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-----Original Message-----
From: ham_amplifiers@... [mailto:ham_amplifiers@...]
On Behalf Of Mike Sawyer
Sent: Monday, November 20, 2006 7:57 AM
To: ham_amplifiers@...
Subject: Re: [ham_amplifiers] Re: AC HiPot tester question

Now, let me ask a question of social and political importance: Is it
possible to 'remove' the whisker by introducing and hi(er)-voltage to the
vacuum cap? The reason I ask is that this was practice used to recover
Ni-Cad batteries. A whisker would develop between anode and cathode and a
charge capacitor, several times the actual output of the cell would be
placed across it, (reversed polarity if memory serves me), and physically
'burn' the offending whisker.
Mod-U-Lator,
Mike(y)
W3SLK
----- Original Message -----
From: craxd <mailto:craxd@...>
To: ham_amplifiers@... <mailto:ham_amplifiers@...>
Sent: Monday, November 20, 2006 12:29 AM
Subject: [ham_amplifiers] Re: AC HiPot tester question


It sounds to me that Jennings wants things two different ways. First
they say don't momentarily test a vacuum cap with DC, but it's okay
to put one in service as a DC blocking cap, de-rated or not. From
everything I've ever read, the whisker problem is caused over time,
not in one minute of use. A DC blocker cap could be in an amp for a
lifetime. That's a good sales gimmick to sell hipots though. I've
never seen that warning either by Comet, or by any of the Russian
manufactured caps. If the cause is over the copper being too soft
used for the plate cups, it ought to be changed to a hard copper or
an alloy to stop it. I can see soft copper for the bellows, but not
the plate cups.

Thank God Rich and I didn't question the Jennings engineers on the
other list as we would have been sent another e-mail by the unknown
admin chastizing us for ever questioning these professionals with
published papers, etc....

Best,

Will

--- In ham_amplifiers@...
<mailto:ham_amplifiers%40yahoogroups.com> , R L Measures <r@...> wrote:

On Nov 19, 2006, at 2:59 AM, pentalab wrote:

--- In ham_amplifiers@...

<mailto:ham_amplifiers%40yahoogroups.com> , R L Measures <r@...> wrote:

...
RICH SEZ.... A 20kV rated C to block 7000V DC sounds like over-

engineering since the actual AC potential across the blocker is
minimal.

#### Rich... Jenning's engineers tell me when using either fixed
glass/ceramic vac cap as a PLATE Blocker.... MIN V rating of the
FIXED vac cap has to be a MINIMUM of 3 X No load plate V......
other wise u get "whisker's" growing on the OFC plates on the
cap..... which will REDUCE the caps V rating.

So why does the statement I quoted in the Jennings catalog on p.4
about DC operation state otherwise?

### Ur gonna get "whisker's" anyway in plate block service for
a fixed vac cap.... so the 20 kv cap... will actually be over
time... a lot lower than 20 kv. IF u Hi-pot test a fixed vac cap
that has been used for plate block service.... u will see it no
longer hi pot tests to 20+ kv any more.

### other than 11m ops... I never see fixed vac caps used as plate
blockers.

I do, Jim. Even 500pF is plenty for a DC blocker at 1.8MHz (XC =

190-

ohms) in typical amplifiers since RL is in the kilo-ohms range.

11m ops don't need much C for 11m... 100-250 pf max
is what they typ use. Their requirements are for a plate block cap
that handles LOTS of RF... esp for 4x20's, etc.

Tom Rauch apparently knows a Ham who mistakenly used a 100pF DC
blocker in a homebrew amplifier. The amplifier produced the

expected

output from 1.8 to 28 MHz. Sometime later, when a friend was

being

shown the amplifier, he noticed that there were only twp zeros

after

the 1 on the blocker cap. When a 1000pF cap was substituted for

the

100pF cap, the output did not increase although the tuning changed
slightly on the 1.8MHz band.
...
R L Measures, AG6K, 805.386.3734
r@... <mailto:r%40somis.org> , rlm@..., www.somis.org