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[Amps] HV fuses.. don't fuse the B-

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Subject: [Amps] HV fuses.. don't fuse the B-
From: "Jim Thomson" <Jim.thom@telus.net>
Date: Wed, 17 Feb 2010 03:04:56 -0800
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Date: Tue, 16 Feb 2010 16:10:37 -0800
From: "Steve Katz" <stevek@jmr.com>

Why not fuse the negative lead instead, which is at potential close to
ground?  If you do this, you need to be sure the HV ammeter shunt is on
the power supply side of the fuse and not the "load" side, so that point
remains close to ground even when the fuse blows....

### I HIGHLY  don't recommend  putting HV fuses in the B- lead.
The cathode of the tube  will soar to almost full B+  potential,
if the B-  ever opens up for any reason. You want the B-  from the
HV  supply, to be one solid connection, right through to the RF
deck, and then solid connection to the bias circuit, and also
through the RX bias cutoff/ spst vac relay
 [ two x 100 k mof's in parallel]...... then  finally terminating
 on the CT  of fil xfmr. 

##  you can also install a simple 3agc CATHODE fuse in the
CT of the fil xfmr... provided you shunt the 3agc fuse holder
with another pair of 100 k, 3watt mof's.  Cathode  fuse has
to be  a super fast type, and sized to handle BOTH  normal plate
current PLUS  normal grid current.  A  2 amp, super fast  3agc 
fuse  will work perfectly for a 3CX-3000A7.   3agc fuse holder
is normally mounted on rear panel. If the cathode fuse blows open
the pair of 100 k resistors across it, are now in the circuit, and amp
is simply biased to cut-off. 

##  For  a GRID fuse on a 3CX-3000A7,  install another  3agc
 fuse holder on rear panel, with a super fast  250-400 ma fuse.
 Fuse is wired between  chassis.. and NEG  terminal  of grid meter.
 This time, do NOT install a pair of 100k resistors across the GRID fuse. 
If grid fuse blows open, NO path for  dc grid current.  Amp can't be
driven. INPUT swr  rises to infinity. Power output drops to ZERO. 

##  IF a resistor is  installed across the grid fuse, it won't bias
itself off, if it blows open.  DC grid current will find a back door path
from chassis, then up through RVS connected safety diodes [ between
B- and chassis].. then  through plate meter, CT  etc.   Amp will be in 
Class C  [ from V drop across grid fuse], and plate meter  will now 
read combined grid and plate current !   So nothing gets installed across 
the  grid fuse. 

##  NO resistor installed between  B- and chassis  either.  Use 4-8
RVS connected  6A10's  between  B-  and chassis.  Parallel 2-4
x  diodes to make.. 'an assy'.   Make 2 x assy's.    Then, the
B-  can never be  more than +/- .7v   from  chassis  potential. 
A resistor  will simply explode. 

## install HV fuse in B+ lead.... in HV supply,  followed by the glitch R.
For a 3CX-3000A7,  a pair of  100 ohm,   225 watt  wire wounds  will
make a good  glitch R.  

## install a 2nd HV fuse... between plate xfmr sec... and FWB
[one leg only, either one] .    





Or use primary fuses or circuit breakers to protect against major
faults, and a B+ glitch resistor to protect against momentary tube
arcs...?

##  not fast enough. If the plate xfmr is big enough, the follow
on energy will fry the glitch R.  I'd still use a fast mag hydraulic
breaker in the 240 line.  Now, if he has a B+ to grid arc, or B+
to chassis,  50 ohm glitch limits the fault current to 100A,
and 100A  will blow open the HV fuse very fast.. <2 msecs. 

later... Jim  VE7RF




The plan is to fuse the HV output of a 5KV 2A supply to a 3X3, which of

course will only be used at the legal limit or into a dummy load. 
 
Steve  Gilbert
K1SG
K1SG@AOL.com
508-435-9133
FN42fe  

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