Jim Barber wrote:
>
>I said:
>
>>>Speaking of glitches..
>>>None of my 8877's have been hot for some time now. (6 months to
>>>2-3 years, depending on the tube) I seem to remember reading here
>>>that someone recommended 4-5 hours on the filaments before
>>>applying B+ . Does that sound right?
>
>And Rich answered:
>
>>There is no such statement on the tech spec sheets. If a seal is
>>leaky, the tube is history.
>
>Ah. Then the tales of heating the tubes for a period after
>extended storage are just that. I have no reason to believe
>any seal is leaky, but I had heard that the potential for a
>startup glitch could be reduced that way.
>
No, they're not just tales.
There are two ways gas can get into a vacuum tube. One is through a
faulty seal, as Rich said. But even if a tube has perfect seals, the
inside surfaces of the tube can very slowly release gas.
Ceramic-metal tubes such as the 8877 have a "getter" attached to the
cathode, which chemically mops up the gas when heated. In a glass
transmitting tube the getter is typically a coating on the anode. The
getter is designed to handle internal outgassing, but it can't cope
with heavy leakage through a faulty seal.
So yes, if a tube hasn't been used for a long time, running for a few
hours with heaters only to activate the getter and clean up the gas can
be beneficial. (Of course you don't actually know what the quality of
the vacuum is, so it may not always be necessary; but it can do no harm
either.)
It's worth remembering that even a tube with a "good" vacuum" will
typically contain millions to billions of gas atoms in the "empty" space
inside (depending on the size of the tube and the quality of the
vacuum). It is very much an active chemical system.
Hang on, actually you *can* check the quality of the vacuum. A few years
ago there were some messages here about running tubes in "ion-gauge
mode". The control grid is biased positive so that it collects electron
current from the filament, while the anode/screen is biased negative and
collects any current due to positive ions. AFAIK it doesn't need high
voltages, only maybe 50-100V. The trick would be to know what operating
conditions to use for each type of tube, and what kind of ion current a
good tube should give.
An archive search for "ion gauge" should find this, but AFAIK nobody has
investigated it further. It would be a great project for someone with a
bucket of new and used tubes!
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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