On Fri, 2008-04-04 at 14:41 -0400, Rick Denney wrote:
> 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.
So any time you hit the tune button or run CW and don't switch to
transmit manually, you abuse the relays with hot switching. Worse if the
input relay was a little faster than the output relay, you may have
drive on the PA with no load. A recipe for a flash over in the tubes or
the tuning capacitor.
>
> 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:
While the tubes can take averaging the power dissipation they are still
peak voltage limited, whether for one cycle, one millisecond, or one
hour, over voltage leads to arcs.
>
> 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 watt meter 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.
That is NOT protection for anything. Tuning and loading is setting the
load line for the tubes while the drive level sets the swing along that
load line. When you load heavy, you reduce the load R seen at the tubes
in an attempt to prevent the voltage swing from getting too big. In the
process you actually increase the loaded Q of the tank and so the
circulating current and its that circulating current (Q times the simple
voltage drop across the coil) that heats the coil and band switch.
>
> 4c. A good peak-reading watt meter 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.
As will a peak reading voltmeter.
>
> 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:
The biggest problems with 572 is that the leads are longer so the
inductance on that "grounded" grid is higher, making for poorer
stability. The second biggest problem is that the elements are bigger
leading to a larger C making tuning on 10 meters harder. The 572 dates
from the 30s like the 811.
>
> 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.
Not only danger to the power supply but to the output tank. And an
output tank with taps set for loading 811 right won't load 572 any
heavier without moving it away from the designed transformation ratio
and that really takes moving the taps on every band.
>
> 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.
The biggest problem is at the beginning of the output pulse before the
relays have switched. Time other than heating components with continuous
power that were chosen for intermittent voice service is not so much of
a consequence.
>
> 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.
And you should set the exciter ALC to limit the power to that level.
>
> 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 little 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).
Once you have the tuner tuned, the amplifier ought to tune at EXACTLY
the same settings for any part of any band it has been tuned to before.
So if you can set up a tuning chart with tuning positions to 2 or 2.5
digit precision, it should be practical to tune it cold to perfection
every time. Failing a 100 digit per half turn scale, perhaps some paper
rubber cemented behind the knob would accept sharp pencil marks for each
band segment.
>
> 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.
And it probably was the old parasitic resistors that had drifted in
value to cause all your problems. They let the PA oscillate unloaded,
that lead to excessive voltage that caused the tubes to flash over from
plate to grid, which damaged the grid, then the arc current showed up as
excessive plate current that melted the diode.
>
> Rick, KR9D
>
73, Jerry, K0CQ
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