On Nov 18, 2006, at 1:42 AM, pentalab wrote:
--- In ham_amplifiers@..., "craxd" <craxd@...> wrote:
For checking vacuum caps, you really need HV DC to test them with
since that's really what they'll be blocking. I'm not familiar
with
that brand Hipot, but that sounds like the control is for
capacitive
reactance compensation. I'm not sure that will be that accurate
testing those with an AC current. You might look for a manual on
the
net you can download and see how to use that with the control in
mention.
Best,
Will
### Will... re-read his post. He used a JENNINGS brand HI-POT
tester... not brand .."X" I remember reading in the old
Jennings engineering application section... that it was strongly
suggested to only use AC hi-pot test devices. Apparently... when
HV DC is applied to any vac cap... the cap will grow whisker's
from the soft OFC copper stator/rotor plates. plus bellows on
variable caps. Jenning's told me, IF you are going to use.. say
a FIXED vac cap... as a plate block cap [don't laff... LOADS of
11m QRO amps use fixed ceramic/glass vac caps as plate
blockers... mounted vertical... connected with a special clamp, to
the center anode typ .82" solid pin] that the vac cap has to be
DE-RATED by 60% for V. IE: For an amp with say 7 kv no load
HV supply.... use a min of a 20 kv test rated vac cap.
A 15kV - tested Tune-C would do the job - provided that the DC
blocker cap was ahead of the Tune-C. With a 7000V anode supply, and
a g-g triode, the AC anode potential would be c. +/¨C 6700V-peak.
Since the RF rating of a vacuumis 60% of it's DC rating, or 9000V in
this case, a 15kV cap would do the job. - note - The "Plywood Box"
amplifier used a 15kV tested Tune-C and a 9000V NL anode supply. The
Tune C did not arc.
### One would think for a brief Hi-Pot test.. that DC HV hi-pot
tester would be ok... it's not.
Page 6, Jennings Vacuum and Gas Capacitors catalog:
"DC: Vacuum capacitors should not be operated in DC applications
above the peak RF working voltage."
I'll try and dig out my
Jenning's notes to find out why.
I think the DC HV types can be
used...
Vacuum capacitors don't come in DC and in AC types.
but certain conditions were applicable in their usage. ...
R L Measures, AG6K, 805.386.3734
r@..., rlm@..., www.somis.org
