Thanks to those of you who helped me solve my amplifier problem. In
testing so far, the Ameritron 811 is now functional, though I'm still
waiting on an opportunity to test it on the air. The dummy load is
successfully toasty, however, and there are no more smells of smoke in
my shack.
There has been some positive response to the thread, so I hope those
who weren't so positive will indulge a summary. Here is what I
learned, for other folks who are not amp experts and those considering
adding a small amp to their Omni V/VI rig, and to make sure I learned
the right things:
1. Ameritron AL-811 amps have a diode in the meter circuit. When that
diode fails shorted, it bypasses a small resistance to ground (on the
order of a couple of ohms) in the B- line. That resistance bumps up
the potential of the B- line a few volts. The shorted diode pulls B-
to a hard ground, which will (ultimately) prevent the amp from
accepting input power. So, it's not a fuse in the circuit-breaking
sense, but it does serve as a protection device when things go awry in
the amp. In my amp, that diode failed shorted, but there was no
apparent damage to my meters.
2. The relay output of an Omni V triggers after RF is being produced,
and if you trigger it with CW or the output of the Tune button, the
amp will be hot switching. For occasional tune-ups in SSB use, it's
probably not an issue--others have set up their amps this way with no
issue. For CW, however, that hot-switching will occur at the start of
every break-in after a delay of more than half a second or so. I am
now using the amp switch in my Heil footswitch to key the amp. As Bob
said, the AL-811 does not have QSK and there's no reasonable way to
make it have QSK.
3. The alternative to the above is to trade the Ameritron 811 in on a
used Centaur, if the objective is a cheap half-gallon with QSK. A fine
reader on this list has sold me one, and the money is on its way.
Considering the cost, that's the route I should have gone in the
beginning.
4. There is a point in power production at which the amp goes
non-linear, and another point at which tubes flash over or start to
melt (especially 811's versus 572b's). Using the Tune button will find
both points. In low-duty-cycle use, however, the tubes can take more,
because the average power consumption is much lower than the peak
power consumption. These amps designed for SSB and CW cannot take much
constant carrier, and that was my real Big Mistake. The problem is
that a low-duty-cycle signal causes the plate and grid current meters
to wiggle too much to get a clear reading during tune-up, especially
for those of us new to tuning hollow-state amps. But there are some
other things I learned concerning that issue:
4a: The wiggling will surround average values, assuming a 50% duty
cycle. A string of dits seems close enough to that.
4b. If one tunes for peak power output (as recommended by Jerry and
others), with an eye on the meters to prevent excessive average plate
and grid current, and an eye on the wattmeter for excessive power
production, then it's still easy enough to tune the amp, and a lot
safer. Per Tom Rauch, I advanced the load slightly from the peak to
reduce power a few percentage points as a protection for the output
tank components. As the AL-811 manual states, never reduce grid and
plate current by reducing the load setting below peak.
4c. A good peak-reading wattmeter will read peak power just fine on a
string of dits. As a result of my learning experiences, I purchased an
Ameritron AWM-30, which is an active peak-reading meter of reasonable
accuracy. The peak-reading aspect of it works as it should, unlike
passive meters. I've decided that the $150 for that meter is the
minimum additional purchase for low-priced amps that don't have such
meters internally. Consider it a bargain tax, and even with it these
amps are still cheap.
5. I didn't buy 572b tubes to make more power. I bought them to be
more forgiving. The graphite plates on the 572's won't melt like the
sheet-metal plates on 811's. Yes, at 1600-1800 volts, the plate bias
is a bit low, and some complain that this will cause non-linearity.
Other complain that they won't produce the same power. I have not
found that either of these factors are true, as long as the amp is
operated within the capabilities of its power supply. Which leads me
to:
6. I think I've achieved sufficient exciter power when the tuned amp
has reached its rated power output on a proper peak-reading meter. I
stop adding drive power when my string of dits reads a PEP of 600
watts (450 on ten and twelve meters and 500 on 160), and then after
tuning I back it down to where I want it. Those who try to get more
than rated power are probably running into the limits of the power
supply, which causes the amp to clip and that's what causes splatter.
Those who buy 572b tubes and claim that they made significantly more
power are in the danger zone, it seems to me.
7. I might still use the Omni's Tune button for a quick check on
tuning after tuning up on a string of dits, but not for more than
about five seconds. So far, the key-down output from using Tune has
exactly confirmed the tune-up using a string of dits and a
peak-reading meter.
8. The AL-811 achieves rated power with an exciter drive power of
about 55 or 60 watts. That's between 1 and 2 O'clock on my RF Power
knob on the Omni V, and the forward power meter on my V reads
accurately at this setting.
9. My tuning procedure is now: Set Load and Plate at nominal values
for that band and check band switch. Set Omni V RF power to about 10
or 11 O'clock. With the amp in Bypass (or off), tune the antenna. Put
the amp in operation mode. Key the amp and send a string of dits. The
plate and grid current meters should wiggle around a quarter of the
scale or less. Adjust the Plate control to peak output power on a
proper peak-reading meter. Advance the Omni V's RF power control to
about a littel over 50 watts (about 1 O'clock) and send a string of
dits. Adjust the load control for peak power, advancing it slightly
above the peak. Adjust the plate control for peak power. In my
experience, the peak-reading meter will now be reading 600 watts (450
on 10/12 and maybe 500 on 160). Switch to SSB and set mic gain as
usual to prevent excessive ALC action (being conservative here is a
good thing).
Half-gallon amps are designed for low price points and as such don't
have as many protections. People also run them wide open. Learning how
to stay within safe limits has cost me a set of tubes, a replacement
parasitic suppressor board, a diode, and about 20 hours of complete
enjoyment.
Rick, KR9D
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