On Feb 14, 2005, at 7:38 AM, TexasRF@aol.com wrote:
Hi alex and all,
This is a true story: About a year ago I was given an unused 8877 that
was
about 20 years old. After all the warnings about firing up old tubes,
I used my
newly acquired solid state 0 to 10kv module and trusty Triplett 630
VOM to
test it for leakage. It showed leakage from plate to grid beginning
at about
600vdc.
Every tube ever made has leakage at 600VDC, the question is how many nA
at how many V ?
I ran the filament only for about a week, testing leakage daily. The
leakage voltage slowly increased day by day and after a week had
reached about
5kv.
Leakage V does not increase or decrease -- leakage current does.
I know really good tubes are better than this but decided to stop the
burn in.
At this point it was installed in a 2m PA and fired up at half plate
voltage, about 2000vdc. It seemed to operate ok at that voltage so
after about an
hour the plate voltage was increased to 3000vdc and then the normal
4000vdc.
There was an immediate "incident" that blew the 2A fuse in the B+
circuit. The
fuse was replaced and the amp has been performing perfectly since then.
I am tempted to retest the tube leakage again but it is just way too
much
trouble to disassemble the cavity to free the tube for a test.
Since then I have tested an old TWT this way and found the same 600vdc
starting point for leakage. After only one day with the filament only,
this tube
would then go all the way to 8kv before onset of leakage. The tube is
rated at
4kv so it probably will be ok when I finally get around to testing it
with
rf.
The moral of this? I will always test tubes for leakage, old or new,
in the
future, before use. I can see that it will prevent some of the big
KAPOWS!
seen in the past.
73,
Gerald K5GW
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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