> From: Gary Myers <gmyers@cetlink.net>
> I had one at one time. It is very rugged looking and internally. It does
> take power to get it up to its full capability.
The 1200 tube is a moderate gain tube, because the driving impedance
is high . That increases negative feedback. They typically take about
100 watts for full drive with 3300 volts of loaded anode voltage.
They have about the same gain as a 3-1000Z or a pair of 3-500Z's
operated at the same voltage.
An 8877 on the other hand has a lower driving impedance, and has more
gain. 50 watts will typically drive an 8877 to 1500 watts.
The price paid for higher gain is shorter life and less
reliability. Higher gain tubes have focused beam construction
requiring much tighter mechanical tolerances, metal oxide cathodes
more prone to emission related failures, and gold plated grids are
easily damaged by excess grid current and material tolerances.
>Finally the killer for me is its
> efficiency falls off on 20 and above hitting an abysmal 45% on 10
> meters!
There were some mods to the early AL-1200's to improve efficiency,
but I've never seen on that low. About the lowest I've seen was 50%
or so on ten meters, with about 1200 REAL watts output.
The mods involved lowering the impedance of the input coax to the
tube sockets to 25 ohms, neutralizing the tube, and using a socket
with less grid inductance (better grounding).
Regarding the measurements, a bunch of people just went through
a similar exercise on the antenna newsgroup where there was a lack
of understanding (translation, no one reads the meter specs) of how
tolerances affected SWR measurement results.
A Bird 43 watt meter has a factory tolerance of + or - 5% of full
scale anywhere on the scale (you can find that in the manual). That
means a 2500 watt slug can read up to 125 watts high or low
anywhere on the Bird's scale. Even a specially calibrated Bird is
only 2.5% FS anywhere on the scale.
Your 900 watts, on a properly calibrated Bird, could have
really been any power between 775 and 1025 watts.
The plate input power (using the internal meters) was likely to have
been anywhere from 1800 to 2200 watts.
So the efficiency could likely have been anywhere from 35 to 57%.
The question begs to be asked...what meter did you use for the
power output reading, and the power input reading? Did you use the
amps internal meters, or an external plate voltage and current meter?
IMHO, we live in a world of exact answers (to the percent) with very
non-exact data (lucky to fall within 10% of what you estimated).
Perhaps every result should include the tolerance range.
73, Tom W8JI
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