At 11:50 PM -0400 9/1/00 W8JI wrote:
>Tube faults like that are intentionally removed by controlled arcing,
>and uncontrolled flash will do the same thing.
>
If the tube repeatedly arcs, it needs to be replaced.
MY EXPERIENCE SUPPORTS THIS PRACTICE, BUT ONLY IN VERY HIGH POWER
TUBES IN PARTICULAR:
The 'debarnacling' of tubes is a routine practice, esp at higher
operating voltages (>10 kV). We routinely do this to every new tube
that we receive from manufacturers, per their own instructions. One
thing that must be followed is their rating on the amount of energy
per spark over. This is, of course, determined by the size of the
energy storage, the speed of the AC shutoff circuitry, and the series
R or L from the power supply. If the energy is not enough, then the
barnacles may not get polished over. If too much, then holes will
appear in grids and even cathode material will be blown off. A lot of
the processing is needed because manufacturers do not have the time
and facilities to spend many hours on each and every tube they ship.
They have to eventually make a 'pack it and ship it' determination,
based on a certain time in their test set, or maximum number of
arcdowns. It is up to us, the customer, to complete the processing
that we have to do, before we put them into 24 hour a day service at
high power.
Gas is another source of tube faults, which also must be processed.
It is not usually a problem in smaller transmitting tubes but becomes
a major factor in operating with 100 kW and up. The volume of the
tube envelope, the complexity of the internal structure, the type of
getter, the way it is gettered, the amount of RF heating in the tube
elements, and other similar factors will cause gas to release over
the life of the tube, but this can be pushed up by processing the
tube at high dissipation, with an energy limited anode power supply
to prevent damage when the gas evolves in the envelope. By pulsing
the tube at low power at first, then slowly increasing the duty
factor and RF power, you can get there in a short time (hours,
sometimes days for bigger bottles). This is even written into the
warranty conditions for some of our tubes from their respective
suppliers.
Maybe evolves is the wrong word, desorbs?
So there are two processes for high power tubes, debarn'ing and gas
processing. For smaller tubes like 3-500Z and 8877, and <5kW
tetrodes, the normal warm up time and instructions from the
manufacturer are usually sufficient. However, occassionally a bad
tube gets out, and acts up in the same manner as the bigger siblings
normally do. I am not certain that you should have to debarn a
smaller tube at all, at those plate voltages.
John
K5PRO
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|