[Amps] L7 FIXED (was L7 problems)

[Jim W7RY]

Jim... You must have missed this statement from W8JI's site...

"The grid, being directly between the anode and cathode, shields the filament-cathode from the anode. Very little cathode fault current flows unless the grid is floated on resistors, chokes, or fuses. F1 in the grid is actually a very bad idea.

73
Merry Christmas
Jim W7RY


-----Original Message----- 
From: Jim Thomson 
Sent: Monday, December 12, 2016 9:09 AM 
To: amps at contesting.com 
Subject: [Amps] L7 FIXED (was L7 problems) 

Date: Sun, 11 Dec 2016 19:21:23 -0600
From: "Jim W7RY" <jimw7ry at gmail.com>
To: "Paul Kraemer" <elespe at lisco.com>, <amps at 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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