Date: Wed, 21 Sep 88 13:54:40 edt
From: uwvax!alliant.Alliant.COM!ll-xn!cook@csd1.milw.wisc.edu (Dale C. Cook)
Subject: Tube Failure Mechanism

Other than becoming gassy, noisy and microphonic -- characteristics that are
present to some extent from birth -- the primary thing that happens to tubes
over their life is that the cathode gradually disappears.  (I think it
gradually deposits itself on the anode, but it HAS been a few years since
engineering school.)  This, in turn, results in a gradual steady loss of
Gm or transconductance.  Which is gain.

Can you hear it?  Depends.  In vintage tube amplifiers, most of the stages
used heavy negative feedback to gain linearity and to compensate for the
(comparitively) wide variation in transconductance between tubes of the
same type.  In those circuits the active element gain should be able to 
vary quite a bit before you hear it.  In the front end, however, you
might not have as much leeway.

As far as the premium tubes go, I'd guess that you are paying for: (a) the
limited (and falling) demand which means small production runs vs ''the
good old days'' when tubes were in greater demand and (b) more rugged
construction which should mean lower noise, less tendency towards micro-
phonics and longer life.

Finally, as to Tom's question about DC filaments, by all means!  I seem to
remember some of the premium amplifiers of audio's golden age touting this
as a feature.  Any other gray beards out there remember?