Bill M wrote:
I think 'copying' would have had legal complications. Usually there was
a licensing arrangement involved. In such a case I imagine the license
holder would have provided detailled information.

It sure did have legal complications, and RCA loved suing people! They
had more lawyers than engineers, it seemed! When the beam power tubes
came out, their patent infringment folks were working three shifts, I think.

Of course Brand B could come up with its on version and get it
registered as their own tube. But the differences between it and Brand
A would have to be sufficient so that they didn't get sued.

Which is why you get the 25L6, which sounds like it's a 6L6, but it's really
not.

RCA was by far the largest license holder and they weren't hesitant to
use their lawyers!

They were the Microsoft of their day.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."