This is certainly an interesting post - good luck with the endeavor! In the
early eighties, the USAF was still using the Collins 208U-3 and 208U-10
amplifiers at their Global Command and Control System sites. I worked on
these for a living as a young ham... what a job and a privilege.
Regarding your post, the 208U-3 was an auto-tuning, 3KW, 2 - 30 MHz
amplifier driven by 1 volt RMS input (20 mW) from the transmitter box on the
other side of the transmitter room. This was accomplished with three stages
of amplification, a 4CX350 driving another 4CX350, which in turn drove a
4CX3000. This wonderful amp was designed by some genius at Collins, or
group of them, in the 50's. They were robust and wonderful.
Perhaps you could pattern your amp after this wonderful design? Yours would
have to include updated components and engineering design principles.
Good Luck and 73!
"The real thrill in amateur work come not from talking to stations in
distant lands, not from receiving multitudes of 'QSL' cards from all the
world, although these are things to stir your imagination, but from knowing
that by careful and painstaking work and by diligent and systematic study,
you have been able to accomplish some feat or establish some fact that is a
new step toward more perfect communication."
- From Radio Age magazine, May 1926, by Arthur Collins, age 16, founder of
Collins Radio Company
Date: Wed, 14 Dec 2005 12:49:14 -0500
From: "Tom Cathey" <K1JJ@comcast.net>
Subject: [Amps] Designing the Cleanest Linear with RF Negative
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
I'm planning to build up a 75M monoband amplifier that has at least -45db
to -50db 3rd order IMD using heavy RF negative feedback. 10MW in, 1500W out.
I wanted to get opinions here what others thought was the best general
configuration and perhaps suggest some real world tubes.
It seems that the best amps that use RF feedback start with a much larger
tube than generally used in open loop linears and run it about half power .
I plan to use the 10 milliwatt output from my FT-1000D as a source. This is
the low level stage just before it goes into the power amp board. I'm hoping
this is class A stuff and a much cleaner starting point than its -32db IMD
200W solid state power amplifier board output. Any opinions on that?
For this amplifier I am thinking of either a chain of grounded grid stages
so that they have individual built in feedback and an additional loop of
feedback around the whole chain.
OR, use a tetrode final with regulated screen voltage with stages going back
to amplify the 10MW level. Hopefully, use up to 15-20db of loop feedback
assuming the gain is available.
I would appreciate any pointers to URLs showing working schematics of the
big commerical amps that Collins, Harris, etc use, and ideas you may have
for a design.
For example... I don't know what good low level tubes to use, but lets just
say it went something like, 6AU6, 12BY7, 6146, 811A, 3-500Z, GS-35B - all
in grounded grid with an additional feedback loop.
Or, for grid driven: The same low level stages, then 4CX-150, 4-1000A,
4CX-3000. Looking for conservative ratings at 1500W and high IMD figures.
Another thing I wonder about is if a tube like the 6146 or 4-1000A which has
modest IMD figures open looped, can successfully participate in a -45
to -50db IMD chain when big feedback is used?
I will be building this amp over the next few months and will post the
results once completed. Thanks in advance for any help!
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