5718/EC71 vs 5703/6S6B

[OneRoomStudio]

Every tube is an audio tube.
These parts work with simple physics, no quantum effects or internal impedance pre-matching.
Some tubes were targeted for AF as their high internal capacitances precluded RF work.
Av different issue is noise, and special care, i.e. quality assurance, would have to be used in production. Is the noise effect in tubes a random event, or how is it controlled? Cathode mixture composition, eveness of application, thickness? I'm sure much of the art is lost, likely mfg proprietary process stuff.
Time to break out the scanning electron mucroscope, or just get some JFET's.
Just because a tube is not marketed towards AF does not mean it would not do a great job.

To be clear, I wasn’t saying that tubes designed for UHFs aren’t good for audio, I was just pointing out that the EC70 and EC71/5718 were classified differently, and were likely different tubes, not the same as had been suggested.

 

[LevinGuitar]

Even in the EC70 family, there were different variations and military versions and all "sound" different