[Boatanchors] SB-220 with parasitics
Rodger Singley
wq9nsc at live.com
Fri Dec 24 16:55:28 EST 2021
A tube with a badly failed seal would behave as you described, a large amount of oxygen leading to fireworks and those tubes are useless. Low power/receiving style tubes that have the getter deposited on the glass envelope are easy to recognize when they have this issue because the getter is generally ash white and flaking away showing the tube is useless.
The conditioning is useful for power tubes where there has been a tiny amount of ingress over a period of time OR when contaminants have outgassed from a tube element during the long period of inactivity. In either of these situations, the amount of contamination is tiny so it is within the capability of the getter to capture it.
That is the purpose of conditioning the tube, to avoid a destructive flashover from a tube that is basically good but has had a significant amount of gas that is in play from the long period of storage.
Rodger WQ9E
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From: w5jo at brightok.net<mailto:w5jo at brightok.net>
Sent: Friday, December 24, 2021 2:54 PM
Cc: Boat Anchors List<mailto:boatanchors at mailman.qth.net>
Subject: Re: [Boatanchors] SB-220 with parasitics
Could someone explain this to me? If a tube leaks, it will also have oxygen
in the envelope with any other gas, wouldn't it?
If it has oxygen in it, wouldn't the filament go flash and burn into?
Over many years I have inserted NOS tubes in transmitters and, following the
proper warm up period just applied power and never had a problem. Just
recently I replaced the 4-250 in my Globe King 500 with a glass bottom tube
made in the 70s without a problem. After 2 or 3 minutes of warm up I turned
on the HV and it worked just fine.
Any tube will have just a minute amount of gas in it, that cannot be
avoided, but that is the purpose of the getter to hold it during the
manufacture of the tube.
In construction this is what takes place and I don' think very many hams can
do it.
>From AI3V: Quote
What is going on is the tantalum is heated hotter than red hot (orange or
even white) while the tube is originally manufactured and connected to the
vacuum pump.
During normal operation the anode is never allowed to get that hot, so it
never outgasses any more.
This superheat or "bombardment " is a fundamental and critical part of
creating a high vacuum.
The other problem with attempting to correct a bad vacuum by sammy hammy
"gettering" is the tube, more or less cold will not work properly.
As you attempt to "getter" the rise in temperature will liberate more gas,
and this gas will condense on any part of the tube that is cooler than the
anode, you have both an electrical runaway condition, and no way possible to
heat the glass red (or orange or white) hot to get the gas back into the
anode.
Unquote
That is the best, short explanation that I have read and why I don't bother
to worry. If the tube has substantial gas it won't go away and you will
have trouble.
Merry Christmas to all,
Jim
W5JO
-----Original Message-----
I half expected them to not
have full vacuum.
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