Date: Thu, 16 Nov 2017 06:05:16 -0600
From: "Jim W7RY" <jimw7ry@gmail.com>
To: "AMPS" <amps@contesting.com>
Subject: Re: [Amps] Glitch resistor + fuse?
<I've never had a wire wound glitch resistor "fail" during normal operation.
<10 watt or 25 watt, HV or 120/240 VAC.
<73
<Jim W7RY
## I have not had any of my 50 ohm at 50 watt wire wounds let go in any of the
drake amps, but they also have that .82 ohm quasi HV fuse in series with the
50 ohm.
I just looked at 2 of my L4PS supplies a few mins ago. They are def a .5 watt
CC resistor.
A 2 watt CC is huge by comparison. The .82 ohm R opens up, end of event.
## Having said that, a buddy has a 7900 vdc supply, with a 160 uf filter in
it, and the big dahl xfmr.
Glitch R assy consisted of 2 x 100 ohm wire wounds in parallel, and each was
225 watt. He used
a Buss HVU-3 HV fuse in series, and inboard of the glitch assy. While trbl
shooting the new amp,
the B+ arced to chassis, and metal that was too close, and other misc mishaps.
All that ever happened
was the HV fuse opened up. He only had a few Buss fuses to begin with, so
started rebuilding the blown ones,
but used too big a gauge of fusing wire. He used a single strand from the
braid of 213 U, and also a single strand
from the belden 18 gauge HV test prod wire. Both, although they look tiny,
were still too big for this application.
## Something went wrong, and B+ arced to the 160m padding assy aluminum
support sandwich. HV fuse did not open in time.
The pair of 225 watt WW resistors were burnt to a crisp, with flakes all
over them, both went open. The 4992 joules stored
in the filter caps, + any follow on current was too much for the 225 watt wire
wounds.... at least with the oversized HV fuse.
## The fast.... fix, was to install 4 x 200 ohm wire wounds, also 225 watt
each, fan cooled, and a new Buss HVU-3 fuse.
4 in parallel = 50 ohms, which was the target value.
Problem solved, correct sized HV fuse always opens up 1st. A better glitch
R of course would be a Globar, type AS.
2nd choice would be a Globar, type SP. Wire wounds are the 3rd choice.
## Either way, the HV fuse, IF sized correctly, takes a huge load off the
glitch assy. Later, another buddy tested various
wire gauges in his lab at work, to determine melt characteristics. Small,
tiny wire gauges are deceiving.
Increase the diam by just 41%, and the cross sectional area of the wire is
now double. He eventually nailed it, and came
up with the correct sized gauge for his specific application.
Jim VE7RF
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