Some tubes, such as TH327/347, caution in the data sheet to not run the
filament without air flow. In their case, the filament power is about 200
watts. If 200 watts is bad for a tube then it seems latent anode heat might
also be ba
You're right about tube data sheets cautioning the need for airflow with
only the filaments. But that's because the filaments disspate enough heat to
raise the temperature of the tube seals too high without cooling. However,
that's a different situation from the latent heat of the anode, since that
can't increase once the power is killed to the amp, assuming the filament
power is killed simultaneously. Of course, if the filaments remain on with
no cooling, then that's asking for trouble!
On the issue of thermal shock with blower left on, remember every time you
switch to standby the same scenario exists. No problem there, so how can
there be a problem at shut down?
Good point, Gerald. I think you're right.
In my view, the largest gain in tube life comes with careful management of
filament/heater current at turn on. I keep thinking about how light bulbs
typically fail at turn on and how that applies to transmitting tubes. One
tube manufacturers' web site suggests a loss of 60 hours of life during
every on/off cycle. Yikes!
I've wondered about the light bulb analogy. Obviously, reducing the inrush
current at turn on is desirable, but I've hardly ever known a transmitting
tube to have a burned out filament (excepting really old tubes, like the
250TH, or tubes that were gassy). Actually, I did lose the filament on an
8877 years ago, but that was because of a bad weld to the filament support
structure. Eimac replaced the tube, even though it was long out of warranty.
More typically, filaments (or cathodes) lose emission, or the grid becomes
contaminated, or the tube becomes gassy or shorts internally. That said, I
still think filament inrush protection is a good idea, particularly with
thoriated tungsten filaments.
73,
Jim W8ZR
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