[R-390] russian tubes?

[atfu]

Dear All

What's this thing about "never, ever install Russian tubes in an R390A!"
(Chuck Rippel)? I only recently got hold of an R390A and so missed the
good old days when it was still easy and cheap to build up a stock of
tubes. Now I see only tubes with somewhat dubious credentials in brittle
boxes at astonishing prices. By contrast, certain types from Russian
production are in plentiful supply, NOS, with a plausible price tag. In
particular, 6K4P looks like a good equivalent to 6BA6 (European EF93).
Well, is it? Anyone having any experience with this?

73 de andre dm1aa