>On Jul 28, 2006, at 8:49 PM, GGLL wrote:
>
>> Ian White GM3SEK escribió:
>>
>>>
>>> It also explains why transmitting tubes generally need to be pre-
>>> heated
>>> after a long period out of use. The process of slow diffusion to the
>>> surface of the materials means that gas will probably have
>>> accumulated,
>>> and the getter needs some time at a high temperature in order to
>>> do its
>>> job.
>>
>> Very clear explanation, Ian, thank you very much. I want to ask
>> something about.
>> This pre-heating process (in, for example, NOS tubes or those used
>> in good
>> condition but long time stored ones) must also be done by making
>> the anode
>> orange hot in high power tubes (those with "gettered" anodes) or
>> it's just
>> enough to run them with filament on for many hours?.
It depends how careful you want to be. The full routine recommended by
the Eimac tube designer was:
a. Bring the filament up to rated voltage slowly, to limit the shock of
turning it on.
b. Then after some time, bring the filament voltage to the specified
value, and run it for about 100 hours.
c. Then, apply the anode voltage through some current limiting means
for a while, and, finally,
d. Run the tube at a dissipation level near the specified limit for a
few hours.
The reason for this is that the getter (or getters - there may be more
than one) needs to experience a spectrum of temperatures, and be given
enough time to mop up different species of trace gases at different
temperatures.
Since no-one can be sure of the condition of any particular NOS tube,
no-one can say exactly "how long is long enough", or which of those
steps could perhaps be shortened.
Rich replied:
>
>Good question, Guillermo. If gettering really works as claimed
>hereabouts, why do large tubes depend on built-in ion pumps?
Small tubes DO in fact have a built-in pump to maintain the quality of
their vacuum. It's called a getter.
Very large tubes have getters too, but the outgassing problem is much
more severe than for smaller tubes, so they need other kinds of vacuum
pumps as well.
Here's John Lyles, from the AMPS archives in 1997:
"For super power triodes, we have a pair of internal Vacion pumps
running all the time....
"We can monitor the vacuum while the tube is running RF, and we do
witness occasional arcs inside the tubes. The background vacuum varies
as indicated by the ion pump power supply current. The tubes will be
sitting at about 10^-7 Torr [mmHg], then a burp occurs (an arc), then it
jumps to 10^-5 or so. It recovers rapidly (minutes) either due to the
getter action or the ion pumps.
"In these large ceramic metal tubes (beer keg sized WITHOUT handles,
with crane lifting hooks), one cannot leave the tube on the shelf for
years without vacuum degradation. The warranty requires checking the
vacuum with external power supply every few months. We put a power
supply on every tube's ion pump, and store them continuously like this.
"When we first turn on the RF, the vacuum does a dance until it settles
back to the background level. It is imperative that we start slowly, and
not slam full RF power (full plate swing) on it until the outgassing can
be accommodated by the ion gauges and the getter."
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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