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Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback

Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
Date: Wed, 14 Dec 2005 13:10:14 EST
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2 cent reply de k5gw:
You will need a reliable test setup to know when you have reached your  
design goals. You might consider starting with the test setup and do a thorough 
evaluation of your FT1000D before beginning the amplifier project. Depending on 
the results, you might have to adjust your expectations. One possibility is 
that  the FT1000D power output could be reduced to say 50 or 100w and achieve 
IMD  levels you are wanting. This could eliminate two or three stages of the PA 
 chain. No way to know without testing and adjusting bias levels.
Gerald K5GW
In a message dated 12/14/2005 11:49:58 A.M. Central Standard Time, writes:

Hi  All,

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!

Tom,  K1JJ

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