Date: Mon, 07 Oct 2013 10:23:20 -0700
From: Bill Turner <dezrat1242@yahoo.com>
To: Amps <amps@contesting.com>
Subject: Re: [Amps] high voltage fuses
ORIGINAL MESSAGE: (may be snipped)
On Mon, 7 Oct 2013 16:24:51 +0000, WA4LAV wrote:
>When the fuse blows the plasma will conduct the rest of the current straight
>to ground acting
>as a self sacrificial crowbar.
REPLY:
Not a good idea. The HV positive does not want to go to ground, it wants to
go to HV negative. In nearly all amplifiers the path from ground to HV
negative is through the grid meter. Check your schematic.
If you surround the fuse with a conductive tube, connect the tube to HV
negative, not to ground.
73, Bill W6WRT
## who cares ? With 4-6 paralleled 6A10 diodes between B- and chassis, its
not
even an issue, been there, done that. Then 2 more paralleled 6A10 diodes, also
between B- and chassis, but this time flipped 180 degs. The fault current
will simply
travel from the chassis, up through the 4-6 x paralleled diodes then directly
to the B- of the
filter caps, completing the loop. The grid meter wont be damaged at all.
Even with rvs
connected diodes across the grid meter, the predominant fault path is from
chassis through
the 4-6 x paralleled diodes, then directly to the B- of the filter caps.
## The only time I ever had problems with plasma arc is when I tried a real
small ga wire
suspended between 2 x ceramic standoffs. Wire melts asap, real fast, but the
plasma forms
a cloud, that then vapor deposited itself onto the out board insulator. The
HV fuse would blow
every time the supply was turned on. A dentist mirror then revealed the vapor
deposit down the
backside of the outboard insulator. Just like an ultra thin coat of silver
paint, .3 inch wide, and top to
bottom of the insulator. The tip off was the ohm meter showed zero ohms
between the top of the
insulator and the chassis its mounted too. I mean like how can a new 1.75
inch tall insulator read
zero ohms ? Dentist mirror revealed the problem.
## The fix was easy. Just slide some heat shrink, OR teflon spaghetti tubing
over the small ga
fuse wire 1st. I used real small stuff in either case..that just barely fit
over the wire. What
also works is just to wrap the wire with 33/88 tape. Now what I just
described has been tested
6 ways to sunday..and with 6800 vdc + a 150 uf filter cap. In all cases, the
hb fuse was 1st ....
then followed by the 200 watt 50 ohm glitch resistor. This was with a 10 kva
CCS xfmr.
## These days I use Buss 5 kv rated sand filled HV fuses... which are 5 inchs
long. That’s 5 kv AC rated
Those things will easily open off sky high fault currents. They are easily
refurbished by removing the
end caps, pouring out the sand, then replacing the fuse wire with whatever
gauge you want. You can also
refurbish em by laying the new wire down the outside the glass body, then
wrapping it with 33/88 tape.
Then solder both ends.
## The 50 ohm glitch R LIMITS the fault current to a safe value, the HV fuse
interrupts the fault current.
The mess of paralleled 6A10 diodes simply provides a direct patch back to B-
in cases where the B+
arcs-shorts to the chassis.
Jim VE7RF
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