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.
I'm in the process of creating the demo for an upcoming video on a 1974 Fender Twin Reverb. This is the point where I'm editing the video and audio layers all together with green screening. The studio-captured audio is not added yet. This is just a rough video mix with the RAW camera guide audio for the drums and guitar ONLY! So this is what the drums and rhythm guitar are sounding like in the room by themselves with no close miking and no post processing (other than a bit of compression and EQ). I am actually kind of shocked how good this is sounding even though one of the camera mics (the one for the drums) is from a DJI Action Cam and the other (for the guitar) is a Panasonic camcorder mic. Usually when making this kind of demo, the raw camera mic audio acts as a guide when lining up video clips on the editing timeline and then gets deleted from the final timeline before publishing. In some cases, I will even mix in a hint of the raw camera microphones with the studio mix to give ...
