Date: Wed, 23 Nov 2011 16:21:46 -0500
From: "Les" <Listwa@comcast.net>
Subject: [Amps] Arcing 8877
Hi All,
I have a 2nd hand 8877 based 144 Mhz amp, built like a rock. Has worked
fine for over a year, then bang. I saw a flash and of course the fuse did
its job. The tube in it was maybe 20 years old.
It took out the shunt( resistor and diode) in the Grid and Meter which I
replaced. I figure I would replace the tube since I had a newer used tube.
I " burned" it in for about 18 hours with the filament and fan running and
another 6 hours with the HV on, just for good luck.
I keyed it and slowly brought the power up, worked fine at 5 W and at 10W
in I measured 230W on the bird meter and it flashed again, fuse again. I
replaced the fuse and powered it up and again at 10 Watts showing 230W, but
the metering was gone and I looked through the vent screen and saw it arcing
intermittently at the age of the fins, above the Anode strap that goes
around it.
I am looking for advise. Easy to fix the meter shunts, but is it a problem
with the tube(s), the anode strap is as tight as it goes.... any guesses.
73s
Les
W2LPL
### Put some back to back diodes directly between B- of HV supply and
chassis.
The 6A10's have a 400A surge rating. Use 2 in parallel to make one assy. Make
2 x identical assy's. Wire em rvs connected as per above. Then do the same
again across
your grid and plate current meter. Put a 50 ohm/50-100 watt wire wound in
series with your
b+ lead. Install a Hv fuse just prior to the 50 ohm glitch resistor. Now you
can't destroy
anything thing inside the amp. The 50 ohm glitch R will limit the fault
current, and the
HV fuse will interrupt the fault current. The safety diodes will protect
the meters. The safety
diodes between B- and chassis will not let the B- wander more than +/- .7
vdc.
later... Jim VE7RF
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