Amps
[Top] [All Lists]

[AMPS] Parasitic Experience? What to do? (LONG)

To: <amps@contesting.com>
Subject: [AMPS] Parasitic Experience? What to do? (LONG)
From: elmore@nssl.noaa.gov (Kim Elmore)
Date: Mon, 28 Jan 2002 10:28:55 -0600
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.


--
FAQ on WWW:               http://www.contesting.com/FAQ/amps
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com


<Prev in Thread] Current Thread [Next in Thread>