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. 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. 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
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