Actually, 35Z5 (not 25Z4, I hope) and 35L6 or 25L6 or 50L6, as well
as the 12SC7 and other 12-volt tubes, are all _easier_ to find than
their 6-volt equivalents.  Most radios made during the late 1930s and
very early 40s were "AC/DC" and used series-strung tubes, so the high-
voltage types are literally sold by the boxful at radio swap meets.

Since your recorder is series-strung, with no transformer, be careful
not to touch anything grounded while holding the mike, or you may get
the (last) shock of your life, depending on which way the power plug
is stuck in the wall socket.  And be really careful working on the
chassis.

Your circuit is so small that just replacing all the wax paper
condensers with modern capacitors (Sprague Orange Drops are the best)
without bothering to test the old ones may be quicker.

By the way, I heard, from a respected radio collector, the story of why
there are so many replacement tubes easily available.  An industrial
study in the mid 1930s concluded that the average radio would burn its
way through four sets of tubes before being junked.  So tube manufactur-
ers were advised to make four more tubes for every one they sold (by the
carload!) to radio manufacturers!  And, unlike the expendable materials
of player pianos, tubes at rest last forever.

Mike Knudsen