On Nov 20, 2006, at 12:17 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.
####### The AC potential across the cap has nothing to do with
it.
#### 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.
RICH SEZ... So why does the statement I quoted in the Jennings
catalog on p.4 about DC operation state otherwise?
#### Dunno. Eimac sez the same thing, de -rate vac caps to
1/3 for plate block use !
RICH SEZ.... I did not read this in any Eimac literature,
#### You don't have friends at Eimac... like me.
.. and Tom Rauch?
RICH SEZ...and it does not make good sense
### sure it does. Apply continuous DC HV to a vac cap.,. and
eventually... DCV rating.. due to whisker growth...
I have seen whisker growth in vacuum caps that have been in long-term storage. However, when such caps are high-potted, after a few tinks the stickers apparently burn off and the piv capability returns.
DROPS...hence size the caps DC HV rating at least 3 x the plate
supply's unloaded v to start with. That's what Jenning's sez...
So why does the Jennings Vacuum and Gas Capacitor catalog say on p.6:
"DC -- Vacuum capacitors should not be operated in DC applications above the peak RF working voltage."
"DC Plus RF -- For DC plus RF applications, the sum of the DC plus the peak RF voltage should not exceed the peak RF working voltage."
that's what Eimac sez... that's what 11m ops say... or else it's
flash kablamo time ..... end of story.
hardly. Page 6 still has you by the short hair.
RICH SEZ...since the prima facie consideration for a DC blocker is
the current rating at 29MHz - since at 10m, the DC blocker
typically carries c. 90% of the tank's RF circulating current.
#### "prima facie consideration" ? Are you OJ's lawyer too ?
Johnny's dead, I'm not quite there yet - despite the plenitude of fervent prayers of Mormons, Muslims, Jehovah's Witnesses, SDAs, and Roman Catholics. .
- prima facie - adv.
1. At first sight; before closer inspection: They had, prima facie, a legitimate complaint.
### yes... 10m and above is the worst case scenario for a plate
block cap.... that's what I have been saying all along.
The 11m ops also highly agree. The
11m ops tell me if they try and use a 16 kv test rated fixed vac
cap with 10.5 kv dc on it..... it will eventually arc.
RICH SEZ... Probably true since Jennings advises that 60% of the DC
test V is the max WV when using DC + RF.
### what ? Jenning's sez the max AC RF working V is 60%
of the DC test V.
Page 6. The Vacuum and Gas Capacitor Catalog:
"RF Working Voltage is the maximum RF voltage that can be applied to the capacitor without affecting its ability to withstand instantaneous overloads, It is generally 60% of the Peak Test Voltage rating. ..."
They are asuming and also implying you have
employed a plate block cap already.
Staying below the peak RF WV ability is the prime concern no matter the application.
What they are refering too is
using a vac cap as a C1/C2 tune/load cap... or using a vac cap to
pad a tune/load cap. In any event... the AC voltage across a
plate block cap is so low... u can ignore it.... UNLESS you insist
on using really small value plate block caps... then you end up
with high AC voltages across em.... as in plate current X Xc
of plate block cap.
Agreed
### for 10.5 kv dc no load... you want at least a 25-30 kv test v
rated fixed vac cap.
According to page 6,
"DC Plus RF -- For DC plus RF applications, the sum of the DC plus the peak RF voltage should not exceed the peak RF working voltage."
Example -- A 1000pF DC blocker, using an 8281 running at 10.5kV with a typical tank Q, would have c. 20A of tank circulating current flowing through the DC blocker at 29MHz, so there would be c. 105volts drop across it. Thus, according to p.6, a capacitor with 10,500v plus 105v = 10,605 / 0.60, so a 17,500 DC-test V or higher unit should be used. However, amplifiers with 10,500v anode supplies are not all that common in Hamdom. Also, 10,500v is getting pretty close to the land of x-rays.
...
RICH SEZ... 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.
#### No way. 500pf = 176 ohms of XC.
RICH SEZ.... The rule of thumb is a 5 to one ratio for a coupling
reactance. 176- ohms of XC and a RL of 176-ohms x 5 = 880-ohms or
higher RL should be okay.
### No way. Where did u cook up this 5 to one ratio ??? Not
in any of my engineering books. Any big metal tube operating
with a 880 ohm plate load Z... is gonna be sucking loads of plate
current. 4-5 A typ. 5 A x 176 ohms XC = 880 vac You just
lost it right there!
#### 100 pf = 884 ohms XC [1.8 mhz] gimme a break !
RICH SEZ... With a pair of 3-500Zs, RL is c. 2k, so 884-ohms would
probably work okay if the Tune-C could tune it out.
#### Now it gets even worse ! 5 A x 884 ohms = 4400 vac dropped
across the plate block cap..
Where did the 1.5A come from?
...
### a 100 pf plate block doorknob cap wouldn't last 30 seconds
in a 3x3
RICH SEZ... Why would the capacitor dissipate heat?
#### lemme see. With say 2.5 A of plate current at 5 kv under
load... and on 29 mhz... current through the 100 pf plate block
cap = 32 A.
How did you calculate this, Jim?
A HT-57 100 pf cap is only rated at 13.7 A @
30 mhz [with 13.7 A, the 100 pf cap's temp is 55-65 deg C...149
deg F..... and well over 100+ deg C.. with 32 A flowing through
it] Use multiple metal tubes on 10m or bigger single metal
tubes with lots of anode to chassis C..... look out.
### with 32 A through a cap rated for 13.7 A = meltdown. [5.5 x
more heat. 32/13.7= 2.34 2.34 squared = 5.5 more trbl] Don't
gimme this nonsense about pulse tuning and SSB being a 15% duty
cycle. The correct component here is 6 x 500 pf HT-57's... or
3-4 x 200pf HT-57's. I use 6 x 500 pf HT-57's.... then it
will have low XC on 160m too.
Later... Jim VE7RF
to be continued ...
R L Measures, AG6K, 805.386.3734
r@..., rlm@..., www.somis.org
