tubes age 122
have much of an affect. The getter (hopefully ....) stays nice and cold through the
process. By that I mean that you don't want it to melt or worse yet vaporize.
Generally you pick getter material so that this is not going to happen in any
reasonable combination of conditions. As long as you stay below melting
temperature it's hard to say exactly what the getter will do as a function of
temperature. It's affinity for crud goes up as temperature increases, but so
does the tendency of the crud to turn back into a gas. Is it better hotter or colder
? - depends on the material and what you are trying to "get". You hope the guy
who made the tube knew his stuff. If he did have a choice then I'd guess he
optimized for a bare tube with no shield. Take Care! Bob Camp
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Date: Wed, 23 Apr 2003 15:25:53 -0400
Subject: [R-390] IERC Tube Shields et. al.
Don't forget other common failure modes: heater to cathode short, grid
emission. Grid emission results from electron-emitting coating migrating from
cathode to grid. In applications with appreciable grid circuit resistance this
emission will cause grid to go positive and upset bias point. In our beloved R-
390 series one IF tube having grid emission can pull AGC line, decreasing or
defeating gain control action and causing all controlled tubes to draw excess
plate current.
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Date: Thu, 24 Apr 2003 10:56:50 -0400
Subject: [R-390] Tube Failures
The heater to cathode short is a good one to know about. I had a humongous
amount of hum when I turned the limiter on, but not when it was off. I eventually
found the problem to be the tube. It testedOK (previous posting about testers
with forked toungues is true!) but when I replaced it the problem vanished. With
the high impedance circuit, even miniscule AC leakage into the cathodes was
enough.
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Date: Thu, 24 Apr 2003 11:18:05 -0400
Subject: Re: [R-390] Tube Failures
Surprised it didn't show a short on your tester, Gord. But this is an excellent
example of why a tube tester is a great tool for getting you into the ballpark, but
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