>
>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.
? The stock bandswitch is not ok with a 4kV anode supply.
>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
21uF is about 200% more than is needed for a FWB.
> (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.
>
Silicone rubber mounting helps.
>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.
The 10m coil material is not sufficient to compensate for increase in
skin affect/RF-R at 28MHz.
> 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.
75mA of ZSAC is needed for linear operation.
> 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.
>
More V = higher RL = more tank L , which = less C-tune, and less C-load.
>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.
Glitch diodes are somewhat cheaper than meters.
> A bit more
>inspection showed that the Zener diode providing bias voltage to the tube
>was shorted.
A shorted zener is often the result of an intermittent VHF parasite.
More inductance on L-supp makes for less VHF gain.
>The original Eimac tube shows no grid-to-anything shorts.
A bent filament helix can be found with a high-pot tester.
>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.
Cleanliness, with the exception urine spray from pets, is seldom a factor
in such matters.
>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.
When lint arcs it usually leaves a carbon path.
>So, I'm assuming that I've been
>witness to a parasitic oscillation.
>
Either that or the amplifier is Possessed by The Devil.
>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?
Invest $11.95 in one of our new, improved, callsign-embroidered prayer
cloths to drape over the amp when it not in use. This is guaranteed to
keep demons out of the amplifier when you are elsewhere.
>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).
Safety wire is almost as resistive as Ni-Cr-Fe wire. If you need nasty
flux, Ni-Cr-Fe wire. and/or silver-solder, let me know. If R-supp does
not discolor on 10m, you need more L-supp.
cheers
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
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