3-500 gettering

[Earl Morse]

Gettering is the process of removing gas from the tube envelope.  It is after 
all a vacuum tube and works better with a vacuum in it.  The gas in a gassy 
tube is either outside air seeping in through the seals or the outgassing of 
tube elements in the tube.

A gassy tube is usually noted by a bluish glow within the tube element during 
transmit.  I fired up an SB1000 that had been boxed for 10 years and it blew 
the fuses the minute I fired it up.  I checked everything out and got it down 
to being able to fire it up without the tube installed but would blow the fuse 
when the tube was installed.  The tube ohmed out good and only after I ran out 
of fuses and substituted a slower blowing fuse did I see the glow for only a 
brief second before the tube arced and blew the fuse.

Some investigation showed that I could get some gettering action by running 
the filament to burn off some of this gas.  I did that and was able to now 
fire up the amp but once loaded so far the blue glow would get brighter until 
it arced again.  Further investigation identified that the gettering compound 
is on the plate and the best gettering action is when you heat the plate and 
make it glow.  I need to make a fixture to attempt to find a way to cook the 
tube a little before an arc occurs in a last ditch effort to ressurect the 
tube.

If I was doing it again I would cook the tube with just filament for a day or 
two and then bring the amp up on a variac to protect the power supply.  The 
idea being to cook off as much gas as possible and avoid an arc.  Seeing that 
the tube has arced before it just makes it that much easier to arc again.

Don't know when I will get around to my last ditch effort to cook that tube 
but it is worth a try before pitching it.  I'll keep the list informed as to 
the results.

Earl
N8SS



To everyone who took time to pass along information regarding break-in 
of 3-500 tubes I want to say "thanks", that all the information waas 
very much appreciated.

The advice I received ranged from "a couple hours" filament break-in 
time to 24 hours.  One persone even stated I could "plug and play", 
all absent any high voltage on the plates, and any exciter power.  

Several noted that while they had heard tubes needed a filament break-
in they had never heard of incrementally ramping up the exciter power 
while three (3) stated very emphatically that the tubes should be 
ramped up slowly (but they did state that 10 watts at a time was too 
conservative).

One ham provided a very detailed explanation of the process, a process 
called "egtering".  Not only was this interesting but very helpful as 
well.  Thanks.
Again, my thanks to all who took time to provide comments and 
assistance.

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