Hi Gerald et al,
Just a little reminder: When testing ceramic type tubes and vacuum
components with your high-pot tester:
ALWAYS ALWAYS ALWAYS clean the ceramic insulator with a degreaser product
like Formula 409 BEFORE conducting
ANY high voltage tests for leakage current!!! Otherwise, your leakage
current analysis will be WORTHLESS!!!
Even a SINGLE finger print left on the ceramic material in the wrong place
can cause BOGUS leakage current readings.
An un-cleaned tube under scrutiny may evaporate some of the HV-conductive
ceramic contamination (mostly skin oils) when sitting
on a test fixture with a lit filament over time. This can fool you into
thinking you have gettered the tube!
Vacuum capacitors with oil contamination on their ceramic envelope may look
like crap upon initial HV current leakage testing,
but with a little degreasing wipe-down, they may pass muster perfectly!
This also means clean (degrease) those vacuum components in the
tank circuit after mounting them with your greasy little hands!
One other caveat: many vacuum components are rated for HV testing with 60 Hz
AC. They may need to be de-rated as much as 40% of their
HV test value when using HV DC with your high pot tester. Check the
manufacture's technical support bulletins prior to testing. This is
particularly true with vacuum capacitors.
73, Paul WN7T
>Date: Mon, 14 Feb 2005 10:38:27 EST
>From: TexasRF@aol.com
>Subject: Re: [Amps] 8877 Test
>To: Amps@contesting.com
>Message-ID: <79.3f112e66.2f421ff3@aol.com>
>Content-Type: text/plain; charset="US-ASCII">>
>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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