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Gassy vacuum tubes

Aug 1st 2013, 03:16

WB7DIE

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Can you tell me what causes a vacuum tube to get gassy over time, and what can be done to prevent or remove the condition? Thanks.
Aug 1st 2013, 15:11

AA6E

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
What kind of tubes are you thinking of? W8JI has a discussion of transmitter power tubes and gas problems.

In general, gas accumulates over years of storage because of slightly leaky seals or outgassing from structures inside the tube. There are procedures for sweeping out gas by heating the filament, which causes gas to be absorbed. Sometimes this works, but not always.

73 Martin AA6E
Aug 14th 2013, 13:00

WB1GCM

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Glass is not a perfect envelope; glass is porous to a small extent. Air molecules, one at a time, will leak into a vacuum tube. If a tube has not been used for many years, it may test poorly. If high voltage is applied to a transmitting tube that has been stored for a long time, flash over can occur.

To avoid this, bring up the filament voltage of a suspected gassy tube slowly, perhaps at half the rated filament voltage. Let it cook for a couple of hours, then increase the filament voltage to it's rated voltage and run it until its emission comes up.

I have had many receivers given to me that were completely dead. After letting them cook a while (to burn off the air), they woke up and performed well, considering their long sleep. A low voltage start-up of a receiver that has been sitting a long time is also important for the filter capacitors!

Antique vacuum tubes, such as 201-A types with good filaments but appear to be duds will test good after a day or two of cooking out the gas.

Bob Allison
WB1GCM
ARRL Senior Test Engineer

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