I'm probably about to go with it, even if I don't remember specifically anything I played. Funny how the mind works like that. As for the onboard camera ...
Here's the video of Routh's collected tweets if you'd like to comb through them. https://x.com/BillboardChris/status/1835452723929457000?ref_src=twsrc%5Etfw%7Ctwcamp%5Etweetembed%7Ctwterm%5E1835452723929457000%7Ctwgr%5E17672e4635927c4505689a4865f3cd0e62c6f309%7Ctwcon%5Es1_&ref_url=https%3A%2F%2Fwww.infowars.com%2Fposts%2Fdemocrats-accused-of-incitement-after-second-attempt-on-trumps-life
