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RE: Be prepared for the worse



Message text written by "Edward R. Cole"
>
Well that is the rub.  Most High Power cavities do not adjust easily.  Not
over this far a range, so one almost needs a second amplifier tuned to
2.4G.  So it may be a case of build it and they will come?

A question for Matt: do you know what freq is used for eme in VK-Land?

>The problem, of course is still to generate 100-200W at 2304 or 2400 MHz, 
>and the really only practical way is with a tube cavity amp like the K9EK 
>one.  And even that needs I think in the single-digit of watts drive. 
Maybe 
>if Ed's still monitoring this list he can give us the details.

K9EK is the expert along with VE4MA.  I believe Ed has gotten 70-100w on
2.3 GHz.  But if there are many with 15-25w bricks then with a ten-foot
dish one could try using JT-44/JT-65.  The question is whether JT-44 tight
frequency constraints and high lunar doppler will make it difficult for
this mode.  I have one of Ed's 7289 amps for 1296 and hope with water
cooling to get 125w (?).  Ed states they are rated to 60w with air-cooling.

73s Ed - AL7EB

>Fred W0FMS<
-------------

Hi, Ed and Fred and other high freq people:

Yup, I am still alive and lurking around. Since my family and I moved (we
bought
an older, smaller, fixer-upper, for more money. Go figure.), I've been
pretty
much out of the radio game. No antennas and too many  house rebuilding
projects. But I saw my call bandied about and thought I would 
answer some questions and elaborate a little on my experiences building
13cm amplifiers.

First, let me say that I am flattered to be noted in the same line as
VE4MA, but 
Barry is really the guru of this stuff. I'm just an obstinate tinkerer and
mechanic 
who tried a lot of stuff and kept good notes and made sense out of them.
And
has a large pile of stuff that didn't quite make it.

The 7289 amps were generally the best. With a good tube (very tube
dependent),
I could get 60W+ RF out and 16db gain (yes, even at that power level.
Verified
by others). It took 1300VDC @ 300mA (CW duty cycle, though they were
linear)
with water cooling (a royal pain, but necessary). I ran them biased for
class AB,
because you could drive them with very low power, and drive power is rather
hard
to come by. C is more efficient, but takes a lot more drive. Tube life was
satisfactory
as long as you paid attention and tuning was fairly easy. The question of
how to 
operate in multiple band segments was one I spent a lot of time on. The
answer is,
 well, yes, kind of. Best operation and easiest, least critical anode
cavity tuning was 
with the anode cavity cut to a length where it would resonate only 10-15
MHz higher
than the desired operating frequency. This kept the tuning disc out near
the cavity
wall and made tuning less critical. The further in toward the center
conductor you get
the tuning disc, the more effect a small movement has on the cavity freq
and the harder
it is to keep the cavity tuned. So, a cavity cut for the Japanese standard
2424 MHz
would work reasonably well on 2401.  And it would make watts on 2304, but
the tuning 
was so critical as to make it very difficult (impractically so) to keep the
amp on power.
 And, yes, I tried all manner of cavity tuning and dielectric loading and
some really 
strange stuff, too. 

Not being satisfied with 60W, and finding out (from VE4MA) about Russian
Gi7b and 
Gs9b triodes, I spent 9 months trying to get the Gi7b to be something other
than a water
heater before I abandoned it for the lower power Gs9b. Another 7-8 months
actually
resulted in a cavity design that actually worked. I have seen 150W out on
2304, but 
10 db gain is about it. And the tube has a strange, domed element structure
with a flimsy
grid. Pour the drive power to it and the output power will rocket up and
just as quickly down. 
You chase the power with the anode tuning, or set it so the tube will drift
into the power,
attempting to compensate for change in internal electrode capacitance as
the grid
heats and wiggles around. Very frustrating. I did actually make one design
that had
two anode tuning devices. You set the cavity max freq with one, then locked
it in place,
then tuned with the other. Worked pretty well and usable in any portion of
the band,
but the design was even less efficient and wouldn't do much over 100W.
WA8OUU made an automatic bias adjusting device that slammed the tube toward

class A as the output power dropped, then C as it went up and actually kept
the output
 power somewhat manageable at the 100W+ level. Once again, the tube only
likes
 to operate at the maximum cavity frequency. Efficiency was terrible;
testing was 
typically done at 3000VDC and anode currents routinely exceeded 600mA. I
had 
to put a fan on my water cooling system heat exchanger.  As a sidenote..
you treat 
a power supply like this with utmost respect; I know that one slight
blunder with this
3KV 1amp homebrew box and I would be toast. Dead toast. 

I think that the bottom line is that the tube-type amps with finicky tuning
and lethal
power supplies are a useful but difficult to manage solution. When I
started messing 
with them, they were the only game in town. There were no other options
(that a normal
 person could afford) for obtaining reasonable power on 13cm.  And I sold
quite a few
 of them. If, however, as another person said, there is a possible solid
state alternative that 
will put out 100+watts, I would be the first to recommend it. I'd love to
find one.

Have fun!

Ed K9EK

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