km1h@juno.com wrote:
>
> VERY INTERESTING point Tom. And something I will have to consider. Past
> experience has led me to classify that arc as instability and the tube
> was yanked. How many times should the darn thing arc before it calms
> down......or does it have to be run red and cooked??
Gettering takes place three ways, by absorbtion of gas during dispersal
of the getter, by heating of a getter, and by the actions of an
electrical discharge.
The gettering agent in the 3-500Z is zirconium (a dull gray material
coated on the anode). Zirconium needs to heated to red-hot temperatures
to be fully activated.
Even when the Zirconium is activated, there is no guarantee it will
clean up a gassy tube. It reacts well only with certain gasses and
gettering is a slow process.
Look at the arc with common sense. In order for oscillation to cause an
arc, the peak voltage has to be higher than the breakdown voltage of the
tube. A healthy 3-500Z can withstand over 12 kV without arcing.
Do you really think the anode voltage can reach 10 kV or more while
oscillating, but can not when driven normally? In order reach high peak
voltages, the anode impedance would have to be very high, Q would have
to be high, and the tube driven hard (by having a lot of feedback). If
that was the case, the PA would make one hell of an oscillator ALL the
time, not just for a few nanoseconds once in a blue moon WITH A CERTAIN
TUBE.
When a tube arcs it is a voltage breakdown problem in the tube, nothing
more....nothing less. Sometimes the arc clears the tube's problem,
sometimes not. Sometimes running the tube helps, sometimes not.
On the Chinese (and used) tubes, Eimac now stamps identifying marks on
the tube.
It's getting to be like printing money.
73, Tom
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