filament voltage

[Glen Zook]

One thing is that most modern tubes use heaters
instead of filaments.  It is the purpose of the heater
to actually heat the cathode to "drive off" ("boil")
the electrons.  With a true filament, just applying
voltage to the filament will start this electron flow
and thus a tube with a filament will operate over a
much larger variation in voltage.  Just as soon as the
voltage is applied to the filament this electron flow
starts.  With the indirectly heated cathode, the tube
must "warm up" before the electron flow really gets
started.

Many of the battery sets of the 1920s actually varied
the filament voltage to control the volume instead of
a more "normal" type of volume control.  As the
filament voltage is increased or decreased, the gain
of the tube varies (higher filament voltage equals
more gain) since more, or less, electrons are "boiled"
off of the element.  This technically can happen with
a tube using a heater (indirectly heated cathode), but
only slightly and it is not instantaneous because of
the lag in the heating of the cathode as compared with
a direct filament.

Operating a tube with the cathode not fully "heated"
can eventually cause problems in the chemical "make
up" of the indirectly heated cathode.  Also, as a tube
nears the end of its useful life, increasing the
heater voltage a certain percentage will increase the
actual temperature applied to the cathode and thus
restore at least some of the performance of the tube.
The cathode eventually gets to the point where the
number of electrons that can be "boiled off" is
reduced substantially from when it was new.  By
increasing the actual temperature applied to the
cathode the number of electrons can be increased,
sometimes to what the level was when the tube was new.
 Additional voltage applied to the heater is what
causes this.

This is how the old TV picture tube "rejuvinators"
worked.  They increased the heater voltage 10 to 20
percent and thus got the cathode to "boil off" more
electrons.  Of course, the use of such a device can
shorten the life of the tube.  But, since the picture
tube had already started going "dim", the "trade off"
of reduced life versus getting a few months more
practical use was moot.  The tube was already
basically unusable and adding the device helped for a
while.

Now, most tubes will actually operate with "heaters"
over a fairly large voltage being applied.  Of course,
you want to keep in the middle of the range if at all
possible.  But, most receiving and most power tubes up
to at least the 100 watt range will operate without
damage over at least a +/- 10 percent range, and, for
those tubes that were designed for the two-way radio
market (including the 6146A and later versions), up to
+/- 20 percent without any major problems.

Of course, virtually everyone refers to the "light" in
the tube as a "filament", but in the vast majority of
cases these days, it is actually a "heater".

Glen, K9STH


--- Chris Campbell <clcampbl at TRAVERSE.COM> wrote:

Why not?  I just fired up my 1922-vintage Crosley VI
and was surprised at how far down you can crank the
voltage on each filament without affecting function.
These are old 5-volt filament 201A triodes.

=====
Glen, K9STH

Web sites

http://home.attbi.com/~k9sth
http://home.attbi.com/~zcomco

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