I have read with interest the renewed debate about cycling filaments
on and off between periods of transmitting. This debate rages
periodically and rather than adding my recommendation against short
term cycling of filaments I thought I would pose the question to my
friend, Reid Brandon at Eimac, for a "straight from the horse's mouth"
treatment.
73 John W0UN
------------------------
Tungsten wire undergoes a physical change when raised to its operating
temperature and returned back to room temperature. In tubes with
thoriated tungsten filaments, the end result is that over many on/off
cycles the shape of the filament begins to become distorted, ever so
slightly but depending on how many cycles, the spacing between the
filament and grid (which must be very close to achieve good electrical
characteristics) may decrease, and eventually a short may occur. This
phenomena has been observed long ago, at first in broadcast applications
(primarily AM and TV) where one on/off cycle per day is common, due
to the nature of the operation (American FM stations operate continuously,
and are not concerned with this problem, in most cases.) Reducing the
filament inrush current is no solution, tungsten must still go through a
large excursion in temperature. Reducing filament inrush current is
important for the reduction in the magnetic field between filament wires,
and allows the spring tension that exists across the filament structure to
gently keep the filament aligned (special springs are used for this purpose
in many transmitting tubes.)
Amateurs who use tubes with filaments (3-500Z, 3CX1200A7 and similar)
should avoid turning the tubes on and off frequently. For periods of a couple
of hours or less one should leave a tube on. One must balance the time the
tube is left on against the time it is to be used, as the tube (in the case
of
Thoriated Tungsten filaments) will become decarburized over time
(Carburization
is a process in which Carbon is created in a portion of the filament wire.
Carbon is necessary for the chemical reaction that generates electrons -
when the carbon is reduced, the tube loses its ability to produce rated
output
power.) It would therefore not pay to leave the tube on from 8 AM to 5 PM
if the tube was only used once between these hours, as the loss of carbon
begins to be more of a concern than the physical changes that may occur
in the filament. Fortunately amateur radio is not a standby service.
Tubes with Oxide Cathodes do not have any carburization and fall into a
different category. In this class of tubes, thermal excursions in the
tungsten
heater are still detrimental to long tube life, although to a lesser degree.
After many on/off cycles, the welds where the wire is attached may fail.
This occurs after many cycles - we have seen this in tubes which are around
ten years or older and the tubes were switched on and off several or many
times per day.
RCA designed some tubes for mobile FM service where the filaments were
keyed, in a mobile two-way radio back around the early 1970's. The filaments
were "keyed" to reduce power consumption during receive. I believe Eimac
made a tube for this application also, but the advent of transistors killed
it off
shortly thereafter. I doubt if tubes like this would operate more than 4 -
6 months
in a radio transmitter installed in a police car.
I do not recommend adding a filament inrush circuit to the Heathkit SB220,
as I don't believe it adds materially to the life of tubes in that
equipment; an
individual's operating habits are more likely to affect tube life.
Reid Brandon, W6MTF
Applications Engineer CPI Eimac div.
http://www.eimac.com
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