Hi Les, I just went through the same experience with my 2m 8877 amplifier.
The amp worked very well over the eme contest but did the big BOOM thing
the next time I fired it up, only a couple of days later.
Fortunately, the snubber diodes on the B- rail protected the meter but one
of the diodes gave up it's life in the process.
The tube would start heavy conduction at any plate voltage higher than
about 900vdc. No amount of fiddling with the tube brought any usable results.
I found a spare YU338 (i think that is the number) that was purchased about
20 years ago and squirreled away. A leakage test was made by connecting a
Tripllet 6KV VOM between the cold tube anode and the hv power supply, with
the grid connected to B-. The voltmeter showed about 20% full scale which
is about 12 microamperes of leakage current.
The tube was plugged into the socket, air and heater voltage was applied
for about five hours. A retest of the leakage showed no measurable current
with 4.2 KV so the tube was left in and rf tested. Happily, it works like a
charm now.
I am of the opinion that a leakage test is very important. Further, it does
not take any sophisticated equipment to do a leakage test. Some means of
limiting the current flow is vital and the HV voltmeter is ideal for that
and it gives a meaningful reading in the process.
The current limiting will protect the metering as well as any hv power
supply fuses/breakers.
If a tube doesn't pass this type of test, it is not going to work with rf
either.
73,
Gerald K5GW
In a message dated 11/24/2011 8:56:41 A.M. Central Standard Time,
Listwa@comcast.net writes:
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
Thanks
Les
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