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References: <3rthem$da--(at)--rl6.crl.com>
Date: Sun, 18 Jun 1995 08:57:55 GMT
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Sender: ebarbou--(at)--etcom15.netcom.com
All oxide-cathode power tubes have this problem. Their
distortion varies slightly as the oxide "breaks in"
>from fresh. This is because of the process used to make
the oxide--usually they start with a coating of barium and
strontium carbonates, then the air is evacuated and the
entire tube structure is heated red-hot with an RF
induction coil. This cooks the carbonates down to oxides,
and the CO2 gas is drawn off by the pump.
Unfortunately, no matter how long it is processed, this
oxide is never fully cooked. So when the tube is first
powered up, some time with current coming out of the
cathode is needed to stabilize the outer oxide layer.
Apparently the operation of this kind of cathode is quite
complex, involving electrolytic reaction of the nickel base
with the barium, driving it to the surface where it can
emit electrons effectively.
You can avoid this by using a tube with a directly-heated,
thoriated tungsten filament. This is a simpler cathode
and has little or no break-in effect, and will last longer
than oxide to boot.
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