> isn't clear how linear this topology is
It is just a vacuum tube.
When well-designed, and peak signal voltage is about 20% of supply voltage, distortion is mostly 2nd, about 5% THD, declining roughly as fast as signal.
So for 30V supply, we can do about 6V peak or 4V RMS at 5% THD, 2V RMS at 2.5%, 1V RMS at 1.25%, more or less. It might be 2% at 1V RMS, and can be far worse, but not a lot better unless level is low and NFB is high.
So the patent's proposed plan gives about 1%THD per Volt of signal, which will put a good "shine" on most home hi-fi sounds. The 6Vpk max level is above the level of most home hi-fi interfaces so it won't go "spzzzzt". Many users won't really notice 1% 2nd harmonic.... but if they paid $999 for this magic toy, they can probably hear something happening.
> the russian 6N28b is degnided to work with 50V
ALL small tubes will work with low voltage.
12AX7 is especially good below 50V, except its gain is so high that very-small inputs will overload the output when powered with small supply.
6DJ8 is another good bet. 12AU7 is terrific too.
> what about this, I don't know if this value should work
It won't work for too many reasons to go into. Why is the input cap 100uFd? Why do you have a cathode follower in front of a JFET? Where is the bias on Q1? How much gain do you have between input and volume control, how small a signal will overload before it gets to a turn-down point?
Use a good transparent mike-amp with gain control. "Capture audio without injury." Then after it is safely stored on tape (disk), run it to something like that 30V tube amp to "injure" it to taste. It needs a gain/loss control in front so you can dial the amount of injury from "nicked" to "crushed flat".
What the patent is really about is a Product where the buyer can swap tubes without running into Safety Regulations. Proposed EEC regulations will require all traditional tube voltages to be "No User Service" sealed boxes. But 24V is generally considered "safe", and I guess they can either stretch that to 30V or build down to 24V for the final product. Now Aunt Matilda and her grandson can swap various tubes all day long looking for "The Sound". Sure the kid will sit on or eat the pretty bottles, but that's not an Electrical Reg problem.
