"Scott Dorsey" wrote in message
...

Every attempt I have seen to build active filters with tube op-amp
networks
has turned out badly, usually due to slew rate limitations. But there
is no reason you can't just build a bunch of passive RLC networks, with
tube gain stages between them. Other than S/N of course.

S/N could probably be made acceptable. What's tough is that at impedances
high enough for tubes to be comfortable with, the inductors need to be very
big and expensive. Unless you transformer-couple the tubes and make the
impedances low, but then you're paying for expensive transformers instead of
expensive inductors.

Peace,
Paul