[Amps] tube fils..again

[Jerry Kaidor]

I was thinking about this last night.  AC is hard to regulate.  Has anybody
thought of running filaments off DC?  DC is easy to regulate.  Cheap
switching power supplies can deliver large amounts of efficiently
generated, tightly regulated DC.  Turn-on and turn off ramps can be
precisely controlled.

   Regulating AC OTOH is hard.  Techniques I can think of off the top of
my head include - special transformers with IIRC air gaps that are
controlled
with DC windings that partially saturate them - transformers with multiple
primaries or secondaries chosen by Triacs or SCR's - small motor-controlled
variacs - powerful AC amplifiers with gain feedback...

                        - Jerry Kaidor, KF6VB