To: <amps@contesting.com>
> Date: Tue, 26 May 1998 14:00:53 -0500
> From: Jon Ogden <jono@webspun.com>
> >It's even tough to test for gas, because the gas can be gettered not
> >only by the intentional gettering agents put in the tube, but also by
> >the arc itself.
>
> Interesting. Why would the tube suddenly arc when I was testing it on 15
> meters while it was fine when I had tested it all day on 10 meters?
I don't know. Why does a tire suddenly go flat after 20,000 miles, a
filament suddenly open after hundreds of hours, or a person suddenly
die after fifty years of life?
If you test the tube over and over again, and it always fails only on
15 meters, you might indeed have a valid point. But it also could be
a choke resonance or tank component failing.
> the tube still usable or should I send it back to the guy I got it from?
Why don't you install a proper fault protection resistor and try
again.
> OK. The series resonance of the plate RFC is around 23 to 24 MHz. Is
> this perhaps a little too close to 21 MHz? Perhaps putting the lid on
> lowered the resonance. I'll have to check for that. I was wondering if
> that might be the case.
That's pretty darned close, but workable in most cases. You
absolutely need to measure it with the cover on. I know you like
using the GDO, but there are MUCH better tests.
One is to use a resistor, diode, and signal source to measure choke
impedance. The problem (I hear an echo) with a GDO is it tells you
nothing about choke impedance.
> Chalk it up to inexperience with tubes again. I always thought gassy
> tubes are typically tubes nearing the end of their useful life and that
> during that time, gain and such would begin to fall off.
Might or might not. Brand spanking new tubes with lots of emission
are sometimes gassy, so are old worn out ones.
73, Tom W8JI
w8ji.tom@MCIONE.com
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