Commercially, what is usually done to protect thoriated-tungsten
filaments (for tubes with handles) from excess inrush current is to
limit the current to no more than 2 X the rated current at nominal
operating voltage. This is specified in many CPI/Eimac data sheets. If
you are designing from scratch, all this requires is to ask your
filament transformer mfr to
make the transformer so that it current limits by magnetic design. This
reliable approach is nearly universally used in broadcast and industrial
RF systems with tubes, and is simpler than using PTCs, step start
relays, triac ramp up, variacs, etc.
For really big tubes (tubes with lifting hooks) like the RCA/Burle 7835
triode (with 96 filament bars, each carrying 75 amps) and Thomson/Thales
tetrodes (TH781, TH558, TH526, TH628 for example) it is required to ramp
up the filament from zero using a slowly increasing voltage (500 seconds
typical), to allow the filament structure to mechanically adjust in
length as it is heated, so as not to bulge & touch the nearby control
grid. It is also important, on these tubes, to not apply any DC bias to
the grid until the filament ramp is complete, so that if there is a
momentary short, it doesn't connect filament potential to the bias power
supply and damage supplies or spot weld the filament in this position.
There is no such thing as instant on with these tubes except when there
is a momentary power glitch that comes back within a few seconds, in
which their filaments have enough thermal inertial to remain hot.
For the prototype power amplifier I am preparing to test in a few weeks,
we just installed the filament cables. They are 1100 MCM cables, that
are as thick as your wrist. They are 15 feet long to the power supply,
which fills one equipment rack (20 kW filament). Tube damage from
filament overcurrent is unacceptable in this case, as the tube costs
more than a yearly salary or the cost of a home...
73
John
K5PRO
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