Hi all,
I've been watching the list for a few days to get the feel of it and have
decided to post a recent experience, reveal my foolishness, and ask for advice.
I've owned an AL-80A since about 1987 (S/N 095, before Ameritron was MFJ
and when the amps could be had for ~$670!) and have been happy overall with
it. About 8 or so years ago, I decided I wanted to "soup up" the HV PS to
a bit shy of 4 kV to see just how much smoke a single 3-500Z could
generate. I did enough checking to determine that the components in the
amp could handle this, but that I would be much closer to the limits of
various switches and the tuning capacitors. The amp also has an early S/N
(010?) internal, Ameritron PIN-5 QSK switch.
I ordered a Dahl transformer with a 2900 VAC secondary good for 0.6 A and
used 10 kV K2AW rectifiers in a full-wave bridge. I had trouble fitting in
enough capacitive filtering, and feel the filtering is barely adequate at
21 uF (a series string of 10 210 uF at 450 WVDC each). I altered the HV
metering so that the displayed value must be multiplied by 2, because the
HV meter FS reading is 3.5 kV. I also altered the bias Zener, to increase
the bias voltage to 15 V to compensate for the increased B+. I also
dispensed with the 120 VAC capability, and chose to use only 240 VAC.
During this time, I also followed some of the suggestions of AG6K: I
installed a new nichrome/MF resistor plate parasitic suppressor consisting
of about 3 1/2 turns of nichrome wire wound about a 100 ohm MF resistor
spaced about 1/8 inch the surface of the resistor. I also installed a big
10 ohm WW resistor in series with the B+ as glitch protection. I did not
install anything in the cathode line. Finally, I installed a beefier fan
in the case which, to my dismay, was *very* noisy. There is lots of AC
motor noise transmitted through the case.
When I'd screwed up my courage enough and turned it on, it all worked. It
makes about 1 kW CW output on 40-20 m, a bit less on 15 and only about 700
W on 10 m. It makes more than the tuning capacitors can handle on 160
m. The HV regulation is ~ 10% and runs just below 4 kV with an idling
plate current of about 50 mA. I get good reports on the air (in-town tests
on dead bands), though I have not tested it for IMD components. I found
that the output impedance of the tube changed enough that I had to add some
load capacitance on 160 M, but all else loaded up fine.
Through this all the amp has remained quite stable, with no surprises or
scary fireworks. At least, until a bit over a week ago. I was tuning it
up into a dummy load on 20 m and, just as I was satisfied with everything,
some fireworks went off. Visible blue flashes in the cabinet, lots of
popping, and then the fuse blew. Strangely, there was no smell of anything
burned or burning. Visual inspection showed that the grid current meter
was bent against the negative pin, and fuses were blown. A 1.5 ohm 1% grid
current meter shunt resistor had more or less exploded and was open, while
the other 0.6 ohm 1% meter shunt resistor was fine. I straightened the
meter needle and, to my astonishment, the meter still works. A bit more
inspection showed that the Zener diode providing bias voltage to the tube
was shorted. The original Eimac tube shows no grid-to-anything shorts. All
else seems OK.
I need to add that I have been inactive for about the past 5 years (new
job, new house, two kids, same wife :) and the did not thoroughly clean the
amp interior before placing it back in service. I had tuned it up on all
the bands briefly and used it a couple of times as a test, but that was
all. Upon opening it, I did find bits of lint all over the place, though
if this were the cause of my brief adrenalin rush I'd think the offending
lint would no longer be in evidence. So, I'm assuming that I've been
witness to a parasitic oscillation.
I've secured all the needed new parts and am ready to dive in a make
repairs. I've also acquired a nice, *quiet* brushless ball-bearing DC fan
capable of moving about 85 CFM that will replace the noisy AC fan. The 12
V side of the PS has more than enough capability to handle the addition 375
mA load. Before I dive in, what else should I consider doing or
checking? Everything has appeared to be rock-stable until now. More turns
on the parasitic suppressor? I have no more nichrome, but have plenty of
0.028" stainless steel wire (aircraft safety wire). What else?
Many thanks if you've made it this far!
Kim Elmore, N5OP
elmore@nssl.noaa.gov
Kim Elmore, Ph.D.
"All of weather is divided into three parts: Yes, No, and Maybe. The
greatest of these is Maybe" The original Latin appears to be garbled.
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