[TenTec] 811 Amplifier redux

[Bob McGraw - K4TAX]

Darn good summary and clearly a lot learned.  Glad to hear of it.

73
Bob, K4TAX


----- Original Message ----- 
From: "Rick Denney" <rick at rickdenney.com>
To: <tentec at contesting.com>
Sent: Friday, April 04, 2008 1:41 PM
Subject: [TenTec] 811 Amplifier redux


> 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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