Fellows
I have several fuses installed in most of these linear's.
Like a fast 750 ma 3agc style grid fuse... located between
chassis.. and negative terminal of 0-1 Amp grid meter.
Also have a 3 A cathode fuse [also a 3agc] in the center tap of
the fil xfmr circuit [ along with the 60 x 6A10 6-A , rotary
switched bias diodes. ] This 3 A cathode fuse has a 1k-25
watt ressitor across it.
Question is... what's an easy way to obtain a visual
indication /led/neon/etc.... to let us know said fuses are
blown ???
Most of the "visual indicating" type fuse holders I have seen...
all require use of the line voltage being fed into em... ie: 120
vac, etc.
I was thinking.. perhaps a 1-2 ohm resistor across each fuse
holder... such that the v drop across the resistor [with fuse
open] will trigger a led/etc. [this would amount to installing a
typ grid/cathode electronic overcurrent device..... except it
would only work IF the grid/cathode fuse opens].
Origionaly, we installed a 100 K- 2w-MF across the grid
fuse.... believing if the grid fuse blew.... the resulting bias
developed.. would cutoff the tube... and tube couldn't be
driven. In actual practise... with the typ globs of drive... you
can still drive the tube... albeit.. in class C ! So we
subsequently removed the 100K resistor. Now,,, if the grid fuse
opens... zero output... input swr rises from flat.. to
2:1. ..... No DC grid current.
Here's the real concern. With the grid fuse blown open.... there
is NO return for DC grid current... fine so far. But what
happens to the drive RF [800 ++ watts worth] ?? Are we burning
up the cathode.... and /or the grid ..or both ????
In all cases these RF decks/ power supplies are remote located 30-
50' away from the station. The station can however... read
all 4 bird line sections... and also continuously monitor the
PEP refelcted pwr [part of the high swr shut down circuit].
On a related note.... IF either HV fuse blow open..[1st one
in one leg of sec of plate xfmr... the 2nd in the B+].... and
with tube driven... ALL the electrons flow to the control
grid... instead of the anode... and you guessed it... the grid
current will skyrocket.. [read it will blow the grid fuse asap]
We have seen cases whereby the HV fuse + grid fuse will
blow open... cuz of an antenna problem... and high swr [trips on
high reflected pep power] circuit has NOT tripped the amp
offline.
We have also seen cases, where the high reflected power circuit
trips 1st.... and trips both linears offline.. [which also
applies -10 vdc to the ALC of the xcvr. ]
All depends sometimes... whether it's an arcing ant/badly
installed connector. [resulting in an INSTANTANEOUS wide open/dead
short... infintite swr] OR just a high swr... between 2:1
and 5:1
In some cases the B+ sandfilled fuse blows..... some times BOTH
sandfilled fuses blow.
For some added protection... we are thinking of an adjustable
spark gap between load cap ...and chassis..... set to fire at a
V higher than the normal low swr V.... and a point well below the
rating of the 5 kv vac load cap. I think Rich may have done
this ? I don't want the expensive ceramic vac load cap to ever
internally arc.
Sri for the drivel....... does anybody actually read any of my
posts....... or is this all old news ???
Later... Jim VE7RF
