On Nov 19, 2006, at 2:59 AM, pentalab wrote:
--- In ham_amplifiers@..., 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@..., rlm@..., www.somis.org
