Date: Sun, 11 Dec 2016 19:21:23 -0600
From: "Jim W7RY" <jimw7ry@gmail.com>
To: "Paul Kraemer" <elespe@lisco.com>, <amps@contesting.com>
Subject: Re: [Amps] L7 FIXED (was L7 problems)
Better to put a diode here to protect the meters. See the link:
D2 in the last diagram.
I put a spot for one on my SB220 replacement diode/metering board.
73
Jim W7RY
## Use several 6A10 diodes, in parallel, for the function of D2. That assy
is then wired directly between B- of HV filter caps... and chassis. At a
rated 400A
surge capability....albeit for only 8.3 msecs...is the reason for 2-4 x 6A10
diodes in parallel.
Then the total surge rating is increased..and safe time extended. Fault
current is just B+ divided by
value of glitch resistor. IF you want D2 to actually survive intact, after
one or
more events, paralleled 6A10s are the ticket.
## To keep the B- from wandering too far from chassis potential, use RVS
connected
diodes. IE: install a single 6A10, between chassis and B-, but this time
with cathode, banded
end of diode, facing the chassis. To recap, 2-4 parallel diodes, with
cathodes facing the B-.
And a single diode, with cathode facing chassis.
## Worse case scenario is a B+ to grid arc....or a B+ to chassis...aka
metal arc. In which case the path for
fault current is from grid to chassis... and then from chassis UP through the
paralleled diode assy... and back to the
B- of of the HV supply, completing the loop. If you are relying on the 240
vac fuses or circuit breaker to open off,
during a fault event, that typ takes 35-80 msecs... imo is too long. I install
a HV rated fuse in series with the 50 ohm
glitch R. With 3 kv of B+..and a 50 ohm glitch R, the fault current is just
60A. 60A of fault current will open off
a 1 Amp rated HV fuse in less than 2 msecs. The glitch R limits the fault
current, the HV fuse interrupts the fault current,
and extremely fast..... which takes a huge load off the glitch R. Event over.
## Looking at the W8JI url posted above. ....
http://www.w8ji.com/fault_protection.htm the grid fuse is installed
incorrectly in all his examples. This is a throwback to Rich measures
advocating replacing the grid chokes with either
1/2 watt CC resistors... or 3agc glass fuses...wired directly between a grid
pin and chassis... fubar.
## Correct method is to toss the grid caps and grid RF chokes....and instead
bond all 3 x grid pins of each 3-500Z directly
to the chassis with wide cu strap. Then mount a 3agc type fuse holder on
the lower rear panel. Wire the fuse between chassis and
negative terminal of grid meter..... or grid meter shunt. Grids are always
chassis bonded. Suck too much DC grid current,
and correctly installed grid fuse opens up. With no path for DC grid current,
the PO of the amp drops to zero watts.
Input swr rises to infinity. Xcvr shuts itself down. Simple to replace 3agc
grid fuse. Just use a FAST type 3agc fuse, like
a 250 ma type...and even less for a single 3-500Z.
Jim VE7RF
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