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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